Coordination chemistry and catalysis with secondary phosphine oxides

Albert Gallen a, Antoni Riera bc, Xavier Verdaguer *bc and Arnald Grabulosa *a
aDepartament de Química Inorgànica i Orgànica, Secció de Química Inorgànica, Universitat de Barcelona, Martí i Franquès, 1-11, E-08028, Barcelona, Spain. E-mail: arnald.grabulosa@qi.ub.es
bInstitute for Research in Biomedicine (IRB-Barcelona), The Barcelona Institute of Science and Technology, Baldiri i Reixac 10, Barcelona 08028, Spain
cDepartament de Química Inorgànica i Orgànica, Secció de Química Orgànica, Universitat de Barcelona, Martí i Franquès 1-11, E-08028, Barcelona, Spain

Received 27th July 2019 , Accepted 5th September 2019

First published on 5th September 2019


Secondary phosphine oxides present tautomeric equilibrium between the pentavalent oxide form (SPO) and the trivalent phosphinous acid (PA). This dichotomy is the origin of the rich coordination chemistry of this class of compounds. As the pentavalent oxide form usually predominates, SPOs are air-stable but at the same time metal coordination can shift the tautomerism towards the PA form, making the ligand act as an ordinary trivalent phosphine. For this reason, this class of ligands has found application in numerous homogeneously catalysed reactions, including some enantioselective transformations. This review aims to give an up-to-date account on the synthesis, coordination chemistry and homogeneous catalysis of SPOs.


image file: c9cy01501a-p1.tif

Albert Gallen, Antoni Riera, Xavier Verdaguer, Arnald Grabulosa

Albert Gallen obtained his PhD in Organic Chemistry from the University of Barcelona (UB) in 2019 with Prof. Riera and Dr. Grabulosa, working in asymmetric catalysis. He is now carrying out postdoctoral research at the Inorganic and Organic Chemistry Department of UB with Prof. Martínez in inorganic reaction mechanisms.

Antoni Riera obtained his PhD in Organic Chemistry from the University of Barcelona (UB) in 1987 with Profs. Serratosa and Pericàs. After a postdoctoral stay with Prof. Amos B. Smith III (Philadelphia), he earned in 1988 an Assistant Professorship at the Organic Chemistry Department of UB and in 2003 a Full Professorship at the same institution. Since 2002, he has been a group leader at the Institute for Research in Biomedicine (IRB Barcelona). His main research areas are asymmetric synthesis and preparation of biologically active compounds. He is co-founder of the company Enantia, S.L.

Xavier Verdaguer obtained his PhD in Organic Chemistry from the University of Barcelona (UB) in 1994 with Profs. Pericàs and Riera. After a postdoctoral stay with Prof. Buchwald (MIT), in 2003 he earned an Assistant Professorship at the Inorganic and Organic Chemistry Department of UB. His research focuses on developing efficient P-stereogenic phosphines for asymmetric hydrogenations and Pauson–Khand reactions.

Arnald Grabulosa obtained his PhD in Inorganic Chemistry from the University of Barcelona (UB) in 2005 with Prof. Muller. After postdoctoral periods with Profs. Gros (Nancy), Kamer and Clarke (St Andrews), in 2017 he earned an Assistant Professorship at the Inorganic and Organic Chemistry Department of UB. His research focuses on organometallic chemistry with novel phosphorus ligands for catalysis and other applications.

1. Introduction

Tertiary phosphines have been used in the preparation of countless coordination and organometallic complexes since they efficiently bind to a large number of metallic centres providing homogeneous catalysts.1

The design of phosphine ligands becomes crucial for the catalytic applications of their metallic precursors. Often, however, these phosphines or even their complexes are difficult to handle, due to their air- and moisture-sensitivity. The preparation of the ligands requires laborious multistep methods often including the use of BH3 adducts2 or phosphonium salts.3 The classical approach employed before the use of borane adducts was the use of highly stable phosphine oxides that had to be eventually reduced, normally using silanes under harsh conditions, incompatible with many functional groups and often leading to racemisation of P-stereogenic compounds.

In contrast, a particular type of oxide is the secondary phosphine oxide (SPO), which due to its tautomerism with a P(III) phosphinous acid (PA) can act as a P-donor ligand in the presence of transition metal cations. This fact has been known for more than 50 years4 and the first catalytic applications of SPOs appeared as early as 1986, by van Leeuwen and co-workers.5 However, this type of ligand received a boost in the early 2000s in cross-coupling chemistry6 and it currently represents an important, yet underused kind of ligand in homogeneous catalysis.

This topic has been previously reviewed but it was either some time ago by Ackermann7,8 or in a partial way by other authors.6,9–13 In the present review, the aim is to give an overview of the coordination behaviour of SPOs with transition metals from group 8 onwards which are those that have found further application in catalysis, as will be discussed in the last part. Although the review is not comprehensive, we have attempted to cover the most relevant results in synthesis, coordination chemistry and catalysis with SPOs.

2. Stereoelectronic properties of SPOs

Like the textbook example of keto–enol tautomerism, similar equilibria can be also drawn for some organophosphorus compounds. Secondary phosphine oxides (SPOs) and heteroatom-substituted secondary phosphine oxides (HASPOs) are of particular interest because they formally contain a pentavalent phosphorus atom (σ4λ5), but they can act as potential P-donor ligands due to tautomeric equilibrium with the trivalent phosphinous acid or related compounds (σ3λ3) (Scheme 1).
image file: c9cy01501a-s1.tif
Scheme 1 Tautomerism of (HA)SPOs.

The trivalent tautomeric form, a phosphinous acid (PA), is able to bind Lewis acids such as metal ions, and as a result, SPOs are occasionally referred to as pre-ligands. In general, SPOs are stable towards oxygen, which can be explained by the prevalence of the pentavalent tautomer except in the presence of transition metal cations or silylating agents,14 which can shift the equilibrium towards the trivalent form. This was proved by deuterium exchange experiments and IR spectroscopic measurements in the 1960s.15 In 2004, Pietrusiewicz, Duddeck and co-workers suggested16 from NMR studies that the fast kinetics of the tautomeric equilibrium is due to the migration of an acidic proton. In this line, Hong and co-workers17 proposed two reaction pathways for the transformation of SPOs to the corresponding PAs (Scheme 2), based on DFT calculations.


image file: c9cy01501a-s2.tif
Scheme 2 Proposed routes for the conversion of SPOs to PAs. Unimolecular pathway (top) and bimolecular pathway (bottom).

In the unimolecular pathway, the tautomerism takes place due to the intramolecular migration of the hydroxyl proton, whereas in the bimolecular one the conversion can be rationalised by an intermolecular transfer of two hydrogen atoms in a synchronous exchange.

More recently, Montchamp and Janesko18 have shown that the reaction mechanism involves the simultaneous dissociation of a P–H bond and the formation of an O–H bond. The proton exchange, which leads to the formal reduction of the P(V) species, cannot take place via an intramolecular proton transfer. According to their DFT studies, the unimolecular pathway does not work for those SPOs containing very bulky substituents, since the distorted geometry of the tetrahedral species has very high-energy barriers.

The electronic character of the substituents at the P atom affects the tautomeric equilibrium. In 2010, Börner and co-workers19 studied the tautomerism of five SPOs with different electronic properties (Fig. 1).


image file: c9cy01501a-f1.tif
Fig. 1 SPOs studied by Börner and co-workers.19

They concluded, according to NMR, IR, DFT calculations and X-ray structural analysis, that only with strong electron-withdrawing groups can the phosphinous acid form be observed in equilibrium, whereas for all the other cases the phosphine oxide form completely prevails. This tendency has been clearly observed by 31P NMR spectroscopy, whose chemical shifts and 31P–1H couplings revealed that with the exception of the perfluorinated phosphine in Fig. 1, all other SPOs exist exclusively in the pentavalent form. Indeed, uncoordinated phosphinous acids have only been described with extremely electron-withdrawing substituents.20,21

Montchamp and Janesko18 have suggested an explanation for this behaviour taking into account an additional resonance form in the prototropic tautomerism of SPOs (Scheme 3).


image file: c9cy01501a-s3.tif
Scheme 3 Tautomerism and resonance forms of SPOs.

The phosphonium form depicted in Scheme 3 is destabilized by strong electron-withdrawing substituents. But this simple description allows the rationalization of other minor tendencies such as the influence of the solvent on the tautomerism.19

The work of Martin, Buono and co-workers22 in the assessment of the electronic properties of SPOs is worth mentioning. They concluded that the coordinated phosphinous acids are less donating than the corresponding trisubstituted phosphorus analogues, including not only tertiary phosphines, but also phosphites and aminophosphines. In contrast, deprotonated phosphinous acids (phosphinito ligands)12 showed strong electron-donating behaviour, being among the most electron-rich P ligands, close to phosphonium ylides, and even surpassing N-heterocyclic carbenes.12,22

The clear stereoelectronic differences between the neutral coordinated phosphinous acids and their anionic forms are a crucial factor for the exceptional behaviour of these ligands in some catalytic transformations, in which the abstraction of the OH proton occurs during the reaction.

Interestingly, it has to be mentioned that in the case of P-stereogenic secondary phosphine oxides, the tautomeric equilibrium does not affect the stereochemical integrity of the phosphorus atom7,16,23 but, despite that, very few optically pure P-stereogenic SPOs have been prepared.

3. Synthesis of SPOs

3.1. Achiral or racemic SPOs

Traditionally, SPOs have been classically prepared by the easy hydrolysis of phosphine halides (Scheme 4, 1).24 Other widely used methodologies are the displacement of an alkoxide from a monosubstituted phosphinic ester by a Grignard reagent (Scheme 4, 2) or by the reduction of a disubstituted phosphinic ester by treatment with lithium aluminium hydride (Scheme 4, 3).15,25 Condensation of aldehydes with primary phosphines (Scheme 4, 4) also leads to the corresponding SPOs in refluxing trifluoroacetic acid.26 In addition, the obvious way to prepare SPOs is through oxidation of secondary phosphines with hydrogen peroxide (Scheme 4, 5) or molecular oxygen at 50–70 °C.27 This method, however, is rarely employed because it usually produces undesired oxidised by-products.
image file: c9cy01501a-s4.tif
Scheme 4 Main synthetic strategies, (1)–(5), to prepare SPOs.

All these strategies allow the preparation of a variety of SPOs, especially those containing aryl substituents.28 In contrast, dialkyl-substituted secondary phosphine oxides are more difficult to prepare because due to their basicity they undergo undesired secondary reactions involving the formation of P–O–P bonds. Li and co-workers29 have reported a useful polymer-supported synthesis of dialkyl-substituted SPOs,29 as depicted in Scheme 5.


image file: c9cy01501a-s5.tif
Scheme 5 Polymer-supported synthesis of dialkyl-substituted secondary phosphine oxides.

Polymer-supported aminophosphines can be straightforwardly prepared by treating Merrifield's resin with an excess of tert-butylamine to form the polymer-supported secondary amine. This resin reacts with PCl3 in the presence of triethylamine to give the supported dichlorophosphine precursor. The reaction of this precursor with a variety of organolithium and Grignard reagents leads to complete substitution of P–Cl bonds. Finally, the desired SPOs can be obtained by hydrolysis that cleaves the SPO from the resin.30,31

This methodology has been extended to the preparation of bidentate systems by introducing 1,2-bis(dichlorophosphanyl)ethane instead of phosphorus trichloride.29

In contrast, (HA)SPOs, H-phosphonates or their derivatives, can be directly obtained from the corresponding diamines, diols or aminoalcohols and PCl3 in a one-pot or two-step procedure (Scheme 6).32,33


image file: c9cy01501a-s6.tif
Scheme 6 General methodology for the preparation of (HA)SPOs.

Other strategies have been developed in order to prepare HASPOs containing P–N bonds. A recent methodology, reported by Hong and co-workers,34 is based on the reaction of imines with organolithium or Grignard reagents followed by condensation with a chlorophosphine. The subsequent aqueous work-up of the chloroaminophosphines leads to the desired amino-substituted SPOs (Scheme 7).


image file: c9cy01501a-s7.tif
Scheme 7 Preparation of amino-substituted SPOs.

Both procedures provide an attractive platform for the synthesis of enantiomerically enriched secondary phosphine oxides, since the backbone of the diamine or diol, along with the substituents of the imine, may contain stereogenic elements, as will be discussed in the next section.

The ferrocenyl group has given many interesting ligands and SPOs are not an exception. As an example, Scheme 8 shows the synthesis of a family of ligands described by Hong and coworkers.35 The ligands were prepared by lithiation of ferrocene, followed by phosphination and hydrolysis, and were employed in Pd-catalysed Suzuki–Miyaura coupling reactions.


image file: c9cy01501a-s8.tif
Scheme 8 Synthesis of racemic SPOs bearing a ferrocenyl group. *CTLC = centrifugal thin layer chromatography.

3.2. Non-racemic SPOs

Chiral, non-racemic SPOs can be classified into two broad categories: those that are chiral due to the backbone and P-stereogenic SPOs.

The first example of an SPO with a chiral scaffold was described in 1999 by Fiaud and co-workers36–38 (Scheme 9) and was prepared by diastereomeric resolution using quinine as a resolving agent.


image file: c9cy01501a-s9.tif
Scheme 9 Synthesis of the optically pure diphenylphospholane SPO.

The general method of Scheme 6 employs diamines or diols and inexpensive PX3 precursors. An interesting asymmetric version of this method was reported in 2000 by Enders and co-workers,39 making use of the enantiomerically enriched TADDOL39 (Scheme 10). Several HASPOs have been prepared according to this general synthetic strategy and most of them have been proved to be valuable ligands in nucleophilic catalysis.40,41


image file: c9cy01501a-s10.tif
Scheme 10 Synthesis of the (R,R)-TADDOL-SPO derivative.

In addition, some examples of enantiomerically enriched HASPOs42 have been obtained from the corresponding optically pure 1,2-diamines (Fig. 2).


image file: c9cy01501a-f2.tif
Fig. 2 Selected chiral secondary diaminophosphine oxides.

As can be seen, all the examples presented contain at least one stereogenic element in the backbone. An elegant synthesis of a HASPO with a stereogenic phosphorus atom was reported by Hamada and co-workers (Scheme 11).10,43–45 In this methodology, L-aspartic acid was converted in a few steps into a chiral triamine. A diastereoselective formation of the corresponding triaminophosphine using PCl3, followed by the subsequent hydrolysis, yielded the desired diaminophosphine oxide (DIAPHOX). The last step can be considered as an SN2 reaction, which can be easily achieved through column chromatography with wet silica. It is important to note that these special types of ligands have displayed excellent results in many palladium- and iridium-catalysed asymmetric processes providing high enantiomeric excesses, especially in asymmetric allylic substitution reactions.43,45–49


image file: c9cy01501a-s11.tif
Scheme 11 Synthesis of the (S,RP)-DIAPHOX ligand.

Even though the preparation of SPOs with a chiral backbone is well-established,42 the same statement is not true regarding P-stereogenic SPOs and for this reason only a few examples are known so far.8,50

The use of menthylphosphinates as chiral auxiliaries in the preparation of P-stereogenic SPOs is relatively well developed.51 The first synthesis of a P-stereogenic SPO was reported in 1968,15 when benzylphenylphosphine oxide was prepared by reduction of the corresponding menthol-precursor with lithium aluminium hydride (Scheme 12).


image file: c9cy01501a-s12.tif
Scheme 12 Preparation of the first P-stereogenic SPO and its epimerisation.

Mislow and co-workers,25 however, demonstrated that this method does not constitute a suitable strategy for the preparation of SPOs, since it causes racemization.25 The epimerization of the phosphorus atom can be rationalised by means of a hydride-addition–elimination mechanism involving a pentacoordinated dihydrido species.

In order to overcome these difficulties, Buono and co-workers52,53 developed another synthetic route, which relies on the reaction of diastereomerically pure menthyl or adamantyl phosphinate precursors with organolithium and Grignard reagents (Scheme 13). Replacement of the phosphinate moiety occurs with the expected inversion at the phosphorus atom.54


image file: c9cy01501a-s13.tif
Scheme 13 Asymmetric synthesis of SPOs with organometallic reagents.

Although the substitution reaction works very well for menthylphosphinates (Scheme 13, top)55 obtaining optically pure menthylphosphinates is cumbersome when Ar is different from the usual phenyl group. For this reason, enantiopure adamantylphosphinates, obtained by means of expensive semi-preparative HPLC,56 were employed (Scheme 13, bottom). In addition, the steric hindrance of the organolithium and Grignard reagents and their excess in the reaction media affect dramatically both the yield and the stereochemical purity of the obtained SPOs. More recently, similar results have been obtained using α-D-glucosamine as a chiral precursor57 with ee values comparable to those reported by Buono and co-workers.56

tert-Butylphenylphosphine oxide has traditionally been the most studied P-stereogenic SPO. It can be obtained in an optically pure form using 1-phenylethylamine as a resolving agent,58,59 in a method that starts with the conversion of the racemic SPO into the corresponding phosphanylthioic acid. Resolution of the phosphanylthioic acid with an enantiomerically enriched amine and subsequent desulfurization in the presence of RANEY® nickel58 provides the desired optically pure SPO (Scheme 14).


image file: c9cy01501a-s14.tif
Scheme 14 Preparation of the optically pure tert-butylphenylphosphine oxide by resolution.

Other efficient methods to separate rac-tBuPhP(O)H have been described using (S)-mandelic acid or ephedrine as a resolving agent.60–62 In parallel, Minnaard and co-workers63,64 described an elegant dynamic resolution of tBuPhP(O)H by crystallisation with (−)-dibenzoyltartaric acid, achieving excellent ees.

Complementary to the above-mentioned methodologies, Feringa, de Vries and co-workers have successfully resolved various SPOs through preparative chiral HPLC.36,65,66 This technique has allowed the isolation of several enantiomerically enriched SPOs (Fig. 3).


image file: c9cy01501a-f3.tif
Fig. 3 Enantiomerically enriched SPOs through preparative chiral HPLC.

Other strategies for the preparation of optically pure tert-butylphenylphosphine oxide via asymmetric synthesis were published in 2005 by Buono and co-workers.54 In that work, they described for the first time the synthesis of the two enantiomers of tBuPhP(O)H using an oxazaphospholidine precursor derived from (S)-prolinol. The highly diastereoselective ring-opening of the oxazaphospholidine with tert-butyllithium in THF at low temperature (Scheme 15) constitutes the key step of the methodology.


image file: c9cy01501a-s15.tif
Scheme 15 Synthesis of both enantiomers of tBuPhP(O)H.

While this ring-opening reaction was shown to proceed with retention of the phosphorus configuration, the two enantiomers of the SPO can be obtained by just changing the Brønsted acid during work-up. Therefore, strong Brønsted acids (pKa ≤ 1) lead to retention of the configuration at phosphorus, whereas weaker acids (pKa ≈ 3–5) produce products with inversion. Unfortunately, this elegant strategy could not be extended to other SPOs different from tBuPhP(O)H.

Other approaches have been examined in order to introduce more than one stereogenic element in the structure of SPOs. Dubrovina, Börner and co-workers9,67 prepared a mixture of diastereomeric dodecahydrodibenzophosphole 5-oxides.9,67

In their search for a general method to prepare chiral SPOs from cheap precursors, Han and co-workers68 have described the use of aminoalcohols as chiral auxiliaries, in particular for SPOs containing naphthyl-derived substituents in combination with bulky groups (Scheme 16).


image file: c9cy01501a-s16.tif
Scheme 16 Enantioselective synthesis of chiral SPOs employing aminoalcohols.

This example emphasizes the potential use of aminoalcohols as chiral auxiliaries for the preparation of P-stereogenic SPOs, even though this methodology is, in fact, more general and can be extended to the synthesis of many P-stereogenic ligands.

Very recently, our group designed for the first time the synthesis of enantiopure (S)-tBuMeP(O)H using enantioselective synthesis (Scheme 17).69


image file: c9cy01501a-s17.tif
Scheme 17 Synthesis of (S)-tBuMeP(O)H. Its enantiomer has been prepared analogously.

The phosphinous acid–borane precursor was prepared by hydrolysis of the corresponding amino derivative.70,71 Since the synthesis starts from cis-aminoindanol,70 both enantiomers are equally accessible.

Treatment of the phosphinous acid–borane with tetrafluoroboric acid62,72 affords the boron trifluoride adduct of the desired compound as a stable solid. This adduct constitutes the second reported crystal structure of an SPO–BF3 adduct after the phospholane oxide–BF3 adduct reported by Toffano and co-workers.73 Analysis of the crystal structure supports that the mechanism of deboronation74 takes place by substitution of the hydrogen atoms of the borane group by fluorine, as suggested by McKinstry and co-workers.75 After basic hydrolysis, the desired oxide was obtained in high yield with excellent enantioselectivity.

Very recently, Senanayake and Tsantrizos76 reported the synthesis of a library of SPOs bearing a tert-butyl group, also using a chiral aminoalcohol as a chiral auxiliary (Scheme 18).


image file: c9cy01501a-s18.tif
Scheme 18 Synthesis of t-butyl containing P-stereogenic SPOs.

The synthesis was based on the formation of a P-stereogenic H-phosphinate by condensation of t-BuPCl2 with the chiral aminoalcohol, followed by hydrolysis. Nucleophilic displacement of the auxiliary by Grignard reagents and hydrolysis provided a library of P-stereogenic SPOs. The synthesis is interesting because it avoids the use of highly pyrophoric reagents such as t-BuLi to make the synthesis more amenable for large scale preparations of ligands.

There are also several optically pure ligands bearing the ferrocenyl substituent that deserve to be mentioned, since they produced excellent hydrogenation catalysts. Pugin, Pfaltz and coworkers77 some time ago described mixed phosphine oxide–phosphine ligands with a ferrocenyl substituent, called JoSPOphos, because they were reminiscent of Josiphos (Scheme 19). These ligands had a stereogenic phosphorus atom and a chiral backbone and were obtained from the well-known of Ugi's amine. Recently, Berthold and Breit have reported new JoSPOphos ligands.78


image file: c9cy01501a-s19.tif
Scheme 19 Preparation of JoSPOphos and SPO-Wudaphos ligands.

In another approach, Chung, Dong, Zhang and coworkers,79,80 inspired by the properties of enzymes, designed a special type of SPO (called SPO-Wudaphos), also starting from Ugi's amine (Scheme 19). These ligands were designed to engage ion pair and H-bond noncovalent interactions for highly enantioselective asymmetric hydrogenation of several substrates.

4. Complexation of SPOs

Secondary phosphine oxides can act, as neutral species, as ambidentate ligands by using the soft phosphorus atom in the PA tautomer or the hard oxygen atom in the SPO tautomer (Scheme 20).81,82
image file: c9cy01501a-s20.tif
Scheme 20 Tautomeric equilibrium of PA–SPO and main coordination modes of SPOs.

The coordinative behaviour depends, among other factors, on the affinity of the metal for one site or the other. In general, early transition metals coordinate through the hard oxygen atom, giving M–SPO complexes, and late transition metals (more employed in homogeneous catalysis) through the soft phosphorus atom, producing M–PA complexes.11,83,84 Hence, examples of P-coordinated complexes can be found for Fe, Pd, Ir, Au, Pt, Rh and Ru, whereas the O-coordination has been described for example with Ti, Cr, Mo, W, Mn, Re and Ru, among others. Interestingly, for some cations in the same oxidation state, examples of both types of coordination are found in the literature, such as the complexes in Schemes 24 and 25. This shows how subtle are the factors that influence the coordination of one or another tautomer. It has to be noted, however, that in the vast majority of complexes used in catalysis the P-coordination of SPOs (as PAs) predominates. Despite this, in the literature these metallic systems are usually called complexes with “SPO ligands” when strictly speaking they should be named as complexes with “PA ligands”. In the present review, the usual nomenclature is usually employed except when emphasis on the PA tautomer is intended.

PAs are weak acids that can be easily deprotonated by bases.85 This gives anionic, strong electron-donating phosphinito ligands, which again can generate either P-coordinated or O-coordinated complexes (Scheme 20). Examples of the more common former kind of complex include Mo, W, Mn, Re, Fe, Pt and Au while the latter coordination has been found for Fe, Rh, Ir and Ag.11

A recent review by Igau and his coworker12 deals with complexes of anionic, P-coordinated phosphinito (also called “phosphoryl”) ligands (M–P(O)R1R2), some of which may surpass carbenes in terms of electronic donation. They are interesting ligands not only in catalysis but also in supramolecular chemistry since they are strong hydrogen bond acceptors. Of course, M–P(O)R1R2 can be considered as complexes with SPO ligands (although they are very often not prepared by coordinating the SPO to the metallic precursor)12 and therefore some of them and their catalytic applications are discussed in the present review. However, full account of their synthesis, properties and applications is not given and can be conveniently found in the mentioned review.12

Interestingly, two SPO units attached to a metal centre can form an intramolecular hydrogen bond, acting as an anionic pseudobidentate ligand (Scheme 20). This moiety can be formally considered as assembled by the combination of a neutral phosphinous acid and an anionic phosphinito unit. This process often requires an external base and leads to a stable six-membered ring. Although the formation of such species is both thermodynamically and kinetically favoured, the required cis geometry cannot always be achieved and sometimes trans isomers with two monodentate PAs are obtained instead.22,69,81 Nevertheless, this pseudobidentate coordination mode has spurred interest in the study of the applications of SPOs in catalysis7,11 and it has been found, for example, that dialkyl SPOs are excellent ligands for cross-coupling reactions.6

It is interesting to note that sometimes the phosphinous acid ligand can be modified during catalysis, such as in allylic alkylation,86 in which the OH group is silylated by N,O-bis(trimethylsilyl)acetamide (BSA) and therefore it is no longer a “true” phosphinous acid.

Finally, it has to be mentioned that SPOs are being increasingly used in the stabilisation of nanoparticles (NPs).87 One of the earliest reports in this direction was the observation of Wang and Buhro,88 who noted that dioctylphosphine impurities had a profound impact on the morphology of CdSe nanocrystals. Inspired by this result, several SPO-stabilised metallic nanoparticles have been synthesised and are being tested in catalytic reactions, as will be discussed in the present review.

4.1. Ruthenium

Although the first examples of Ru systems bearing PO–H⋯PO bridges were initially published during the 1980s,89–91 it was not until 1999 when these studies were extended to SPOs92 (Fig. 4).
image file: c9cy01501a-f4.tif
Fig. 4 Ru complexes containing PO–H⋯PO bridges.

Ru(II) compounds containing the dimethyldithiophosphinate ligand were discovered by Robertson, Stephenson and co-workers89 while they were studying the reactivity of PPh2Cl with cis-[Ru(S2PMe2)2(PPh3)2]. Refluxing such a complex in a mixture of acetone and water allowed the compounds depicted in Fig. 4 to obtained. The intermolecular H-bridges could be successfully substituted, forming a chelate, upon treatment of the pseudobidentate complexes with BF3·Et2O.

The hexamethylbenzene Ru(II) complex could be straightforwardly prepared, with good yield by reacting the hexamethylbenzene ruthenium(II) dimer in refluxing methanol and 4 equivalents of (MeO)2P(O)H.90 In contrast, Koelle and co-workers91 prepared the trisubstituted complexes [RuCp*{(PPh2O)3H2}] and [RuCp*{(P(OR)2)3H2}] upon treatment of the Ru(II) dimer [{RuCp*(μ-OMe)}2] with Ph2P(O)H or (RO)2P(O)H. These compounds contain two PPh2OH units and a PPh2O connected by hydrogen bond interactions (Fig. 4).

Almost at the same time, Ru(II) complexes with atropoisomeric biaryl-SPOs were also described (Scheme 21).93–95


image file: c9cy01501a-s21.tif
Scheme 21 Preparation of tethered (η6-arene)–Ru(II) complexes by hydrolysis of phosphorus–carbon bonds.

The synthesis of these aryl triflate ruthenium(II) complexes was carried out by reacting an excess of not thoroughly dry triflic acid with a solution of the Ru(II) acetate precursor in 1,2-dichloroethane. In this reaction, the triflic acid protonates the acetate and a water molecule splits the P–C bond, as described by Pregosin and co-workers.96

Although the preparation of Ru(II)-tethered complexes is a well-known topic,97–101 the special reactivity of phosphinous acid Ru(II)-tethered complexes is worth mentioning, because treatment with HBF4 affords chelate-complexes, similar to those described with O–BF2–O bridges (Fig. 5).


image file: c9cy01501a-f5.tif
Fig. 5 Tethered (η6-arene)-ruthenium(II) complexes containing O–BF2–O bridges.

The first attempts to prepare piano-stool ruthenium(II) structures containing P–OH bonds, in order to achieve pseudobidentate ligation, were described with P(OMe) substituents102 and no catalytic applications were found for these complexes until the work of Ackermann,103 in which they used electron-rich SPOs in ruthenium-catalysed arylation reactions between pyridines or imines and aryl chlorides.

At present, the number of phosphinous acid Ru(II)–arene type complexes is considerably high.13 Their synthesis usually takes place by scission of dimers [RuCl(μ-Cl)(η6-arene)]2 (for arene = p-cymene, mesitylene and hexamethylbenzene, among others) with two equivalents of the corresponding SPO or HASPO (Fig. 6).


image file: c9cy01501a-f6.tif
Fig. 6 Piano-stool Ru(II)–PA complexes.104–107

These reactions are known to take place due to the tautomerism of the (HA)SPOs, followed by the cleavage of the chloride bridges to afford the corresponding mononuclear complexes. However, another synthetic strategy relies on the coordination of a chlorophosphine precursor to the coordination sphere of Ru(II) and later hydrolysis to afford the desired compounds, as described by Cadierno and co-workers.105,107

From a mechanistic point of view, this process appears to be slightly more complex, as demonstrated by Peruzzini, Mealli and co-workers108 in the tautomerism and coordination of compounds HnP(O)(OH)3−n with [RuCp(PH3)2(OH2)]+ fragments. Hence, according to DFT studies, the lowest reaction energy barriers are found for a reaction pathway in which the ligand is initially O-coordinated as an SPO. This coordination mode allows a later rearrangement to a P-coordinated compound, a PA (Scheme 23).

The direct transfer of a P-bound H atom to the terminal oxo oxygen (1,2-proton shift) by a purely acid–base process is energetically prohibited by the uncoordinated SPO, due to the high covalency of the P–H bond. The metal-assisted tautomerisation is much easier and in the studied case occurs via a four-legged piano stool Ru hydride species, formed by oxidative addition of the SPO. From this intermediate, the H ligand is transferred to the phosphoryl group.

A related behaviour can be also observed in the coordination of some SPOs to Ru(II) arene systems, where the O-coordinated analogue is detected in solution and tautomerises upon precipitation.69,106

The use of optically pure SPOs as ligands in asymmetric catalysis is quite scarce, and when it comes to Ru(II) systems, the only precedent was described by Leung and co-workers102 in the coordination of optically pure (S)-tBuPhP(O)H.

Therefore, due to the growing interest in catalysis of the more soluble Ru(II)–p-cymene complexes, compared to other arenes, we envisaged in our group the preparation of an optically pure Ru(II) complex containing (R)-tBuMeP(O)H, following the procedures described by Ackermann and co-workers109 and by Buono and co-workers106 as depicted in Scheme 24.

In our studies it was possible to detect the formation of O-coordinated species, by reacting the dimer with a slight excess of (R)-tBuMeP(O)H. Interestingly, when we tried to isolate this species by precipitation with hexane, only the P-coordinated complex was obtained in low yield. In order to improve the yield, the reaction was carried out in refluxing hexane.109 It turned out that with this procedure the desired complex was obtained in high yield despite the insolubility of the Ru dimer in this solvent.

The coordination of bulky and basic phosphinous acids has been found to be difficult because of the lack of stability of the complexes formed. It is known, however, that electron-withdrawing ligands such as arylphosphines or carbonyl groups tend to facilitate the introduction of more than one SPO in the coordination sphere. This behaviour has been also observed for Ru by Buono and co-workers,81 who reported the synthesis of ruthenium carbonyl complexes bearing SPOs (Scheme 25) which have been successfully applied in the cycloisomerisation of arenynes (Scheme 81).

Our own efforts in the area69 led to the isolation of both O- and P-coordinated complexes following previous methodologies.81 The treatment of [RuCl2(CO)3]2 with (S)-tBuMeP(O)H in THF at room temperature yielded the O-coordinated complex as a stable solid (Scheme 26). Treatment of this complex with another equivalent of (S)-tBuMeP(O)H in toluene at 110 °C for two days produced the bis-coordinated compound, albeit with a low yield. In order to find a more convenient method, the reaction was carried out under microwave conditions, which led to a higher yield in a much shorter time affording the desired complexes with stereochemical retention at phosphorus.

The preparation of Ru(II) dinuclear derivatives was initially disclosed by Gould, Stephenson and co-workers,110 who described trihalide-bridged Ru(II) complexes by hydrolytic cleavage of the coordinated phosphinites into phosphinous acid derivatives, connected by intramolecular hydrogen-bridges (Scheme 27). Hydrolysis of the phosphinite ligands is thought to occur due to the presence of water in methanol solutions.

Nitrosyl–ruthenium(II) dimers forming pseudobidentate architectures have been also described,111 by the reaction of RuCl3 with the corresponding P(O)R(OEt)2 derivatives and diazald (N-methyl-N-nitroso-p-toluenesulfonamide) as a nitrosating agent (Scheme 28).

Another interesting dinuclear Ru(II) was described by Cole-Hamilton and coworkers112 by the reaction of [RuCl2(PPh3)3] and two equivalents of a mixed anhydride–phosphinite ligand (Scheme 29).

This complex, whose structure was elucidated by X-ray diffraction methods, was formed by a rearrangement reaction of the mixed anhydride ligand, as it happens with Rh(III) (Scheme 40).

There are some examples in the literature of phosphinous acid complexes containing Ru centres in other oxidation states. In this regard, the works of Cadierno and co-workers113,114 in the preparation of Ru(IV)-bis(allyl)-SPO complexes (Scheme 30) are worth mentioning.

These complexes were straightforwardly prepared by the direct reaction of several alkyl and aryl SPOs with the chloride-bridged dimeric precursor in dichloromethane. Those complexes containing aromatic SPO units could be also prepared by hydrolysis of the chlorophosphines in THF/H2O mixtures.

As a closing remark to Ru complexes with SPO ligands, it has to be noted that Yakhvarov, Peruzzini and coworkers115 were able to generate a simple but extremely unstable phosphine oxide (H3PO) electrochemically and stabilise its tautomer (phosphinous acid, H2P(OH)) in the form of a Ru(II)–Cp complex.

4.2. Osmium

Despite its close similarity with ruthenium, much less research has been carried out on osmium complexes with SPO ligands. Cadierno and co-workers116 have recently described the coordination of phosphinous acids to the [OsCl26-p-cymene)]2 dimer forming in moderate to high yields complexes of the type [OsCl26-p-cymene)(PR2OH)] and [OsCl26-p-cymene){P(OR)2OH}] (Scheme 31).

The scission of the Os(II)–p-cymene dimer takes place in THF at RT, where the SPO ligands show a clear preference towards the P-coordination mode. As observed for analogous Ru(II)–arene complexes, reactions involving phosphites are kinetically slower than those with SPOs.116

These Os(II)–arene systems could also be straightforwardly prepared in high yields by hydrolysis of the P–Cl bond in complexes [OsCl26-p-cymene)(PR2Cl)] by reflux in wet THF. Similarly, due to the sluggish reactivity of the related [OsCl26-p-cymene){P(OR)2OH}] complexes, their hydrolysis requires more drastic hydrolytic conditions, a behaviour also observed for the cleavage of the P–N bonds in amino-phosphine derivatives [OsCl26-p-cymene){PR2(NMe2)}].116,117

The reactivity of Os(IV) systems was explored by Esteruelas and coworkers118 who isolated the cationic complex [OsH2Cp(PiPr3)(PPh2OH)]PF6 upon the reaction of the neutral precursor [OsClCp(PiPr3)(PHPh2)] in the presence of TlPF6 in wet acetone (Scheme 32).

The reaction seems to occur by P–H oxidative addition of the diphenylphosphine ligand onto the unsaturated intermediate formed upon chloride abstraction with the Tl(I) salt. Further addition of a water molecule to the Os(IV) = PPh2 moiety affords the complex in high yield. The same procedure has been successfully applied to the preparation of pentahydride-diphenylphosphine Os(IV) complexes and the detected intermediates support the proposed mechanism.119

Although other less common oxidation states have been explored in osmium–SPO complexes, such as the porphyrin–Os(VI) derivatives described by Che and co-workers,120 increasing interest in the design of bidentate and pseudobidentate SPO ligands able to coordinate Os(II)–arene fragments has been observed. In this line, Carmona and co-workers121 reported the preparation of P,N-based Os(II) dicationic aquacomplexes containing optically active phosphinooxazoline ligands (Scheme 33).

These compounds appeared as non-separable mixtures of diastereoisomers that were transformed into the related monocationic species in refluxing dichloromethane. Under these conditions, one of the Ph groups migrates from the phosphorus to the osmium centre, giving a coordinated oxazolino-SPO ligand.

Another interesting example of a pseudobidentate benzene–Os(II) complex was prepared by protonation of the bis(dimethylphosphonate) precursor with an excess of trifluoroacetic acid, followed by treatment with an excess of pyridine (Scheme 34). The formation of the heterobimetallic complex can be successfully accomplished by reacting the pseudobidentate species with a Tl(I) salt triggering the H+/Tl+ exchange.122

4.3. Rhodium

Although the first application of Rh–SPO complexes was disclosed in the early 1980s for the hydroformylation of linear olefins,123 more intense catalytic interest appeared when SPOs were successfully applied to asymmetric hydrogenation of imines124 and more recently of functionalised olefins.77 Despite excellent catalytic results, little research about the nature of the Rh species involved in the catalysis has been performed so far.124,125

In particular, there are only a few examples of isolated pseudobidentate Rh complexes in the literature containing SPOs with electron-withdrawing substituents (Fig. 7).125,126


image file: c9cy01501a-f7.tif
Fig. 7 Pseudobidentate Rh complexes reported in the literature.

Börner and co-workers126 managed to prepare a family of neutral Rh(I) SPO complexes by the reaction of two equivalents of the ligand with [Rh(acac)(COD)] in THF. The basicity of the acetylacetonate anion is able to deprotonate the OH group, forming pseudochelated complexes. According to X-ray studies, the shortening of the P–Rh length is correlated with the increase of the electron-withdrawing character of the substituents, as expected. In this line, it is interesting to recall that aryl SPOs tend to react faster with metallic centres, affording more stable complexes. A clear example of this trend was also discussed by Börner and co-workers.126 They observed that while the SPO containing the perfluorinated ligand immediately reacted at −78 °C with Rh, electron-richer di-tert-butylphosphine oxide only produced traces of the expected complex, even at elevated temperatures.

The reactivity of the aryl SPO–Rh(I) pseudobidentate complexes was further studied by Börner's group126 when they prepared some BF2-bridged complexes (Scheme 35) formed in situ by later chelation with a BF2 unit.

Although the reaction of the pseudobidentate complexes with BF3, BF3·Et2O and HBF4 gave only poor yields of the desired complexes,126 the authors circumvented these difficulties by reacting 2 equivalents of [Rh(COD)2]BF4 to directly afford the BF2-substituted analogues.

Other examples of formation of bidentate systems were described by Han and co-workers125 by the reaction of optically pure menthylphenyl- and menthylbenzyl SPOs with [Rh(COD)2]OTf (Scheme 36).

X-ray analysis showed that the two optically pure ligands were linked through the P atoms in a cis fashion as expected, in which the terminal OH units were in turn interacting via a H-bond with the O of the OTf counterion. Furthermore, the stereochemistry of P was preserved.

In spite of the difficulties in the isolation of electron-rich SPO–Rh(I) complexes, Martin, Buono and co-workers22 managed to prepare a Vaska-type complex [L2Rh(CO)Cl] for L = (tBu)2P(O)H and (Cy)2P(O)H (Scheme 37).

The reaction of [Rh(CO)2(μ-Cl)]2 with two equivalents of the ligand affords the corresponding O-coordinated complex, whose formation appears to be reversible, according to 31P NMR spectroscopy. The pseudobidentate Rh(I) complex was prepared by chloride abstraction under a CO atmosphere, which isomerises the trans complex to the cis analogue.

In our group69 we also studied the reaction of [Rh(CO)2(μ-Cl)]2 with 2 equivalents of the optically pure (S)-tBuMeP(O)H in dichloromethane that produced trans-[RhClCO((S)-tBuMeP(O)H)2] (Scheme 38). The trans arrangement of the phosphinous acid ligands was confirmed by NMR spectroscopy.

Rh(III)–SPO complexes are rare in the literature. The earliest examples were given by Stephenson, Roundhill and coworkers more than 35 years ago,127,128 when they isolated triply chloride-bridged Rh(III) dimers with a pseudobidentate diphenylphosphinous acid–diphenylphosphinite ligand by reacting rhodium(I) precursors or rhodium trichloride with several phosphorus compounds in mixtures of simple alcohols and water. Many years later, Reid and coworkers129 reported similar compounds by the reaction of rhodium trichloride with diphenylphosphine in ethanol. Much more recently, Van Leeuwen and co-workers130 revisited and expanded this chemistry by reporting the preparation of a μ-Cl3 bridged dimeric, octahedral Rh(III) complex bearing the same anionic pseudobidentate ligand (Scheme 39).

The reaction of the SPO with RhCl3 in refluxing isopropanol afforded the dimeric complex, which can be later treated with tert-butoxide forming the Rh hydride. These species have been successfully applied as catalysts for transfer hydrogenation reactions.130

In their detailed research on the coordination chemistry and rearrangements of mixed anhydride ligands of the general formula R2POC(O)R′, Cole-Hamilton and coworkers112 serendipitously obtained a Rh(III) dimer containing a propanoyl moiety (Scheme 40). This compound was probably produced by the presence of adventitious water in the reaction medium.

In 2014, Garralda and coworkers131 reported interesting Rh(III)-acyl complexes with an SPO by the reaction of acylrhodium(III) precursors with diphenylphosphinous acid (Scheme 41).

The complexes had a moderately strong intramolecular hydrogen bond between the acyl and the diphenylphosphinous acid. This kind of complex was employed as a homogeneous catalyst in the hydrolysis of ammonia- and amine–boranes to produce hydrogen.

More recently, the same group132 has reported another type of Rh(III)–SPO complex by the reaction of dimeric Rh(III) species with diphenylphosphinous acid (Scheme 42).

The addition of diphenylphosphinous acid to saturated acyl–alkyl–Rh(III) complexes led to the formation of a kinetic product that upon heating rearranged to the thermodynamic product, possibly through a Rh(I) intermediate. The mechanism of the formation of the complexes was thoroughly studied by theoretical methods which can be relevant for several catalytic transformations.

4.4. Iridium

In an early report of Stephenson and coworkers127 they described several Ir(III)–SPO complexes. Despite this report, complexation studies with Ir and SPO ligands are rare in the literature, although it has been found that in situ prepared Ir precursors with SPO ligands are competent systems for asymmetric hydrogenation of ketimines66 and functionalized olefins.124 Indeed, the first preformed Ir(I) complexes described were reported in 2011 by Buono and co-workers22 using achiral (tBu)2POH (Scheme 43). The monocoordinated Ir complex was prepared by reacting (tBu)2POH with the dimeric iridium precursor. Subsequent treatment of this solution under a carbon monoxide atmosphere triggers the substitution of COD by CO. In our group, we managed to characterise in solution the analogous Ir complex with the optically pure ligand tBuMeP(O)H.69 Attempts to prepare complexes with two of these ligands under the conditions of Martin and co-workers were unsuccessful.22

Attempts to deprotonate the coordinated phosphinous acid with triethylamine produced an interesting tetrameric iridium cluster22 (Scheme 44).

Although this procedure is clearly not a preparative method, it strongly suggests that the presence of a base triggers the formation of polymetallic species, containing Ir–Ir bonds.

More recently, van Leeuwen and co-workers133 have been able to prepare, for the first time, a bis-coordinated iridium(I) complex containing two tBuPhP(O)H units forming a pseudobidentate H-bridge (Scheme 45).

The complex was prepared by reacting a solution of the ligand tBuPh(O)H with the [Ir(OMe)(COD)]2 dimer and two equivalents of water in THF at room temperature. The reaction with the analogous iridium precursor [IrCl(COD)]2 containing the less-labile chloride anion affords the monomeric Ir(I) phosphinous acid complex. In contrast, the presence of water in the reaction media triggers the protonation of the methoxide and its subsequent release as methanol, which enables the introduction of the second SPO unit. Very recently, the same group has described134 that upon treatment of the complex with 5 bar of hydrogen a mixture of mono- and dihydride species was obtained, formed by oxidative addition of H2 in a very diastereoselective process. The crystal structure of a dinuclear Ir(III) hydride could be obtained by X-ray diffraction.

There is little research carried out in the preparation of optically pure Ir–SPO complexes for their application in asymmetric catalysis. Pfaltz and co-workers135 prepared a new SPO-oxazoline-type ligand containing a stereogenic centre located at the heterocyclic ring, which was successfully coordinated to [Ir(COD)Cl]2 under basic conditions (Scheme 46).

The corresponding zwitterionic Ir(I) complexes, containing a phosphinite, proved to be quite unstable for use in asymmetric hydrogenation reactions. However, further exploration of their reactivity allowed the synthesis of an iridacycle containing an intramolecular hydrogen bridge. Treatment of the neutral complex [Ir(COD)(PN–SPO)] or [Ir(COD)Cl]2 under strong basic conditions in methanol induces a P–Ar bond cleavage. The coordinated phosphorus unit is transformed into a phosphinic acid methyl ester by addition of methoxide, followed by the oxidation of Ir(I) to the Ir(III) hydride complex. Later release of COD and concomitant coordination of the SPO ligand leads to the formation of the iridacycle, a particular type of compound that has displayed interesting results in asymmetric catalysis.136,137

A few years ago, van Leeuwen, Cano and coworkers82 presented a study describing the reproducible synthesis of iridium NPs stabilised by a few SPOs, starting from the dimer [Ir(OMe)(COD)]2 as an organometallic precursor under a hydrogen atmosphere. They were able to obtain small NPs that were full characterised by a wide range of techniques (including in-depth MAS-NMR studies), in order study the binding mode of the ligands at the surface of the nanoparticles. They found evidence of three coordination modes: the SPO as a purely anionic ligand Ir–P(O)R2, as a neutral Ir–P(OH)R2 ligand and as a monoanionic bidentate H-bonded dimer R2P–OH–H⋯O = PR2, with the last mode being predominant for the more basic dicyclohexylphosphine oxide. The Ir/SPO NPs were used in the hydrogenation of cinnamaldehyde to the parent alcohol (Scheme 87).

4.5. Nickel

When considering catalyst design, the most used d-block metals have been noble metals such as iridium, rhodium, or palladium but at present there is a huge research effort on catalysis mediated by earth-abundant transition metals. The complexation of these metals to SPOs, however, has been rarely described.

For Ni(II), the first complex with a pseudobidentate bridge was described in 1977 (ref. 138) with a few reports on related structures a few years later.139,140

In 2005, Han and co-workers141,142 disclosed the nickel-catalysed hydrophosphinylation of terminal alkynes with a mixture of [Ni(PPh2Me)4] and Ph2P(O)H.141,142 They were able to generate in situ a five-coordinated hydrido phosphinito nickel complex, by means of an oxidative addition of diphenylphosphine oxide and [Ni(PEt3)4], which was characterised by 1H NMR spectroscopy but not isolated (Fig. 8).


image file: c9cy01501a-f8.tif
Fig. 8 Reported Ni(II) complexes with SPOs.141–144

Some years later, Fang and co-workers143,144 reported another catalytic application for “in situ” Ni–HASPO complexes, in this case, having diaminophosphine oxides as ligands which were reacted with Ni(II) sources for the coupling of deactivated aryl halides and tosylates with Grignard reagents (Kumada–Tamao–Corriu reaction).143,144 These nickel complexes exhibited high activities in the Kumada–Tamao–Corriu coupling of electronically deactivated chlorides, fluorides and tosylates with Grignard reagents. Such a broad range of halides is rare in the literature. In addition, a small enantiomeric excess was found when a chiral HASPO was used.

Cationic Ni(II) species are of interest for migratory insertion of olefins to produce polymers139,145–147 but due to their high reactivity and their instability, they are usually generated “in situ” from a nickel(II) halide precursor.148

More recently, Breuil and co-workers149 reported that bis(cyclooctadiene)nickel(0) reacts with sulfonamido-phosphines in the presence of a second phosphine ligand, leading to a self-assembled supramolecular organometallic species. Further studies of the same research group disclosed the preparation of several diamagnetic π-allylic nickel complexes bearing a phosphinito–phosphinous acid system as stable solids (Scheme 47).150

This study150 showed that when [Ni(COD)2] reacts with the SPO ligand in toluene and an excess of COD, to prevent the formation of metallic nickel, the process yields the corresponding pseudobidentate π-allylic nickel(II) complexes. Lewis acid molecules such as boranes can easily substitute this labile intramolecular H-bridge, increasing the rigidity of the six-membered ring. In the case of the parent Ni systems, treatment of the pseudobidentate complexes with B(C6F5)3 triggers the formation of the corresponding bidentate analogues (Scheme 48).

In these studies, it became evident again that basic and bulky aliphatic SPOs are much less reactive to [Ni(COD)2]. Indeed, when they used the bulkier mesityl or iso-propyl SPO analogues only degradation to metallic nickel was observed.150

4.6. Palladium

Dixon and Rattray151 were among the first152,153 to convincingly describe dimeric palladium(II) complexes with two diphenylphosphine oxides acting as anionic pseudobidentate ligands. The research on these Pd–SPO complexes received a boost in 2001 when Li and co-workers30,31 used dialkylphosphinous acids in catalytic cross-coupling reactions.154 In addition, the same type of compound was successfully applied to the Suzuki–Miyaura coupling reactions of aryl chlorides155 and in Heck reactions of 4-chloroquinolines (Scheme 49).156,157

These complexes were prepared by reacting a chlorophosphine with palladium chloride or acetate, followed by the addition of water, and sometimes were obtained fortuitously by decomposition complexes with other ligands, highlighting their stability.158,159 The dimeric nature of the complexes has been established by means of X-ray diffraction methods.158–162

The remarkable air-stability of these Pd systems made them attractive for use as catalysts in C–C bond formation processes. This fact, together with the high activity of such Pd(II) SPO systems, particularly those containing electron-rich and sterically hindered alkyl substituents, spurred research in the field. Wolf and co-workers163 expanded these studies by preparing a family of palladium complexes having both monomeric and μ-chlorido-bridged dimeric structures (Fig. 9) using the same synthetic procedures previously described by Li and co-workers.31


image file: c9cy01501a-f9.tif
Fig. 9 Pd(II) monomeric and dimeric SPO complexes described by Wolf and co-workers.163

Some dialkylchlorophosphine palladium complexes have also been isolated which easily give the corresponding SPO complexes by hydrolysis.161

The whole set of compounds was successfully applied in Suzuki–Miyaura cross-coupling reactions (Scheme 98) displaying good conversions with a wide range of electron-deficient and electron-rich aryl iodides, bromides and chlorides.163

In order to study the influence of electron-deficient substituents on the catalytic activity of these processes, Hoge and co-workers164,165 evaluated the coordination properties of bis-(trifluoromethyl)-, bis-(pentafluoroethyl)-, and bis[2,4-bis(trifluoromethyl)phenyl] phosphinous acids with Pd (Scheme 50).

Dimeric complexes were obtained by reacting the ligands with palladium dichloride. In contrast, treatment of a solution (C2F5)2P(O)H (and other fluorinated SPOs)164 with palladium bis(hexafluoroacetylacetonate) leads to the formation of a monomeric complex.165

Despite the growing interest in the preparation of optically pure complexes for application in asymmetric transformations, only a few examples of chiral Pd–SPOs can be found in the literature. The first attempts to obtain enantiomerically enriched palladium complexes were carried out in Dai laboratories using the optically pure (S)-tBuPhP(O)H (ref. 86) (Scheme 51).

The reaction of [PdCl2(COD)] and two equivalents of SPO in THF affords the dinuclear monocoordinated complex, whereas the use of [PdCl2(CH3CN)2] in the presence of triethylamine as a base leads to the pseudobidentate compound in moderate yield.

Having these literature precedents in mind, in our group we envisaged69 the complexation of optically pure tBuMeP(O)H to Pd precursors, following the methods developed by Li and co-workers30 and by Buono and co-workers166 for related Pt compounds. Therefore, when (S)-tBuMeP(O)H was treated with [PdCl2(COD)] a mixture of cis and trans isomers was observed (Scheme 52).

Addition of NEt3 formed smoothly the desired dimeric phosphinito–phosphinous acid palladium complex with excellent yield.69 This complex appeared to be very stable since after some experimentation it was found that it could be prepared by simply refluxing PdCl2 in THF in air, in the absence of a base. Furthermore, the pseudobidentate bridge could be straightforwardly transformed into a real bidentate unit upon treatment with tetrafluoroboric acid (Scheme 53).69

The lability of the bridging ligand in Pd(II) SPO dinuclear complexes appears to have an important influence on the catalytic processes. Indeed, Buono, Giordano and co-workers166–168 described an unprecedented application of dimeric acetate–Pd complexes, having the tBu/Cy combination in the SPO ligands, in the [2 + 1] cycloaddition reaction/ring expansion of alkynes with norbornadiene derivatives (Scheme 104). An account by Clavier and Buono on Pd- and Pt-phosphinous acids and their applications in this type of reaction has been recently reported.169

Some described complexes contain a bridging-acetato ligand between the two Pd(II) centres and the optically pure tBuPhP(O)H moiety (Scheme 54).

The formation of the palladium dimer is thought to occur by means of a ligand exchange of dba by SPO after oxidative addition of acetic acid to Pd(0) to give the Pd-hydrido monomer (Scheme 55).

The loss of AcOH, coordination of dba and later insertion of the olefin moiety into the palladium–hydride bond afford a Pd(II) enolate intermediate that quickly evolves into a monomeric acetate complex. Subsequent dimerization furnishes the desired μ-acetato bridged system.

These Pd SPO complexes were also studied in our group69 by reacting palladium(II) acetate with optically pure (S)-tBuMeP(O)H (Scheme 56).

Interestingly, when the ligand was treated with Pd(OAc)2 in refluxing toluene according to previous methodologies,35,166,167,170 a mixture of monomeric and dimeric species in a 1[thin space (1/6-em)]:[thin space (1/6-em)]1 ratio was found.

The increase of the bulkiness of the SPO favours the formation of the monomeric species as described by Ackermann and co-workers (Scheme 57).170

Hence, the reaction of palladium(II) acetate with two equivalents of Ad2P(O)H in toluene exclusively forms the pseudobidentate monomeric acetato-complex, which displayed high catalytic activity in the cross-coupling of 2-pyridyl Grignard reagents.170

The promising catalytic activity of these palladium complexes have put these systems under the spotlight, and very recently, Nuel, Giordano and Martinez171 have successfully prepared a family of P-stereogenic biphosphinite palladacycles through a H-transfer-based self-assembly process (Scheme 58).

The reaction is thought to occur through the oxidative addition of an acid HX to Pd(0) followed by the cis coordination of two SPO units.

Hong and co-workers35 described the preparation of ferrocenyl-derived SPOs (Scheme 8) and their application in Suzuki–Miyaura cross-coupling reactions (Scheme 59).

Ferrocenyl SPOs can be synthesised by lithiation of ferrocene and reaction with the corresponding aminochlorophosphine derivative, which after treatment with SiO2/CTLC yields the desired SPO. Alternatively, the ligand can be also obtained by hydrolysis of the ferrocenyl-derived chlorophosphine. Coordination of the ligand with the corresponding palladium source afforded the dinuclear μ-chlorido bridged complex and the monomeric acetato species, respectively.35 Interestingly, the pseudobidentate architecture can be achieved, by also introducing another SPO in the ferrocenyl unit (Scheme 59).35

With the aim of improving the efficiency of several C–C catalytic bond formation reactions, other Pd–SPO systems have been explored. Particularly interesting are those containing heterocyclic-based secondary phosphine oxides, which have promoted excellent results in Heck reactions, as described by Hong, Shaikh and co-workers (Scheme 60).6,172

Cu-Promoted coupling of imidazole and 2-bromoarenes, followed by sequential deprotonation, addition and hydrolysis of a chlorophosphine derivative, yielded the imidazole-derived SPOs. Later coordination with [PdCl2(COD)] triggered the formation of the expected cis Pd(II) complex, albeit without the pseudobidentate coordination. A rare Pd(I) dinuclear cluster containing a Pd–Pd bond was also formed in the reaction course, both using palladium(II) bromide or [PdCl2(COD)] as a starting reagent.

The pseudobidentate chelation has been recently achieved by Hong's group by introducing a tBu substituent at the phosphorus atom (Scheme 61).173

Hong and his co-worker174 also reported the preparation of indolyl-substituted secondary phosphine oxides in which the N of the heterocycle acts as an hemilabile centre (Scheme 62).

In this case, coordination of the heterocyclic-based SPO to [PdCl2(COD)] yielded the μ-chlorido-bridged palladium dimer, rather than the monomer.

Introduction of a bulky quinolyl group in the SPO triggered the formation of the mononuclear complexes (Scheme 63), as reported by the same group.175

The reaction of the allylpalladium dimer or [PdCl2(COD)] did not furnish the expected allyl-Pd and dichloro-Pd species, respectively, but the same pentacoordinated Pd complex instead. This complex displayed an uncommon (for Pd(II) complexes) distorted square-pyramidal geometry, having an apical chlorido ligand and two bidentate SPO ligands with an intramolecular hydrogen bridge.

The preparation of allyl-palladium complexes with SPOs has been scarcely explored. Only Dai and co-workers86 described in 2003 a dimeric optically pure complex containing two allyl frameworks linked by a magnesium centre (Scheme 64).

In the view of these results, our group studied the reactivity of different allyl Pd precursors with the enantiomerically enriched tBuMeP(O)H. Only the 1,3-diphenylallyl group allowed the isolation of the optically pure Pd–SPO complex (Scheme 65).69

Later treatment with HBF4·OEt2 was carried out in order to form a BF2 bridge, leading to the corresponding complex with a bidentate ligand, which was crystallographically characterised.

4.7. Platinum

The coordination chemistry of Pt with SPOs has been comparatively less explored compared to Pd. The first study was carried out by Chatt and coworkers4 with the synthesis of the Pt(II) complex cis-[PtCl2(PPh2OH)(PEt3)] by the reaction of the dimeric precursor [PtCl(μ-Cl)(PEt3)]2 with diphenylphosphine oxide (Scheme 66).

A few years, later Roundhill and co-workers176 studied the reaction of [Pt(PPh3)4], again with diphenylphosphine oxide in benzene. Interestingly, they found that the reaction occurred with an oxidation of Pt(0) to Pt(II) followed by the formation of a phosphinito–phosphinous acid bridge (Scheme 67).

These complexes, along with a rather unnoticed report of Dixon and Rattray151 on Pd and Pt complexes with diphenylphosphinito–diphenylphosphinous acid, sit among the first examples of coordination compounds that bear the combination of a coordinated phosphinous acid and a phosphinito ligand, self-assembled through intramolecular hydrogen bonding.7,32 It is interesting to note that the crystal structure of the complex with diphenylphosphinous acid was not reported until much later.177 Although they were not specifically designed for this purpose, these early examples showed that anionic electron-rich phosphorus species could be prepared using air-stable SPOs.

Inspired by these works, van Leeuween and co-workers5 described the first application of this Pt(II) pseudobidentate complex as a catalyst for the Pt-catalysed hydroformylation of olefins, renewing the interest in the field.5

In 1995, Ghaffar, Parkins and co-workers178 studied Pt(II) complexes containing HP(O)Me2 units (Fig. 10) as catalysts for the hydrolysis of nitriles to amides and some of them in the hydration of the more challenging cyanohydrins.179


image file: c9cy01501a-f10.tif
Fig. 10 Pt(II) complexes as catalysts for the selective hydrolysis of nitriles to amides.

More recently, these studies have been further explored by van Leeuwen and co-workers180 in the use of “in situ” Pt(II) systems modified with chiral non-racemising SPOs, which have been successfully applied in nitrile hydration reactions. They gave some mechanistic insights into the species involved in the catalytic process (Scheme 68).

Hence, in the presence of [PtCl2(COD)] and an excess of the ligand, coordination of two units and formation of the intramolecular H-bridge were expected.180 The trans-labilising effect of the P moieties allows the coordination of another SPO. The final release of the remaining ancillary Cl forms the catalytically active species, which can be alternatively achieved upon treatment of [Pt(PPh3)4] with an excess of the SPO.

Surprisingly, it was not until 2007 when a renewed interest emerged in the preparation of stable Pt–SPO complexes that could be used as preformed catalysts. Inspired by the pioneering work of Li and co-workers in 2001 in the application of Pd–SPO coordination compounds in various catalytic transformations,156 Buono and co-workers181 described the preparation of novel [Pt(η2-acetato){RPhPO}2H] complexes (Scheme 69) and their application to the benzylidenecyclopropanation of norbornadienes.

The preparation of these complexes was carried out by reacting two equivalents of the corresponding SPO with [PtCl2(CH3CN)2] furnishing the dicoordinated trans-Pt isomer as a major product (Scheme 69), as also observed by other authors with bulky SPOs.164 It is worth mentioning that the reaction of optically pure (R)-tert-butylphenylphosphine oxide after 12 h in refluxing THF yielded a mixture of cis- and trans-complexes, in which partial racemisation was observed.181 The authors proved that the equilibrium between the cis- and trans- isomers did not induce racemisation, running the same reaction at room temperature using [PtCl2(COD)] as a starting material, where a 1[thin space (1/6-em)]:[thin space (1/6-em)]1 mixture of isomers was formed but without optical erosion. Later exchange of the chlorido ligand with acetate took place by addition of AgOAc, resulting in the formation of the corresponding cis-neutral complex, and the release of acetic acid182 (Scheme 70).

Treatment of the cis-, trans-mixture with triethylamine triggered the formation of the dinuclear pseudobidentate platinum complex, which displays the two platinum centres in a square planar geometry according to X-ray diffraction studies.

Furthermore, Giordano, Buono and co-workers182 proposed a mechanism for the dimer formation that is depicted in Scheme 71 for a general case.

The formation of the dinuclear complex might be explained by association of two pseudobidentate Pt(II) units, generating a 18 electron intermediate. The first step of the mechanism depends on the stereochemistry of the biscoordinated Pt(II) complexes. Hence, only those containing a cis-geometry are able to form the pseudochelated building block for the assembly process. The trans-analogues (which are formed in the presence of bulky substituents attached to P) appeared to be unreactive towards triethylamine, suggesting a pre-equilibrium between the cis and trans-complexes, which is shifted upon formation of the chloride-bridged product.

As has been mentioned, sterically hindered groups on the P atom, such as tBu, lead to the corresponding trans-Pt(II) complexes. Although that they are unreactive towards NEt3, they do react in the presence of AgOAc and a base to furnish the Pt(κ2-acetato) SPO complexes182 (Scheme 72).

This unusual κ2-acetato complex can be also prepared by refluxing PtCl2 with silver acetate as reported by Giordano, Buono and co-workers.182

The reactivity of the Pt(II) dinuclear complexes has been explored recently by Martinez, Giordano, Nuel and co-workers183 in their studies on the aerobic/anaerobic oxidation of several alcohols (Scheme 73).

Abstraction of the bridging chlorido ligands with silver hexafluorophosphate in wet dichloromethane forms the dicationic complex with two coordinated water molecules, which can be easily deprotonated in basic media to furnish the neutral hydroxo-bridged platinum complexes.

In their search for more efficient catalysts for Suzuki–Miyaura cross-coupling reactions and Catellani reactions, Hong and co-workers184 described the preparation of heterocyclic substituted SPOs and their coordination to Pt systems (Scheme 74).

The reaction of the SPO with a heterocyclic substituent with PtCl2 in THF showed the expected formation of trans-Pt complexes, which display interactions between the terminal OH moiety and the Cl ligands in the solid state.

4.8. Gold

The coordination chemistry of gold(I) with SPO ligands has been little explored. The first examples were reported by Schmidbaur and co-workers185–188 with phosphinous acid ligands (Scheme 75).

The reaction of [ClAu(CO)] with a chlorophosphine affords a complex with loss of CO that was not isolated but formed the corresponding Au(I) phosphinous acid complex by hydrolysis. Further reaction with Me3SiCl leads to the silylated complex. The same reaction has been carried out using SMe2 as a leaving group in the Au(I) precursor and dimethyl phosphonate. Its corresponding gold–phosphinite complex could be obtained upon deprotonation with triethylamine.

Substitution of two phosphine units has been also achieved by Schmidbaur and co-workers185–187 (Scheme 76).

Disubstituted gold(I) phosphinous acid complexes were obtained by the reaction of [(Me2S)AuCl] with two equivalents of Ph2P(O)H in the presence of silver salts of non-coordinating anions.185–187

More recently, van Leeuwen and co-workers189,190 used the complex [tBu(1-naphthyl)P(OH)AuCl]2 as a precursor for gold nanoparticles that proved to be active for the hydrogenation of substituted aldehydes (Scheme 77).

At the same time, Schröder and co-workers191 described the first use of molecular systems in enyne cycloisomerisation and hydroxy- and methoxycyclisation reactions (Scheme 80).

In our group, we prepared the first gold complex with the optically pure, phosphine oxide (S)-tBuMeP(O)H, which easily substituted the tht ligand when reacted with [Au(tht)Cl]192 providing a Au(I) linear complex as a pale grey solid in good yield (Scheme 78).

Crystals of this complex contained three molecules in the asymmetric unit with very similar distances and angles.

In the literature, Au–SPO complexes present symmetric dimeric structures with the gold centres stabilised by aurophilic interactions and interconnected by two terminal O–H⋯Cl hydrogen bonds191 (Scheme 79).

In solution, it has been found that, in the case of disubstituted Au(I)SPO complexes (Scheme 79), the dimeric structures are in dynamic exchange according to NMR, where there is no rupture of O–H⋯O hydrogen bonds or Au⋯Au interactions, inducing only conformational changes in the molecule. The nature of these interactions has been found to have an influence on some catalytic reactions.190,191

5. Non-enantioselective catalysis

The prodigious utility of phosphines and other trivalent derivatives in homogeneous catalysis is somewhat shadowed by their air-sensitivity, due to the stability of the phosphoryl group (P[double bond, length as m-dash]O) in pentavalent phosphorus compounds. SPOs turn this weakness into strength and for this reason are very interesting in catalysis with the hope of avoiding the sophisticated and expensive air-free conditions that traditional catalysis requires.

Since 1986 when van Leeuwen and co-workers reported the first application of SPOs on the Pt-catalysed hydroformylation of olefins,5 the use of these systems had gained great interest. A summary of the main applications of SPOs in this field is presented through the following sections.

5.1. Isomerisations and rearrangements

A few years ago, Fensterbank and coworkers191 described that SPO–Au(I) complexes (Scheme 79) were active in enyne cycloisomerisation and hydroxyl- and methoxycyclisation reactions (Scheme 80).

The complex [ClAu(PtBuPhOH)] displayed good activities in the cycloisomerisation of several enynes under mild conditions, even with deactivated substrates. In the same paper, the same complex is used in a few hydroxy- and methoxycyclisations.

Slightly later, Buono, Clavier and coworkers81 reported the use of carbonyl Ru–SPO complexes (Scheme 25) in the cycloisomerisation of arenynes (Scheme 81).

They found that the dimeric precursor [{RuCl(μ-Cl)(CO)3}2] with silver triflate was inactive in the transformation in contrast to the complexes with SPO ligands. The best results were obtained with the most electron-rich ligands, such as dicyclohexylphosphine oxide.

An interesting reaction is the rearrangement of aldoximes to amides (Scheme 82), catalysed by the tethered ruthenium(II) complex in Scheme 21.193

This reaction is an atom economical process to obtain primary amides which involves a dehydration/rehydration sequence via the nitrile intermediate.194 The aldoximes were completely rearranged with 5 mol% of the catalyst in neat water at 100 °C. The mechanism of the reaction is basically the same as that of nitrile hydration and will be discussed later. Indeed, the same complex and many other Ru complexes have been found to be excellent catalysts for the hydration of nitriles.13,195

Recently, Crochet, Cadierno and coworkers196 described the tandem isomerisation/Claisen rearrangement of diallyl ethers in aqueous sodium hydroxide solution, to yield α,β-unsaturated aldehydes, catalysed by [RuCl26-arene)(PR3)] complexes (Scheme 83).

They used several phosphines and phosphites, but they found that the best results were obtained with triethylphosphite. After some mechanistic studies, they concluded that one of the ethoxy groups of the triethylphosphite readily hydrolyses, generating a diethylphosphite–Ru species (a HASPO) that facilitates the first isomerisation step.

5.2. Hydrogenation

There are also some applications of Ru–SPO complexes in hydrogenation. Focusing on non-enantioselective reactions, Zhang and coworkers197 recently described a tridentate diphosphine–SPO ligand that formed a very stable Ru(II) hydride complex, containing a molecule of carbon monoxide. This complex was very active in the hydrogenation of aldehydes and ketones (Scheme 84).

The complex is especially suited for the selective hydrogenation of α,β-unsaturated aldehydes, because it leaves the olefin untouched (TON up to 36[thin space (1/6-em)]500 and 99% selectivity). Some mechanistic studies revealed that an outer-sphere mechanism was operative, with the formation of a phosphinous acid dihydride intermediate.

As already stated, in recent years SPOs have found increased used in the stabilisation of nanoparticles (NPs)87 and in this line the first applications of these systems are reported. In the case of hydrogenation, van Leeuwen and coworkers198 prepared Ru nanoparticles and used them in the hydrogenation of substituted benzenes (Scheme 85).

They used several SPOs with different electronic and steric properties and were able to obtain NPs with a narrow size distribution. The nanoparticles with the diphenylphosphine oxide were used in hydrogenation reactions of substituted benzenes to the corresponding cyclohexanes in high conversions, often in a neat substrate.

Van Leeuwen and coworkers189 have also reported the use of t-Bu(1-naphthyl)P(O)H-stabilised gold NPs (Scheme 77) in the homogeneous hydrogenation of substituted aldehydes (Scheme 86).

This was the first application of Au/SPO systems in homogeneous catalysis.191 The reaction was remarkably chemoselective and α,β-unsaturated aldehydes were reduced to alcohols without affecting the C–C double bond. An important example of this transformation was the smooth reduction of acrolein to allyl alcohol. The same exclusive reduction of the carbonyl group in aldehydes bearing cyano, nitro, alkynes and other groups usually sensitive to hydrogenation was observed. Interestingly, other Au-NPs stabilised by ligands different from SPOs were completely inactive in aldehyde reduction. Shortly after this report, the same group extended this study to a small library of SPOs.190 They demonstrated that aryl-substituted SPOs presented a strong polarity of the P[double bond, length as m-dash]O bond and showed a high catalytic activity and almost perfect chemoselectivity in the hydrogenation of substituted aldehydes. In contrast, alkyl-substituted SPOs exhibit lower polarity of the phosphoryl bond or the presence of P–OH bonds, slowing the heterolytic cleavage of hydrogen and thus worsening the catalytic results. The perfect chemoselectivity can be explained by ligand–metal cooperative effects, in which the SPO ligand plays a crucial role. Hence by means of DFT calculations199 it has been recently found that the SPO provides an effective frustrated Lewis pair that allows the heterolytic activation of the dihydrogen molecule, which is then added to the aldehyde carbonyl group through a concerted mechanism.

Recently, the group of van Leeuwen has also been active in the development of Ir/SPO systems for hydrogenation. In a first study, they used Ir NPs, stabilised with three different SPOs, Ph2P(O)H, Cy2P(O)H and tBuPhP(O)H, for the hydrogenation of cinnamaldehyde and p-nitrobenzaldehyde (Scheme 87).82

Once again, the Ir-NPs were very chemoselective towards the reduction of the carbonyl of the aldehydes in THF at RT. It was found that the NPs stabilised with tBuPhP(O)H were both the smallest and the most active but very little ligand effect was found. In the case of the hydrogenation of p-nitrobenzaldehyde, concomitant reduction of the nitro group to the give the aniline was observed.

In parallel, the same group133 reported a study in which they compared the catalytic activity of a molecular Ir–SPO complex (Scheme 45) and Ir NPs stabilised with the same SPO, namely tBuPhP(O)H (Scheme 88).

These two Ir–SPO catalysts showed markedly distinct rates and selectivities in the hydrogenation of substituted aldehydes. The Ir–SPO complex showed very high activity and selectivity in the hydrogenation of cinnamaldehyde and p-nitrobenzaldehyde. The Ir/SPO-NPs were less active but more robust than the complex giving high selectivities in the hydrogenation of substrates that poison the molecular catalyst. Very recently, the same group expanded the studies134 on the catalytic potential of the Ir–SPO complex in Scheme 45. They found that it is an excellent catalyst for the hydrogenation of substituted aldehydes, displaying high activities and selectivities for a wide variety of substrates, with TOF values up to 2040 h−1 without the need for bases or additives. The catalysis presumably operates though a ligand–metal cooperative mechanism in which the SPO plays a dual role as a modifying ligand and as a functional ligand, acting as a heterolytic activator of the H2 molecule.

5.3. Transfer hydrogenation

There dimeric Rh(III)–SPO complexes in Scheme 39 were employed130 in transfer hydrogenation of ketones. Under the optimal conditions, acetophenone was reduced with a 92% conversion and a TOF of 1825 h−1. The Rh[thin space (1/6-em)]:[thin space (1/6-em)]SPO ratio was found to be crucial to obtain a good catalytic activity and had to be adjusted for every substrate. DFT calculations were used to characterise several intermediates and showed that the process occurs via a concerted outer-sphere mechanism. When a binaphthol-derived phosphorous acid was employed, up to 89% ee was obtained in the reduction of acetophenone.

5.4. Nitrile hydration

The acid- or base-catalysed hydrolysis of nitriles to amides is an atom-economical organic transformation but as it requires harsh conditions it has the drawback in which the amide is often further hydrolysed to an acid. To obtain better selectivity, transition-metal catalysis is advantageous and SPO ligands seem to be particularly fit for this reaction.200,201 The first use of SPOs in this area was reported in a communication quite some time ago by Parkins and Ghaffar,178,202 who employed platinum complexes of simple monophosphorus ligands in nitrile hydrolysis (Scheme 89).

They found that the most active system has dimethylphosphine oxide in aqueous solutions and proposed a mechanism in which the carbon of the coordinated nitrile is attacked by the hydroxyl group of the phosphinous acid to explain the high activity of SPOs compared to that of tertiary phosphines. With this system, a wide range of nitriles could be hydrated under relatively mild conditions, sometimes in pure water.179,203 For these reason, the catalyst [PtH{(PMe2O)2H}(PMe2OH)], known as the Parkin catalyst, has been used in the synthesis of a number of complex organic molecules13,201,204 and has merited its own review, written a few years ago by Cadierno.205

Pt(II) complexes bearing a chiral SPO ligand (Scheme 68) have been applied to the hydration of aromatic nitriles by van Leeuwen and coworkers (Scheme 90).180

The [Pt(SPO)3Cl]Cl complex (Scheme 68) showed moderate activity in the hydration of m- and p-substituted benzonitriles and was totally inactive in the hydration of o-substituted derivatives. In contrast, the hydride complex formed by the reaction of the SPO and [Pt(PPh3)4] and the cationic complex derived from [Pt(SPO)3Cl]Cl by chloride extraction were found to be much more active for many benzonitriles, including the ortho-substituted ones.

Despite the above results, most of the work has been carried out by using ruthenium(II) complexes13,195 since the first report of Tyler and coworkers a few years ago,104 (Fig. 6) who employed the same dimethylphosphinous acid as a ligand to Ru-p-cymene (Scheme 91).

This SPO was faster than other phosphorus ligands with hydrogen bond accepting capability and was able to hydrate many nitriles in water under neutral conditions. An interesting aspect of this type of catalyst is that it is capable of hydrating cyanohydrins at ambient temperature, which is a challenge because it is prone to decompose to HCN that would poison the catalyst.179,206,207

These studies were extended to many Ru(II)–arene complexes with SPOs and related phosphorus acid ligands by Cadierno and coworkers,105 (Fig. 6) who found that the most active systems where again those based on dimethylphosphinous acid, achieving TOFs up to 32 h−1 at 100 °C. In addition, they performed calculations on the mechanism of the reaction by DFT and showed that the SPO ligand played a key role in the hydration process by the formation of a metallacycle (Scheme 92).

They found that the hydration reaction does not proceed by direct addition of water to the coordinated nitrile but via a five-membered metallacycle formed by intramolecular attack of the phosphinous acid to the nitrile. The hydrolysis of this metallacycle liberates the amide product and regenerates the catalyst.

The same simple SPOs have been also been tested with bis(allyl)ruthenium(IV) complexes (Scheme 30) by the same authors, with very good results (Scheme 93).113,114

The reactions can be quickly performed in neat water without any additive and proceed smoothly for a large variety of nitriles giving the corresponding primary amides. The presence of many functional groups in the nitrile is well tolerated, highlighting the high chemoselectivity of the catalytic system. The elevated activity is possibly due to the high solubility and stability of the systems in water. Indeed, after selective crystallisation of the amide product, the Ru catalyst remains completely dissolved and can be reused. The good properties of the system were used to synthesise some pharmacologically important compounds as well as in the hydration of cyanohydrins.

At this point, it has to be recalled that ruthenium–phosphinous acid complexes can be prepared by hydrolysis of the coordinated chlorophosphines (Schemes 22 and 30). Hence as the nitrile hydration reactions take place in water, the latter precursors can be directly used because they easily generate the desired Ru–SPO compounds under catalytic conditions. This has been demonstrated both for Ru(II)-arene107 and Ru(IV)-bis(allyl)114 complexes. An example of the utility of in situ generated Ru(II)–arene–SPO complexes is in the hydration of β-ketonitriles to β-ketoamides (Scheme 94), whose precedents required the use of enzymes such as nitrile hydratases.13


image file: c9cy01501a-s22.tif
Scheme 22 Preparation of Ru(II)-p-cymene SPO complexes.

image file: c9cy01501a-s23.tif
Scheme 23 Calculated reaction pathway for the formation of complexes [RuCp(PH3)2(SPO)]+.

image file: c9cy01501a-s24.tif
Scheme 24 Preparation of Ru(II)-p-cymene–SPO complexes.

image file: c9cy01501a-s25.tif
Scheme 25 Preparation of bis-coordinated Ru(II)–SPO complexes.

image file: c9cy01501a-s26.tif
Scheme 26 Synthesis of optically pure O- and P-coordinated Ru(II) complexes.

image file: c9cy01501a-s27.tif
Scheme 27 Preparation of Ru(II) dinuclear complexes containing intramolecular H-bonds.

image file: c9cy01501a-s28.tif
Scheme 28 Preparation of pseudobidentate Ru(II) dinuclear complexes.

image file: c9cy01501a-s29.tif
Scheme 29 Preparation of a dinuclear Ru(II) complex.

image file: c9cy01501a-s30.tif
Scheme 30 Preparation of bis(allyl)-ruthenium(IV)-SPO complexes reported by Cadierno and co-workers.113,114

image file: c9cy01501a-s31.tif
Scheme 31 Preparation of Os(II)–p-cymene complexes described by Cadierno and co-workers.116

image file: c9cy01501a-s32.tif
Scheme 32 Synthesis of the diphenylphosphinous acid Os(IV) complex.

image file: c9cy01501a-s33.tif
Scheme 33 Hydrolytic P–C cleavage in cationic Os(II)–arene complexes.

image file: c9cy01501a-s34.tif
Scheme 34 Preparation of pseudobidentate Os(II) complexes.

image file: c9cy01501a-s35.tif
Scheme 35 Preparation of aryl-SPO Rh(I)–BF2 capped complexes.

image file: c9cy01501a-s36.tif
Scheme 36 Rh complexes prepared by Han and co-workers.125

image file: c9cy01501a-s37.tif
Scheme 37 Synthesis of Rh(I) alkyl–SPO complexes.

image file: c9cy01501a-s38.tif
Scheme 38 Preparation of complex trans-[RhClCO((S)-tBuMeP(O)H)2].

image file: c9cy01501a-s39.tif
Scheme 39 Formation of Rh(III) complexes described by van Leeuwen and co-workers.130

image file: c9cy01501a-s40.tif
Scheme 40 Synthesis of the Rh(III)SPO-acyl dimer.

image file: c9cy01501a-s41.tif
Scheme 41 Preparation of Rh(III)–SPO complexes.

image file: c9cy01501a-s42.tif
Scheme 42 Preparation of Rh(III)/SPO-acyl complexes.

image file: c9cy01501a-s43.tif
Scheme 43 Reported Ir–SPO complexes with (tBu)2POH.22

image file: c9cy01501a-s44.tif
Scheme 44 Deprotonation of an Ir–phosphinous acid complex.22

image file: c9cy01501a-s45.tif
Scheme 45 Synthesis of the bis-coordinated Ir(I) complex.133

image file: c9cy01501a-s46.tif
Scheme 46 Preparation of optically pure Ir(I)–SPO-oxazoline complexes.

image file: c9cy01501a-s47.tif
Scheme 47 Ni–SPO allylic complexes prepared by Breuil and co-workers.150

image file: c9cy01501a-s48.tif
Scheme 48 Reactivity of SPO–nickel complexes.

image file: c9cy01501a-s49.tif
Scheme 49 Pd(II) SPO complexes reported by Li and co-workers.31

image file: c9cy01501a-s50.tif
Scheme 50 Electron-poor SPO–Pd(II) complexes prepared by Hoge and co-workers.165

image file: c9cy01501a-s51.tif
Scheme 51 Preparation of Pd(II) complexes with (S)-tBuPhP(O)H.

image file: c9cy01501a-s52.tif
Scheme 52 Preparation of Pd complexes with the optically pure (S)-tBuMeP(O)H.

image file: c9cy01501a-s53.tif
Scheme 53 Substitution of the OH bridge by BF2 in the dinuclear optically pure Pd complex.

image file: c9cy01501a-s54.tif
Scheme 54 μ-Acetato dimeric Pd complexes reported by Buono, Giordano and co-workers.166–168

image file: c9cy01501a-s55.tif
Scheme 55 Proposed reaction pathway for the formation of the Pd dinuclear complex.

image file: c9cy01501a-s56.tif
Scheme 56 Preparation of Pd acetate complexes with (S)-tBuMeP(O)H.

image file: c9cy01501a-s57.tif
Scheme 57 Preparation of the Pd(II) acetate complex with (Ad)2P(O)H.

image file: c9cy01501a-s58.tif
Scheme 58 Chiral bisphosphinite palladacycles described by Nuel, Giordano and Martinez.171

image file: c9cy01501a-s59.tif
Scheme 59 Preparation of Pd/SPO complexes bearing a ferrocenyl substituent.

image file: c9cy01501a-s60.tif
Scheme 60 Preparation of imidazole-based SPO–Pd complexes.

image file: c9cy01501a-s61.tif
Scheme 61 Preparation of pseudobidentate imidazole-derived Pd complexes.

image file: c9cy01501a-s62.tif
Scheme 62 Synthesis of indolyl-SPOs and formation of the dimeric Pd complex.

image file: c9cy01501a-s63.tif
Scheme 63 Mononuclear pentacoordinated Pd complex with a heterocyclic SPO.

image file: c9cy01501a-s64.tif
Scheme 64 Preparation of the optically pure allyl Pd complex reported by Dai and co-workers.86

image file: c9cy01501a-s65.tif
Scheme 65 Preparation of optically pure allyl Pd complexes with tBuMeP(O)H and substitution of the OH bridge by BF2.

image file: c9cy01501a-s66.tif
Scheme 66 Synthesis of the Pt(II) complex cis-[PtCl2(PPh2OH)(PEt3)] reported by Chatt and co-workers.4

image file: c9cy01501a-s67.tif
Scheme 67 Synthesis of the Pt(II) complex bearing a pseudobidentate bridge reported by Roundhill and co-workers.176

This well-known ability of Ru(II) complexes to catalyse transfer hydrogenation (TH) reactions was used to develop a useful tandem hydration/TH reaction allowing the direct conversion of β-ketonitriles to β-hydroxyamides with a Ru(II)-p-cymene SPO complex with sodium formate in water.208 Interestingly, the reactions had to be performed under slightly harsher conditions and with a higher catalyst loading compared to the hydration of nitriles since the reduction of the ketone was found to be the rate-determining step.

The exact same transformation has been recently reported by the same authors using the Ru(IV)-bis(allyl)-SPO complex for the nitrile hydration reaction followed by an enzymatic reduction with ketoreductases, in a tandem hydration/bioreduction process (Scheme 95).209

This process took place in aqueous phosphate buffer in high yields and with very high enantioselectivities. It should be noted that the enantioselectivity is obviously imparted by the enzyme and for this reason this reaction is not included in the enantioselective catalysis part. This reaction nicely illustrates the possibility to combine classic transition-metal catalysis with enzymatic catalysis.

Other Ru–SPO complexes have been found to be active in the hydration of nitriles, such as the cationic tethered complexes in Scheme 21 but with poorer results compared to the neutral Ru(II) and Ru(IV) complexes just described. In this case however, they were found to be active in the rearrangement of aldoximes to primary amides,193 as discussed earlier.

The related Os–SPO complexes have been much less explored in the hydration of nitriles and the existing studies have been carried out with [Os(η6-p-cymene)(PR2OH)] complexes (Scheme 31).116 Like their Ru counterparts, they are competent catalysts in nitrile hydration in pure water without the need for any additive and are thought to follow the same mechanism with the cyclic five-membered intermediate. The most active system is again with the dimethylphosphinous acid. This compound is very active and chemoselective and for the less reactive aliphatic nitriles is superior to its Ru analogue. It seems that this better performance is due to subtle differences in the ring strain of the Os intermediate metallacycle. Finally, it should be mentioned that in parallel to the Ru analogues, the catalytically active Os(II)–arene SPO complexes can be prepared by in situ P–Cl hydrolysis of the parent complexes with coordinated chlorophosphines.116 Interestingly, however, they could also be prepared by P–N bond hydrolysis of complexes with a coordinated aminophosphine.117

5.5. Hydrolysis of amine–boranes

A report from Garralda and coworkers (Scheme 41)131 described the synthesis of rhodium(III) acylphosphine-hydrides stabilised by intramolecular H bonds involving a coordinated SPO, which catalysed the hydrolysis of amine–boranes (Scheme 96).

The complexes were evaluated in the hydrolysis of amine–boranes to release hydrogen, a very interesting reaction in hydrogen storage applications. The reaction occurred under mild conditions (40 °C) with low catalyst loading and the catalyst could be easily reused.

5.6. Hydroformylation

The hydroformylation of olefins is an extremely important reaction, especially industrially. As early as 1983, Matsumoto and Tamura123 applied diphenyl- and dioctylphosphine oxides in the Rh-catalysed hydroformylation of linear olefins. Although they used triphenylphosphine as a ligand, the addition of the mentioned phosphine oxides increased the stability of the catalyst, probably by formation of [Rh2(CO)2(PPh3)2(R2P(O))2].

A few years later, van Leeuwen and coworkers5 reported that Pt/SPO hydride complexes (Scheme 67) catalysed the hydroformylation of 1- and 2-heptenes yielding linear aldehydes (along with some alcohols) with 90 and 60% selectivity, respectively. In further studies, several alkyl and acyl intermediates could be identified210 and it was concluded that diphenylphosphinous acid was an interesting ligand with peculiar properties, capable of activating the hydrogen, which seems to be the bottleneck in Pt-catalysed hydroformylation.211 It has to be noted that prolonged hydroformylation produces inactive dimeric Pt complexes containing a phosphido and a hydrido bridge.210

Much more recently, Börner and coworkers212 revisited the Rh/SPO-catalysed hydroformylation. They used diphenylphosphine oxide and three other diarylphosphine oxides with electron-poor aryl groups for the hydroformylation of 1-octene and cyclohexene (Scheme 97).

The four SPOs performed better than the typically used triphenylphosphine although rather low n-selectivities were observed. Interestingly, methanol and other protic solvents gave better results, as they preserve the bidentate coordination mode.126 The same authors33 extended the study of the Rh/HASPO-catalysed hydroformylation, in the context of the study of the degradation/hydrolysis of several phosphites used in industrial hydroformylation.213 The results were poorer than those with the SPOs just described and in addition a side reaction involving the originated aldehydes generated α-hydroxyphosphonic acid diesters, reducing the concentration of the ligand and hence the activity of the resulting catalysts.

5.7. Cross-coupling

The most important application of SPOs in homogeneous catalysis is their use in Pd-catalysed coupling reactions. The first reports of the area are due to Li and coworkers at DuPont and date back to 2001.30,156 They showed that the Pd–SPO complexes mentioned before (particularly di(tert-butyl)phosphine oxide) were excellent ligands for a variety of cross-coupling reactions with vinyl and aryl chlorides, including Suzuki–Miyaura, Mizoroki–Heck, Buchwald–Hartwig aminations and related reactions. In addition, SPOs with nickel were also active in Kumada–Tamao–Corriu reactions (Scheme 98).

This is a remarkable result, because aryl chlorides are rather inert substrates for cross-coupling reactions that usually require air-sensitive trialkyl- or dialky(2-biphenylyl)phosphines. The success of SPOs in these reactions was attributed to the formation of electron-rich anionic species in the basic media under catalytic conditions, facilitating the rate-limiting oxidative addition of inactivated aryl chlorides to Pd(0) precursors.

These results spurred fruitful research in Pd-catalysed (and also Ni-catalysed) cross-coupling reactions: Suzuki–Miyaura coupling reactions35,172,174,214–219 as well as Mizoroki–Heck,157,159,175,220–222 Stille,157,223 Sonogashira,224,225 Hiyama,226,227 Kumada–Tamao–Corriu,31,228–230 Negishi,155,231 and other reactions with mechanisms related to cross-coupling reactions.155,175,225,230,232–236 In many of the cases but not all,35,172,174,175,220–222,225,230,231,236 the ligand of choice was di(tert-butyl)phosphine oxide. Remarkably, some of these reactions were carried out in neat water.11,223,224,227,234

To close this section, the work of Bedford and coworkers,237 who studied Suzuki–Miyaura coupling reactions with ortho-metallated phosphites and phosphinites is worth noting. They found that sometimes under catalytic conditions hydrolysis of the ligands generated SPOs that played a significant role in the catalytic activity, depending not only on the ligand but also on the interplay between the ligand and the palladium precursor. It can be concluded, therefore, that as SPOs can be generated in situ by ligand hydrolysis, they can influence the catalysis even when they are not used on purpose as ligands.

5.8. C–H bond arylation

The activation of unfunctionalised aromatic C–H bonds is a huge area of tremendous interest238–240 that has attracted intense research efforts,241,242 yielding a wealth of systems based on many transition metals.243

In the case of activation of C(sp2)–H bonds, the reaction is usually known as arylation. In this field, the relatively inexpensive ruthenium-based systems have been profusely used244–246 and those with SPOs are particularly successful that they have merited their own reviews.247,248 This area has advanced thanks to the research of Ackermann and coworkers, who some time ago pioneered the field with the Ru(II)-catalysed ortho-diarylation of 2-phenylpyridines and the monoarylation of ketimines with aryl chlorides in the presence of diadamantylphosphine oxide (Scheme 99).103,249

In contrast to cross-coupling reactions, this reaction was found to be quite insensitive to the nature of the aryl chloride but the reactions had to be carried out at 120 °C to achieve high yields. The same group expanded the scope of the reaction by arylating aryltriazoles250 and phenoxypyridines with p-bromoanisole251 although the best results were obtained when mesitylcarboxylic acid instead of diadamantylphosphine was used as a ligand.249

When the electrophilic reagent was changed to aryl tosylates, Ackermann and coworkers252 obtained excellent results with a very bulky diaminophosphine oxide, a HASPO (Scheme 100).

With this ligand, 2-phenylpyridine was almost exclusively monoarylated, in contrast to the results in Scheme 99. With the same method, they also reported the arylation of o-tolyloxazolines. These arylations are thought to proceed through a concerted metallation/deprotonation mechanism assisted by the deprotonated SPO.

In more recent studies, Ackermann and coworkers109 have used well-defined [RuCl26-p-cymene)(PRR'OH)] acid systems (Fig. 6), the same as those used in nitrile hydration, in arylation reactions. They found that the simple SPO dibutylphosphine oxide gave the best results of activity and chemoselectivity in the arylation of o-tolyloxazolines and 2-vinyl-pyridines with aryl bromides and tosylates (Scheme 101).

In addition, they showed that this chemistry is so tolerant to functional groups that it can be applied to the preparation of drug precursors. At the same time, Clavier and coworkers106 used the same type of complex in the arylation of 2-phenypyridine with chlorobenzene and found that t-BuPhP(O)H was the best ligand, yielding an 89% yield in 24 h at 80 °C. The results of the preformed catalysts outperformed those of in situ experiments and a marked halide effect was found.

5.9. Cycloadditions

Pd- and Pt-phosphinito–phosphinous acid complexes have been found to be excellent catalysts for [2 + 1] cycloadditions between norbornadienes and alkynes to afford various types of methylenecyclopropane derivatives. This chemistry has been mainly developed by Clavier and Buono, who wrote their own review on the topic.169

In 2005, Buono and coworkers167 disclosed that Pd–SPO complexes that have been heavily used in cross-coupling reactions were also active in [2 + 1] cycloadditions between terminal alkynes and norbornadienes, to give functionalised alkylidenecyclopropanes (Scheme 102).

This reactivity contrasts with the hydroalkylation that was observed when the well-known Herrmann–Beller phosphapalladacycle was used.253–255 The presence of an acetate rather than a chloride was required for the reaction and this was attributed to the strong coordination ability of the latter. Regarding the substituents at phosphorus, in subsequent studies255,256 they found that CyPhP(O)H was the best SPO for the reaction, with a particularly broad spectrum of substrates, including ynamides.257 Interestingly, it could also be confirmed that an SPO was really needed since no reaction occurred with triphenylphosphine or typical diphosphines.

The mechanism of the reaction has been studied by experimental and computational methods167,255,258 to ascertain why the Herrmann–Beller catalyst cleanly forms the hydroalkylation product while the SPO-based catalyst gives the [2 + 1] cycloaddition product. It seems that in both transformations a key syn-carbometallated intermediate is formed which in the case of Herrmann–Beller palladacycles is rapidly hydrolysed to give the alkylation product (Scheme 103).167,255

In contrast, with phosphinito–phosphinous acid Pd complexes the carbon–carbon triple bond inserts again into the Pd–C bond to form vinylidene species that after acidolysis furnish the [2 + 1] cycloaddition product. The formation of Pd-vinylidene species was supported by deuterium labelling experiments. It can be concluded that the phosphinito–phosphinous acid ligand is unique for this transformation because the two phosphorus atoms are inequivalent, but interchangeable. The asymmetric version of the reaction was explored with optically pure P-stereogenic SPOs and will be commented in the enantioselective catalysis section.

When the [2 + 1] cycloaddition reaction was attempted with propargyl acetates, it was found that these substrates triggered a tandem cycloaddition–ring expansion reaction giving various functionalised bicyclo[3.2.1]octadienes (Scheme 104).168

This unusual tandem reaction has been observed with several norbornadienes168 and applied to the synthesis of seven-membered carbocycles.259 It has to be noted that the reaction involves a vinylidenecyclopropane intermediate that can be generally isolated, which rearranges to the ring-expanded product.

In parallel, the same group developed the Pt-catalysed version of the reaction in Scheme 102,181 which constituted the first example of carbon–carbon bond formation catalysed by Pt–SPO species. The reactions proceed under mild conditions (55 °C) and require the presence of acetic acid (Scheme 105).

It was found that in most cases Pt-based systems performed better than Pd-based analogues. Like Pd-catalysed reactions, Pt-vinylidene complexes were proposed as intermediates, but with important differences between the mechanisms for Pd- and Pt-catalysed reactions.260

When using propargyl acetates as substrates, a Pt-catalysed regio- and diastereoselective tandem [2 + 1]/[3 + 2] intermolecular cycloaddition reaction was developed (Scheme 106).261

This tandem reaction has a wide scope of substrates and allows the formation of tricyclic compounds in good yields and selectivities. The mechanism of the [3 + 2] cycloaddition261 reaction is thought to involve platinacyclopentenes formed by oxidative coupling between the alkyne and the methylenecyclopropane formed by the [2 + 1] cycloaddition.

5.10. Additions to allenes

A few years ago, Breit and coworkers262 reported a Rh(I)-catalysed coupling of benzotriazoles with allenes, a C–N bond forming reaction. Due to the equilibrium between the N1 and N2 tautomers of 1,2,3-triazoles, a mixture of N1- and N2-substituted products is usually found. An exceptionally high N1-selectivity was found in the case of the SPO ligand named JoSPOphos in contrast to the high N2-selectivity obtained with DPEphos (Scheme 107).

The catalytic system was robust and tolerated many modifications on the benzotriazole and the allene. Given that JoSPOphos is a chiral, P-stereogenic SPO, some enantioinduction could be expected and indeed a 46% ee was observed.

The mechanism was studied by labelling experiments262 and a plausible explanation for the formation of the two regioisomers could be found (Scheme 108).

Further work from the same group expanded the Rh(I)/JoSPOphos catalysis to the coupling of allenes with pyrazoles263 and tetrazoles,264 the coupling of alkynes to triazoles78 and the hydroamination of alkynes265 but as these reactions gave very good enantioselectivities, they are described in the enantioselective catalysis section.

6. Enantioselective catalysis

Despite the fact that P-stereogenic SPOs are in general configurationally stable, they not have been profusely used in asymmetric catalysis and the same happens with other types of chiral SPOs. The last few years, however, have witnessed the appearance of very interesting results in asymmetric catalysis with SPOs, which bodes well for the future. Most of the best results in enantioselective catalysis with SPOs have been achieved with bidentate systems, although the most heavily studied chiral SPO is the P-stereogenic tert-butylphenylphosphine oxide.

6.1. Hydrogenation

The most thoroughly studied reaction in asymmetric homogeneous catalysis is by far hydrogenation. Unsurprisingly, several reports have described the use of SPOs in hydrogenation of several unsaturated substrates.

The first report on the use of SPOs in asymmetric catalysis came from Minnaard, Feringa, de Vries and coworkers,66 who used several simple P-stereogenic SPOs (including tert-butylphenylphosphine oxide) obtained by preparative HPLC (Fig. 3) in the Ir-catalysed hydrogenation of imines (Scheme 109).

With this system, they were able to reduce several ketimines with relatively good conversions and selectivities.

Immediately after this report, the same group applied the same ligands to the Rh- and Ir-catalysed reduction of functionalised alkenes124 (Scheme 110).

It was found that the typical tert-butylphenylphosphine oxide was a versatile ligand in the Ir-catalysed hydrogenation of β-branched dehydroamino esters and in the Rh-catalysed hydrogenation of an enol carbamate. Interesting solvent effects on the absolute configuration of the hydrogenated product were found.

After these initial results, there was little activity in the field of hydrogenation with SPOs38,67 because it became clear that in order to obtain better results competitive with the best ligands in asymmetric hydrogenation more elaborate systems were needed. In this line, in 2014 Han and co-workers,125 who described the in situ Rh-catalysed hydrogenation of α-acetamidocinnamates using optically pure H-menthylphosphinates, achieved enantioselectivities up to 99.6% with (RP)-menthylbenzylphosphinate. They were able to grow suitable crystals for X-ray determination of a solution with [Rh(COD)2]OTf and the ligand tBuPhP(O)H and they found that the complexes formed did not contain the expected pseudobidentate bridge, but a OTf moiety instead (Fig. 11).125


image file: c9cy01501a-f11.tif
Fig. 11 Rh(I)-Catalysed asymmetric hydrogenation of MAC with chiral SPOs.

The best results in the field, however, were reported by Pfaltz and co-workers,77 who considered that the scarce research done was due to the insufficient affinity of the SPO for Rh. With this in mind, they successfully combined an SPO with a phosphino moiety, which should not only lead to a stronger coordination to the metal but also should give better-defined complexes. Indeed, they successfully prepared two SPO–P ligands with chiral elements on the backbone as well as at the phosphorus atom of the SPO (Fig. 11). The first SPO–P family (JoSPOphos, Scheme 19) was based on a chiral ferrocenyl backbone, which leads to ligands similar to the well-known Josiphos,266–268 while the second (TerSPOphos) contained a menthyl substituent. Both families were tested in the hydrogenation of α-acetamidocinnamates achieving the best results reported for these SPO systems, displaying enantioselectivities up to 99% ee and very high TOFs, up to 20[thin space (1/6-em)]000 h−1.

Pugin, Pfaltz and coworkers77 also employed JoSPOphos in combination with [{RuCl(μ-Cl)(η6-p-cymene)}2] in the hydrogenation of β-ketoesters. They found that it was difficult to draw structure–selectivity relationships but managed to obtain a 92% ee in the hydrogenation of 2-oxopentanoate at very low catalyst loading under 1 bar of hydrogen. It was found that for these ligands the absolute configuration of the phosphorus atom controls the sense of the optical induction.

A couple of years later, Ding and coworkers269 studied the Rh/phosphoramidite-catalysed hydrogenation of α-substituted ethenylphosphonic acids and serendipitously discovered that the hydrolysis products213 of certain phosphoramidites, which were HASPOs, were indeed excellent ligands for the reaction (Scheme 111).

The evolved catalysts showed excellent enantioselectivity and catalytic activity in the asymmetric hydrogenation of many α-substituted ethenylphosphonic acids producing enantiopure phosphonic acids with biological interest. The authors hypothesised that the H-bonding interactions had a positive effect on catalysis (poor conversions were observed in protic solvents) and that two HASPOs produced a pseudobidentate ligand, as shown by X-ray crystallography.

The same authors expanded this work to the preparation of bioactive building blocks by Rh(I)/HASPO-catalysed asymmetric hydrogenation of acrylic acids (Scheme 112).270–272

Interestingly, they observed a synergistic effect273 with sharp improvements on both activity and enantioselectivity when achiral triphenylphosphine and related ligands were employed along with the chiral HASPO ligand.

Recently, Dong, Zhang and coworkers79,80,274 used their ferrocenyl-based ligands SPO-Wudaphos (Scheme 19) in the hydrogenation of α-methylene-γ-keto carboxylic acids80,275 and α-substituted ethenylphosphonic acids274 obtaining exceedingly high enantioselectivities and very good conversions towards the hydrogenated product, which in many cases had biological applications (Scheme 113).

Both experimental and computational methods showed the important role of ion-pair and H-bond non-covalent interactions in the excellent performance of the ligands.

Van Leeuwen and coworkers276 reported on the enantioselective hydrogenation of ketones by Ir-NPs stabilised by an atropoisomeric SPO (Scheme 68) that was unable to racemise under catalytic conditions (Scheme 114).

This study represented the first example of asymmetric hydrogenation on SPO-stabilised NPs and the first asymmetric hydrogenation catalysed by non-supported Ir-NPs. Although the enantioselectivities were modest, only one ligand was studied so there is ample room for improvement as more ligands are studied.

6.2. Nitrile hydration

One of the most important applications of SPOs in catalysis is in the hydration of nitriles, as has been described in this review. The development of an asymmetric, version, however, is very rare. In 2004, Minnaard, Feringa and de Vries277 attempted kinetic resolution in the hydrolysis of racemic nitriles with Pt-catalysis using enantiopure tBuPhP(O)H but no resolution was found, possibly due to racemisation of the ligand during the reaction.

Much later, a report of van Leeuwen and coworkers,180 who used the chiral Pt–SPO complexes in Scheme 68 in the hydration of aromatic nitriles (Scheme 90), offered more promising results. One of the substrates they studied was the axially chiral [1,1′-binaphthalene]-2,2′-dicarbonitrile. When this dinitrile (in racemic form) was mono- and dihydrated, a successful kinetic resolution was achieved (Scheme 115).

In this reaction, the unconsumed dinitrile, the monocarboxamide and the biscarboxamide were formed in non-racemic forms, especially the latter, which could be obtained in very high enantioselectivity.


image file: c9cy01501a-s68.tif
Scheme 68 Substitution on Pt(II) SPO complexes.

image file: c9cy01501a-s69.tif
Scheme 69 Preparation of Pt-acetato SPO complexes described by Buono and co-workers.181

image file: c9cy01501a-s70.tif
Scheme 70 Coordination behaviour of platinum SPO complexes reported by Buono and co-workers.182

image file: c9cy01501a-s71.tif
Scheme 71 Mechanism of dimerisation of Pt–SPO complexes.

image file: c9cy01501a-s72.tif
Scheme 72 Formation of κ2-acetato Pt SPO complexes.

image file: c9cy01501a-s73.tif
Scheme 73 Reactivity of dinuclear Pt–SPO systems.

image file: c9cy01501a-s74.tif
Scheme 74 Preparation of SPO-heterocyclic Pt complexes.

image file: c9cy01501a-s75.tif
Scheme 75 Synthesis of Au(I)–SPO complexes.185

image file: c9cy01501a-s76.tif
Scheme 76 Preparation of disubstituted Au(I)–SPO complexes.

image file: c9cy01501a-s77.tif
Scheme 77 Preparation of gold(I) nanoparticles described by van Leeuwen and co-workers.189,190

image file: c9cy01501a-s78.tif
Scheme 78 Synthesis of the optically pure Au(I) complex.

image file: c9cy01501a-s79.tif
Scheme 79 Aurophilic interactions in Au(I) SPO complexes.

image file: c9cy01501a-s80.tif
Scheme 80 Au/SPO-catalysed reactions involving enynes.

image file: c9cy01501a-s81.tif
Scheme 81 Ru-Catalysed cycloisomerisation of arenynes.

image file: c9cy01501a-s82.tif
Scheme 82 Rearrangement of aldoximes into amides catalysed by a Ru complex.

image file: c9cy01501a-s83.tif
Scheme 83 Ru-Catalysed tandem isomerisation/Claisen rearrangement of diallyl ethers.

image file: c9cy01501a-s84.tif
Scheme 84 Hydrogenation of aldehydes by a tridentate diphosphine–SPO ligand.

image file: c9cy01501a-s85.tif
Scheme 85 Hydrogenation of aromatic hydrocarbons by SPO-stabilised NPs.

image file: c9cy01501a-s86.tif
Scheme 86 Hydrogenation of functionalised aldehydes.

image file: c9cy01501a-s87.tif
Scheme 87 Ir/SPO-hydrogenation of aldehydes.

image file: c9cy01501a-s88.tif
Scheme 88 Ir/SPO-hydrogenation of aldehydes.

image file: c9cy01501a-s89.tif
Scheme 89 Hydrolysis of nitriles catalysed by Pt–SPO complexes.

image file: c9cy01501a-s90.tif
Scheme 90 Pt/SPO-catalysed hydration of aromatic nitriles into carboxamides.

image file: c9cy01501a-s91.tif
Scheme 91 Hydrolysis of nitriles catalysed by a Ru-p-cymene–PMe2OH complex.

image file: c9cy01501a-s92.tif
Scheme 92 Involvement of the SPO in the Ru-catalysed hydration of nitriles.

image file: c9cy01501a-s93.tif
Scheme 93 Hydration of nitriles by Ru(IV)-bis(allyl) complexes.

image file: c9cy01501a-s94.tif
Scheme 94 Catalytic synthesis of β-ketoamides and β-hydroxyamides from β-ketonitriles.

image file: c9cy01501a-s95.tif
Scheme 95 Tandem nitrile hydration/bioreduction reaction to yield β-hydroxyamides from β-ketonitriles.

image file: c9cy01501a-s96.tif
Scheme 96 Hydrolysis of amine–boranes catalysed by Rh(III)/SPO-complexes.

image file: c9cy01501a-s97.tif
Scheme 97 Rh/SPO-catalysed hydroformylation.

image file: c9cy01501a-s98.tif
Scheme 98 Pd/SPO- and Ni/SPO-catalysed cross-coupling reactions of aryl chlorides.

image file: c9cy01501a-s99.tif
Scheme 99 Ru(II)-p-Cymene/Ad2P(O)H-catalysed arylation reactions.

image file: c9cy01501a-s100.tif
Scheme 100 Ru(II)-p-Cymene/HASPO-catalysed arylation reactions.

image file: c9cy01501a-s101.tif
Scheme 101 Arylation reactions catalysed by well-defined Ru(II) precursors.

image file: c9cy01501a-s102.tif
Scheme 102 Pd/SPO-catalysed [2 + 1] cycloadditions.

image file: c9cy01501a-s103.tif
Scheme 103 Key intermediates of the [2 + 1] cycloaddition of norbornadienes and alkynes.

image file: c9cy01501a-s104.tif
Scheme 104 [2 + 1]-Cycloaddition/ring expansion tandem reaction.

image file: c9cy01501a-s105.tif
Scheme 105 Selected examples of Pt-catalysed [2 + 1] cycloadditions.

image file: c9cy01501a-s106.tif
Scheme 106 Pt/SPO-catalysed tandem [2 + 1]/[3 + 2]-cycloaddition reactions.

image file: c9cy01501a-s107.tif
Scheme 107 Rh-Catalysed coupling of benzotriazoles with allenes.

image file: c9cy01501a-s108.tif
Scheme 108 Mechanism for the Rh-catalysed coupling of benzotriazoles with allenes.

image file: c9cy01501a-s109.tif
Scheme 109 Asymmetric Ir/SPO-catalysed hydrogenation of imines.

image file: c9cy01501a-s110.tif
Scheme 110 Asymmetric Rh- and Ir/SPO-catalysed hydrogenation of functionalised olefins.

image file: c9cy01501a-s111.tif
Scheme 111 Rh/SPO-catalysed hydrogenation of ethenylphosphonic acids.

image file: c9cy01501a-s112.tif
Scheme 112 Rh(I)/SPO/PPh3-catalysed hydrogenation of acrylic acids.

image file: c9cy01501a-s113.tif
Scheme 113 Rh/Wudaphos-catalysed hydrogenations.

image file: c9cy01501a-s114.tif
Scheme 114 Hydrogenation of ketones catalysed by Ir/SPO-stabilised NPs.

image file: c9cy01501a-s115.tif
Scheme 115 Kinetic resolution of a racemic axially chiral dinitrile.

image file: c9cy01501a-s116.tif
Scheme 116 Pd/SPO-catalysed asymmetric allylic alkylation of the model substrate.

image file: c9cy01501a-s117.tif
Scheme 117 DIAPHOX-HASPO ligands and their tautomerisation induced by BSA.

image file: c9cy01501a-s118.tif
Scheme 118 Asymmetric Pd/DIAPHOX-catalysed formation of quaternary stereocentres.

image file: c9cy01501a-s119.tif
Scheme 119 Pd/SPO-catalysed asymmetric [2 + 1] between norbornadienes and alkynes.

image file: c9cy01501a-s120.tif
Scheme 120 Rh(I)/JoSPOphos-catalysed asymmetric addition of pyrazoles to allenes.

image file: c9cy01501a-s121.tif
Scheme 121 Rh(I)/JoSPOphos-catalysed asymmetric addition of tetrazoles to allenes.

image file: c9cy01501a-s122.tif
Scheme 122 Rh/JoSPOPhos-catalysed hydroamination of alkynes or allenes with pyrazoles.

image file: c9cy01501a-s123.tif
Scheme 123 Asymmetric Rh(I)-catalysed allylation of alkynes or allenes.

image file: c9cy01501a-s124.tif
Scheme 124 Asymmetric hydrocarbamoylation of allylic formamides.

6.3. Allylic substitution

Enantioselective allylic substitution is a flagship reaction in asymmetric catalysis, especially with Pd-based catalysts. In spite of this, it has been scarcely explored with (strictly speaking) SPO ligands. To the best of our knowledge, only Dai and co-workers86 studied the use of tBuPhP(O)H in the Pd-catalysed allylic alkylation of rac-3-acetoxy-1,3-diphenyl-1-propene, the model substrate, with dimethylmalonate (DMM) in the presence of bis(trimethylsilyl)acetamide (BSA) (Scheme 116).

The solvent, base, and other parameters of the reaction were varied and up to 80% ee was accomplished using an “in situ” generated palladium complex.

The situation is completely different when it comes to HASPO ligands because of a very successful family of HASPOs developed by Hamada and coworkers in 2004,46 the P-stereogenic DIAPHOX ligands (Scheme 11), afforded very good results in allylic substitution. For the model substrate (Scheme 116), they obtained48 94% conversion and 99% ee with the presence of BSA and zinc acetate. 31P NMR studies demonstrated that BSA triggered the tautomerisation of DIAPHOX to the trivalent diamidophosphite (Scheme 117).

The results of some experiments showed that two molecules of DIAPHOX coordinated the Pd centre in a bis(monodentate) fashion and that the presence of zinc acetate was required in order to obtain good results. It was hypothesised that the exocyclic aminoethyl group has a directing effect upon the attacking nucleophile through an attractive secondary interaction.47,48

The most interesting result, however, was that the ligands were also very good in the formation of quaternary stereocentres46 in high enantioselectivity (Scheme 118), an extremely important challenge, yet difficult, in organic synthesis.

Given the promising results obtained with the DIAPHOX ligands, after the first report,46 both the applicability and the mechanism were studied in depth by Hamada and coworkers.43,47,48,278–285 In addition, the reaction has been expanded not only to several types of Pd-catalysed allylic alkylations43,282,283 and aminations,278,280,285 but also to Ir-catalysed allylic alkylation281 and amination.279 These reactions have been applied to the formation of chiral centres in synthesis of biologically active products285 and even to total syntheses.284,286

It should be noted that Nemoto and Hamada10,45,49 have reviewed the application of DIAPHOX ligands to allylic substitution and the reader is directed to these reviews to have the full account of their applicability.

6.4. Cycloadditions

The [2 + 1] cycloaddition products in Scheme 102 are chiral and display a particular chirality called geometrical enantiomorphic isomerism (also known as geometric isomerism or cistrans isomerism) due to the E and Z configurations of the C–C double bond. Using optically pure P-stereogenic SPOs, Buono and coworkers166,167,169 developed the asymmetric version of the Pd-catalysed [2 + 1] cycloaddition between norbornadienes and alkynes (Scheme 119).

Several simple chiral SPOs were employed but the most enantioselective was tBuPhP(O)H, with up to 59% ee for the reaction between norbornadiene and phenylacetylene.167 An improvement of the enantioselectivity was possible by changing the reaction conditions and adding (S)-(+)-mandelic acid. With the optimised conditions, up to 95% ee could be obtained.166

6.5. Additions to allenes

In the Rh-catalysed addition of benzotriazoles to allenes, Breit and coworkers262 reported a single result of 46% ee with the JoSPOphos ligand. Shortly after, the same group263 reported the Rh(I)-catalysed asymmetric N-selective coupling of pyrazole derivatives with terminal allenes to give secondary and tertiary allylic pyrazoles, which are important intermediates of medicinally important targets (Scheme 120).

The reaction was highly regio- and enantioselective with the ligand JoSPOphos and tolerated a broad range of substituted pyrazoles and allenes.

The same group expanded this chemistry to the asymmetric coupling of tetrazoles with allenes,264 allowing the synthesis of tertiary and quaternary allylic C–N bonds in high enantioselectivity (Scheme 121).

Several typical bidentate phosphines such as DIOP and BINAP were used, but the best was JoSPOphos, providing good yields for many substrates and enantioselectivities of up to 97% ee.

Further research from the same group265 explored the enantioselective Rh-catalysed hydroamination of alkynes or allenes with pyrazoles (Scheme 122).

This methodology allowed the functionalisation of both terminal and internal alkynes and also allenes with a broad range of pyrazoles. High chemo-, regio- and enantioselectivities were obtained towards the allylated pyrazoles, with remarkable functional group compatibility.

Quite recently, the same group78 expanded their chemistry to the Rh-catalysed allylation of triazoles with alkynes or allenes (Scheme 123).

It was clear that the Rh/JoSPOphos system was very good in the regio- and enantioselective addition of triazoles to alkynes and terminal allenes. A broad substrate range of triazoles, alkynes and allenes provided N-allylated triazoles in very good yields and enantioselectivities. In this paper, the synthesis of three new JoSPOphos ligands is described.

6.6. Hydrocarbamoylation

Donets and Cramer287 reported Ni/Al-catalysed intramolecular hydrocarbamoylation of allylic formamides with a diaminophosphine oxide HASPO (Scheme 124).

The bulky HASPO, inspired by the work of Hamada and coworkers with DIAPHOX (see the allylic substitution part), was readily accessible and is believed to simultaneously bond to the Ni centre and the aluminium Lewis acid. This bimetallic catalyst required the presence of a monodentate phosphine to displace the cyclooctadiene. With this system, excellent results in the asymmetric catalytic synthesis of chiral γ-lactams were obtained by C–H activation of formamides.

7. Conclusions

This review, although not in a fully comprehensive way, aims to give a good grasp of the amazing variety of architectures that SPO ligands create when they coordinate to transition metals as well as their modularity and the variety of roles that they play in catalysis. Indeed, usually the introductions of papers exploring the catalytic applications of SPOs in catalysis highlight the air-stability of SPOs in contrast to classic phosphines. After analysing the literature on SPOs over the last 50 years, it is clear that SPOs are much more than “air-stable versions of phosphines” but they have their own idiosyncrasy and hold immense potential in catalysis. In addition, some SPOs have given impressive catalytic results including in asymmetric catalysis.

Despite all this, the truth is that SPOs still remain infrequently used in catalysis. This can be attributed to several factors. The first is that there is a lack of synthetic methods for SPOs. The most common synthesis is still the classic hydrolysis of chlorophosphines, an efficient method but only for the simplest SPOs, whose chlorophosphine precursors are commercially available. The scarcity of efficient synthetic methods is especially true for chiral SPOs and to make things worse, in the case of P-stereogenic SPOs the hydrolysis method cannot be used since enantiopure chlorophosphines are very difficult to obtain. The good news is that the wealth of new synthetic phosphorus chemistry appearing in recent years is also impacting the preparation of SPOs, especially the chiral ones and hence exciting new SPOs can be expected. Another factor that hampers a more widespread use of SPOs in catalysis is that the stereoelectronic properties of SPOs are still poorly understood, especially compared to phosphines and carbenes and the same can be said about the mechanism of metal-assisted tautomerisations. In addition, the coordination chemistry of SPOs is complicated and frequently changes in the course of a catalytic reaction. At present, the detailed factors that determine the coordination mode of SPOs remain speculative at best. Needless to say, much better knowledge of all these aspects is required to understand the exact role of an SPO in catalysis. Fortunately, our knowledge is continuously improving not only with experimental results but also with increasingly insightful theoretical studies, usually employing DFT-based methods. We are confident, therefore, that when the synthetic and mechanistic challenges associated with SPOs are met, many new accomplishments will be seen in the use of SPOs in catalysis.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

We thank MINECO (grant numbers CTQ2015-65040-P and CTQ2017-87840-P) and IRB Barcelona for financial support. We gratefully acknowledge institutional funding from Severo Ochoa Award (MINECO) and from the CERCA program (Generalitat de Catalunya).

References

  1. Phosphorus(III) Ligands in Homogeneous Catalysis: Design and Synthesis, ed. P. C. J. Kamer and P. W. N. M. van Leeuwen, Wiley, 2012 Search PubMed.
  2. M. Ohff, J. Holz, M. Quirmbach and A. Börner, Synthesis, 1998, 1391–1415 CrossRef CAS.
  3. M. R. Netherton and G. C. Fu, Org. Lett., 2001, 3, 4295–4298 CrossRef CAS PubMed.
  4. J. Chatt and B. T. Heaton, J. Chem. Soc. A, 1968, 2745–2757 RSC.
  5. P. W. N. M. van Leeuwen, C. F. Roobeek, R. L. Wife and J. H. G. Frijns, J. Chem. Soc., Chem. Commun., 1986, 31–33 RSC.
  6. T. M. Shaikh, C.-M. Weng and F.-E. Hong, Coord. Chem. Rev., 2012, 256, 771–803 CrossRef CAS.
  7. L. Ackermann, Synthesis, 2006, 1557–1571 CrossRef CAS.
  8. L. Ackermann, in Phosphorus Ligands in Asymmetric Catalysis: Synthesis and Applications, ed. A. Börner, Wiley-VCH, Weinheim, 2008, vol. 2, pp. 831–847 Search PubMed.
  9. N. V. Dubrovina and A. Börner, Angew. Chem., Int. Ed., 2004, 43, 5883–5886 CrossRef CAS PubMed.
  10. T. Nemoto and Y. Hamada, Tetrahedron, 2011, 67, 667–687 CrossRef CAS.
  11. T. Achard, Chimia, 2016, 70, 8–19 CrossRef CAS PubMed.
  12. P. Sutra and A. Igau, Coord. Chem. Rev., 2016, 308, 97–116 CrossRef CAS.
  13. J. Francos, D. Elorriaga, P. Crochet and V. Cadierno, Coord. Chem. Rev., 2019, 387, 199–234 CrossRef CAS.
  14. D. Moraleda, D. Gatineau, D. Martin, L. Giordano and G. Buono, Chem. Commun., 2008, 3031–3033 RSC.
  15. T. L. Emmick and R. L. Letsinger, J. Am. Chem. Soc., 1968, 90, 3459–3465 CrossRef CAS.
  16. D. Magiera, A. Szmigielska, K. M. Pietrusiewicz and H. Duddeck, Chirality, 2004, 16, 57–64 CrossRef CAS PubMed.
  17. C.-H. Wei, C.-E. Wu, Y.-L. Huang, R. G. Kultyshev and F.-E. Hong, Chem. – Eur. J., 2007, 13, 1583–1593 CrossRef CAS PubMed.
  18. B. G. Janesko, H. C. Fisher, M. J. Bridle and J.-L. Montchamp, J. Org. Chem., 2015, 80, 10025–10032 CrossRef CAS PubMed.
  19. A. Christiansen, C. Li, M. Garland, D. Selent, R. Ludwig, A. Spannenberg, W. Baumann, R. Franke and A. Börner, Eur. J. Org. Chem., 2010, 2733–2741 CrossRef CAS.
  20. B. Kurscheid, W. Wiebe, B. Neumann, H.-G. Stammler and B. Hoge, Eur. J. Inorg. Chem., 2011, 5523–5529 CrossRef CAS.
  21. N. Allefeld, M. Grasse, N. Ignat'ev and B. Hoge, Chem. – Eur. J., 2014, 20, 8615–8620 CrossRef CAS PubMed.
  22. D. Martin, D. Moraleda, T. Achard, L. Giordano and G. Buono, Chem. – Eur. J., 2011, 17, 12729–12740 CrossRef CAS PubMed.
  23. D. Magiera, W. Baumann, I. S. Podkorytov, J. Omelanczuk and H. Duddeck, Eur. J. Inorg. Chem., 2002, 3253–3257 CrossRef CAS.
  24. C. A. Busacca, J. C. Lorenz, N. Grinberg, N. Haddad, M. Hrapchak, B. Latli, H. Lee, P. Sabila, A. Saha, M. Sarvestani, S. Shen, R. Varsolona, X. Wei and C. H. Senanayake, Org. Lett., 2005, 7, 4277–4280 CrossRef CAS.
  25. W. B. Farnham, R. A. Lewis, R. K. Murray and K. Mislow, J. Am. Chem. Soc., 1970, 92, 5808–5809 CrossRef CAS.
  26. A. J. Bloomfield, J. M. Quian and S. B. Herzon, Organometallics, 2010, 29, 4193–4195 CrossRef CAS.
  27. M. M. Rauhut and H. A. Currier, J. Org. Chem., 1961, 26, 4626–4628 CrossRef CAS.
  28. L. D. Quin and R. E. Montgomery, J. Org. Chem., 1963, 28, 3315–3320 CrossRef CAS.
  29. G. Y. Li, P. J. Fagan and P. L. Watson, Angew. Chem., Int. Ed., 2001, 40, 1106–1109 CrossRef CAS PubMed.
  30. G. Y. Li, Angew. Chem., Int. Ed., 2001, 40, 1513–1516 CrossRef CAS.
  31. G. Y. Li, J. Organomet. Chem., 2002, 653, 63–68 CrossRef CAS.
  32. L. Ackermann and R. Born, Angew. Chem., Int. Ed., 2005, 44, 2444–2447 CrossRef CAS PubMed.
  33. A. Christiansen, D. Selent, A. Spannenberg, M. Köckerling, H. Reinke, W. Baumann, H. Jiao, R. Franke and A. Börner, Chem. – Eur. J., 2011, 17, 2120–2129 CrossRef CAS PubMed.
  34. C.-Y. Hu, Y.-Q. Chen, G.-Y. Lin, M.-K. Huang, Y.-C. Chang and F.-E. Hong, Eur. J. Inorg. Chem., 2016, 3131–3142 CrossRef CAS.
  35. L.-Y. Jung, S.-H. Tsai and F.-E. Hong, Organometallics, 2009, 28, 6044–6053 CrossRef CAS.
  36. F. Guillen and J.-C. Fiaud, Tetrahedron Lett., 1999, 40, 2939–2942 CrossRef CAS.
  37. F. Guillen, M. Rivard, M. Toffano, J. Legros, J. Daran and J. Fiaud, Tetrahedron, 2002, 58, 5895–5904 CrossRef CAS.
  38. A. Galland, C. Dobrota, M. Toffano and J.-C. Fiaud, Tetrahedron: Asymmetry, 2006, 17, 2354–2357 CrossRef CAS.
  39. D. Enders, L. Tedeschi and J. W. Bats, Angew. Chem., Int. Ed., 2000, 39, 4605–4607 CrossRef CAS PubMed.
  40. M. R. Nahm, X. Linghu, J. R. Potnick, C. M. Yates, P. S. White and J. S. Johnson, Angew. Chem., Int. Ed., 2005, 44, 2377–2379 CrossRef CAS PubMed.
  41. M. R. Nahm, J. R. Potnick, P. S. White and J. S. Johnson, J. Am. Chem. Soc., 2006, 128, 2751–2756 CrossRef CAS PubMed.
  42. A. De la Cruz, K. J. Koeller, N. P. Rath, C. D. Spilling and I. C. F. Vasconcelos, Tetrahedron, 1998, 54, 10513–10524 CrossRef CAS.
  43. T. Nemoto, L. Jin, H. Nakamura and Y. Hamada, Tetrahedron Lett., 2006, 47, 6577–6581 CrossRef CAS.
  44. T. Nemoto, T. Harada, T. Matsumoto and Y. Hamada, Tetrahedron Lett., 2007, 48, 6304–6307 CrossRef CAS.
  45. T. Nemoto, Chem. Pharm. Bull., 2008, 56, 1213–1228 CrossRef CAS PubMed.
  46. T. Nemoto, T. Matsumoto, T. Masuda, T. Hitomi, K. Hatano and Y. Hamada, J. Am. Chem. Soc., 2004, 126, 3690–3691 CrossRef CAS PubMed.
  47. T. Nemoto, T. Fukuda, T. Matsumoto, T. Hitomi and Y. Hamada, Adv. Synth. Catal., 2005, 347, 1504–1506 CrossRef CAS.
  48. T. Nemoto, T. Masuda, T. Matsumoto and Y. Hamada, J. Org. Chem., 2005, 70, 7172–7178 CrossRef CAS PubMed.
  49. T. Nemoto and T. Hamada, Chem. Rec., 2007, 7, 150–158 CrossRef CAS PubMed.
  50. K. M. Pietrusiewicz and M. Zablocka, Chem. Rev., 1994, 94, 1375–1411 CrossRef CAS.
  51. Y. Zhang, S.-Z. Nie, J.-J. Ye, J.-P. Wang, M.-M. Zhou, C.-Q. Zhao and Q. Li, J. Org. Chem., 2019, 84, 8423–8439 CrossRef CAS PubMed.
  52. A. Leyris, J. Bigeault, D. Nuel, L. Giordano and G. Buono, Tetrahedron Lett., 2007, 48, 5247–5250 CrossRef CAS.
  53. D. Gatineau, L. Giordano and G. Buono, J. Am. Chem. Soc., 2011, 133, 10728–10731 CrossRef CAS PubMed.
  54. A. Leyris, D. Nuel, L. Giordano, M. Achard and G. Buono, Tetrahedron Lett., 2005, 46, 8677–8680 CrossRef CAS.
  55. Q. Xu, C. Zhao and L. Han, J. Am. Chem. Soc., 2008, 130, 12648–12655 CrossRef CAS PubMed.
  56. D. Gatineau, D. H. Nguyen, D. Hérault, N. Vanthuyne, J. Leclaire, L. Giordano and G. Buono, J. Org. Chem., 2015, 80, 4132–4141 CrossRef CAS PubMed.
  57. L. Copey, L. Jean-Gérard, B. Andrioletti and E. Framery, Tetrahedron Lett., 2016, 57, 543–545 CrossRef CAS.
  58. J. Michalski and Z. Skrzypczynski, J. Organomet. Chem., 1975, 97, C31–C32 CrossRef CAS.
  59. R. K. Haynes, T. Au-Yeung, W. Chan, W. Lam, Z. Li, L. Yeung, A. S. C. Chan, P. Li, M. Koen, C. R. Mitchell and S. C. Vonwiller, Eur. J. Org. Chem., 2000, 3205–3216 CrossRef CAS.
  60. J. Drabowicz, P. Lyzwa, J. Omelanczuk, K. M. Pietrusiewicz and M. Mikolajczyk, Tetrahedron: Asymmetry, 1999, 10, 2757–2763 CrossRef CAS.
  61. F. Wang, P. L. Polavarapu, J. Drabowicz and M. Mikolajczyk, J. Org. Chem., 2000, 65, 7561–7565 CrossRef CAS PubMed.
  62. M. Stankevič and K. M. Pietrusiewicz, J. Org. Chem., 2007, 72, 816–822 CrossRef PubMed.
  63. J. Holt, A. M. Maj, E. P. Schudde, K. M. Pietrusiewicz, L. Sieron, W. Wieczorek, T. Jerphagnon, I. W. C. E. Arends, U. Hanefeld and A. J. Minnaard, Synthesis, 2009, 2061–2065 CAS.
  64. F. A. Kortmann, M.-C. Chang, E. Otten, E. P. A. Couzijn, M. Lutz and A. J. Minnaard, Chem. Sci., 2014, 5, 1322–1327 RSC.
  65. O. Cervinka, O. Belovsky and M. Hepnerova, J. Chem. Soc. D, 1970, 562 RSC.
  66. X. Jiang, A. J. Minnaard, B. Hessen, B. L. Feringa, A. L. L. Duchateau, J. G. O. Andrien, J. A. F. Boogers and J. G. de Vries, Org. Lett., 2003, 5, 1503–1506 CrossRef CAS PubMed.
  67. N. V. Dubrovina, H. Jiao, I. Tararov Vitali, A. Spannenberg, R. Kadyrov, A. Monsees, A. Christiansen and A. Börner, Eur. J. Org. Chem., 2006, 3412–3420 CrossRef CAS.
  68. Z. S. Han, H. Wu, Y. Xu, Y. Zhang, B. Qu, Z. Li, D. R. Caldwell, K. R. Fandrick, L. Zhang, F. Roschangar, J. J. Song and C. H. Senanayake, Org. Lett., 2017, 19, 1796–1799 CrossRef CAS PubMed.
  69. A. Gallen, S. Orgue, G. Muller, E. C. Escudero, A. Riera, X. Verdaguer and A. Grabulosa, Dalton Trans., 2018, 47, 5366–5379 RSC.
  70. T. León, A. Riera and X. Verdaguer, J. Am. Chem. Soc., 2011, 133, 5740–5743 CrossRef PubMed.
  71. S. Orgué, A. Flores-Gaspar, M. Biosca, O. Pàmies, M. Diéguez, A. Riera and X. Verdaguer, Chem. Commun., 2015, 51, 17548–17551 RSC.
  72. L. McKinstry and T. Livinghouse, Tetrahedron Lett., 1994, 35, 9319–9322 CrossRef CAS.
  73. A. Galland, J. M. Paris, T. Schlama, R. Guillot, J.-C. Fiaud and M. Toffano, Eur. J. Org. Chem., 2007, 863–873 CrossRef CAS.
  74. T. Imamoto and T. Oshiki, Tetrahedron Lett., 1989, 30, 383–384 CrossRef CAS.
  75. L. McKinstry, J. J. Overberg, C. Soubra-Ghaoui, D. S. Walsh and K. A. Robins, J. Org. Chem., 2000, 65, 2261–2263 CrossRef PubMed.
  76. S.-G. Li, M. Yuan, F. Topić, Z. S. Han, C. H. Senanayake and Y. S. Tsantrizos, J. Org. Chem., 2019, 84, 7291–7302 CrossRef CAS PubMed.
  77. H. Landert, F. Spindler, A. Wyss, H. Blaser, B. Pugin, Y. Ribourduoille, B. Gschwend, B. Ramalingamm and A. Pfaltz, Angew. Chem., Int. Ed., 2010, 49, 6873–6876 CrossRef CAS PubMed.
  78. D. Berthold and B. Breit, Org. Lett., 2018, 20, 598–601 CrossRef CAS PubMed.
  79. C. Chen, Z. Zhang, S. Jin, X. Fan, M. Geng, Y. Zhou, S. Wen, X. Wang, L. W. Chung, X.-Q. Dong and X. Zhang, Angew. Chem., Int. Ed., 2017, 56, 6808–6812 CrossRef CAS PubMed.
  80. C. Chen, S. Wen, X.-Q. Dong and X. Zhang, Org. Chem. Front., 2017, 4, 2034–2038 RSC.
  81. L. V. Graux, M. Giorgi, G. Buono and H. Clavier, Organometallics, 2015, 34, 1864–1871 CrossRef CAS.
  82. I. Cano, L. M. Martínez-Prieto, P. F. Fazzini, Y. Coppel, B. Chaudret and P. W. N. M. van Leeuwen, Phys. Chem. Chem. Phys., 2017, 19, 21655–21662 RSC.
  83. D. M. Roundhill, R. P. Sperline and W. B. Beaulieu, Coord. Chem. Rev., 1978, 26, 263–279 CrossRef CAS.
  84. B. Walther, Coord. Chem. Rev., 1984, 60, 67–105 CrossRef CAS.
  85. J.-N. Li, Y. Fu and Q.-X. Guo, Tetrahedron, 2006, 62, 4453–4462 CrossRef CAS.
  86. W. Dai, K. K. Y. Yeung, W. H. Leung and R. K. Haynes, Tetrahedron: Asymmetry, 2003, 14, 2821–2826 CrossRef CAS.
  87. F. S. S. Schneider, M. Segala, G. F. Caramori, E. H. da Silva, R. L. T. Parreira, H. S. Schrekker and P. W. N. M. van Leeuwen, J. Phys. Chem. C, 2018, 122, 21449–21461 CrossRef CAS.
  88. F. Wang and W. E. Buhro, J. Am. Chem. Soc., 2012, 134, 5369–5380 CrossRef CAS PubMed.
  89. I. W. Robertson and T. A. Stephenson, Inorg. Chim. Acta, 1980, 45, L215–L216 CrossRef CAS.
  90. W. Kläui and E. Buchholz, Inorg. Chem., 1988, 27, 3500–3506 CrossRef.
  91. T. Rüther, U. Englert and U. Koelle, Inorg. Chem., 1998, 37, 4265–4271 CrossRef.
  92. R. Torres-Lubián, M. J. Rosales-Hoz, A. M. Arif, R. D. Ernst and M. A. Paz-Sandoval, J. Organomet. Chem., 1999, 585, 68–82 CrossRef.
  93. T. J. Geldbach, D. Drago and P. S. Pregosin, Chem. Commun., 2000, 1629–1630 RSC.
  94. C. J. den Reijer, M. Wörle and P. S. Pregosin, Organometallics, 2000, 19, 309–316 CrossRef CAS.
  95. T. J. Geldbach, F. Breher, V. Gramlich, P. G. Anil Kumar and P. S. Pregosin, Inorg. Chem., 2004, 43, 1920–1928 CrossRef CAS PubMed.
  96. T. J. Geldbach, P. S. Pregosin and A. Albinati, Organometallics, 2003, 22, 1443–1451 CrossRef CAS.
  97. J. Hannedouche, G. J. Clarkson and M. Wills, J. Am. Chem. Soc., 2004, 126, 986–987 CrossRef CAS PubMed.
  98. T. J. Geldbach, G. Laurenczy, R. Scopelliti and P. J. Dyson, Organometallics, 2005, 25, 733–742 CrossRef.
  99. P. Clavero, A. Grabulosa, M. Font-Bardia and G. Muller, J. Mol. Catal. A: Chem., 2014, 391, 183–190 CrossRef CAS.
  100. M. Navarro, D. Vidal, P. Clavero, A. Grabulosa and G. Muller, Organometallics, 2015, 34, 973–994 CrossRef CAS.
  101. F. Martínez-Peña, S. Infante-Tadeo, A. Habtemariam and A. M. Pizarro, Inorg. Chem., 2018, 57, 5657–5668 CrossRef PubMed.
  102. E. Y. Y. Chan, Q. Zhang, Y. Sau, S. M. F. Lo, H. Y. Sung, I. D. Williams, R. K. Haynes and W. Leung, Inorg. Chem., 2004, 43, 4921–4926 CrossRef CAS PubMed.
  103. L. Ackermann, Org. Lett., 2005, 7, 3123–3125 CrossRef CAS PubMed.
  104. S. M. M. Knapp, T. J. Sherbow, R. B. Yelle, J. J. Juliette and D. R. Tyler, Organometallics, 2013, 32, 3744–3752 CrossRef CAS.
  105. E. Tomás-Mendivil, V. Cadierno, M. I. Menéndez and R. López, Chem. – Eur. J., 2015, 21, 16874–16886 CrossRef PubMed.
  106. L. V. Graux, M. Giorgi, G. Buono and H. Clavier, Dalton Trans., 2016, 45, 6491–6502 RSC.
  107. R. Gonzalez-Fernandez, P. J. Gonzalez-Liste, J. Borge, P. Crochet and V. Cadierno, Catal. Sci. Technol., 2016, 6, 4398–4409 RSC.
  108. G. Manca, M. Caporali, A. Ienco, M. Peruzzini and C. Mealli, J. Organomet. Chem., 2014, 760, 177–185 CrossRef CAS.
  109. D. Zell, S. Warratz, D. Gelman, S. J. Garden and L. Ackermann, Chem. – Eur. J., 2016, 22, 1248–1252 CrossRef CAS PubMed.
  110. R. O. Gould, C. L. Jones, W. J. Sime and T. A. Stephenson, J. Chem. Soc., Dalton Trans., 1977, 669–672 RSC.
  111. T. G. Southern, P. H. Dixneuf, J. Y. Le Marouille and D. Grandjean, Inorg. Chem., 1979, 18, 2987–2991 CrossRef CAS.
  112. J. Coetzee, G. R. Eastham, A. M. Z. Slawin and D. J. Cole-Hamilton, Dalton Trans., 2014, 43, 3479–3491 RSC.
  113. E. Tomas-Mendivil, F. J. Suarez, J. Diez and V. Cadierno, Chem. Commun., 2014, 50, 9661–9664 RSC.
  114. E. Tomas-Mendivil, J. Francos, R. Gonzalez-Fernandez, P. J. Gonzalez-Liste, J. Borge and V. Cadierno, Dalton Trans., 2016, 45, 13590–13603 RSC.
  115. D. Yakhvarov, M. Caporali, L. Gonsalvi, S. Latypov, V. Mirabello, I. Rizvanov, O. Sinyashin, P. Stoppioni and M. Peruzzini, Angew. Chem., Int. Ed., 2011, 50, 5370–5373 CrossRef CAS.
  116. R. González-Fernández, P. Crochet, V. Cadierno, M. I. Menéndez and R. López, Chem. – Eur. J., 2017, 23, 15210–15221 CrossRef PubMed.
  117. R. González-Fernández, P. Crochet and V. Cadierno, ChemistrySelect, 2018, 3, 4324–4329 CrossRef.
  118. M. A. Esteruelas, A. M. López, J. I. Tolosa and N. Vela, Organometallics, 2000, 19, 4650–4652 CrossRef CAS.
  119. M. A. Esteruelas, A. Lledós, M. Martín, F. Maseras, R. Osés, N. Ruiz and J. Tomàs, Organometallics, 2001, 20, 5297–5309 CrossRef CAS.
  120. J. Xie, J.-S. Huang, N. Zhu, Z.-Y. Zhou and C.-M. Che, Chem. – Eur. J., 2005, 11, 2405–2416 CrossRef CAS PubMed.
  121. D. Carmona, C. Vega, N. García, F. J. Lahoz, S. Elipe, L. A. Oro, M. P. Lamata, F. Viguri and R. Borao, Organometallics, 2006, 25, 1592–1606 CrossRef CAS.
  122. U. Schubert, R. Werner, L. Zinner and H. Werner, J. Organomet. Chem., 1983, 253, 363–374 CrossRef CAS.
  123. M. Matsumoto and M. Tamura, J. Mol. Catal., 1983, 19, 365–376 CrossRef CAS.
  124. X. Jiang, M. van den Berg, A. J. Minnaard, B. L. Feringa and J. G. de Vries, Tetrahedron: Asymmetry, 2004, 15, 2223–2229 CrossRef CAS.
  125. X.-B. Wang, M. Goto and L.-B. Han, Chem. – Eur. J., 2014, 20, 3631–3635 CrossRef CAS PubMed.
  126. A. Christiansen, D. Selent, A. Spannenberg, W. Baumann, R. Franke and A. Börner, Organometallics, 2010, 29, 3139–3145 CrossRef CAS.
  127. J. A. S. Duncan, D. Hedden, D. M. Roundhill, T. A. Stephenson and M. D. Walkinshaw, Angew. Chem., Int. Ed. Engl., 1982, 21, 452–453 CrossRef.
  128. J. A. S. Duncan, T. A. Stephenson, M. D. Walkinshaw, D. Hedden and D. M. Roundhill, J. Chem. Soc., Dalton Trans., 1984, 801–807 RSC.
  129. B. Patel, S. J. A. Pope and G. Reid, Polyhedron, 1998, 17, 2345–2351 CrossRef CAS.
  130. P. M. Castro, H. Gulyas, J. Benet-Buchholz, C. Bo, Z. Freixa and P. W. N. M. van Leeuwen, Catal. Sci. Technol., 2011, 1, 401–407 RSC.
  131. V. S. Nacianceno, L. Ibarlucea, C. Mendicute-Fierro, A. Rodríguez-Diéguez, J. M. Seco, I. Zumeta, C. Ubide and M. A. Garralda, Organometallics, 2014, 33, 6044–6052 CrossRef CAS.
  132. V. San Nacianceno, L. Ibarlucea, C. Mendicute-Fierro, A. Rodríguez-Diéguez, J. M. Seco, A. J. Mota and M. A. Garralda, Inorg. Chem., 2018, 57, 5307–5319 CrossRef CAS PubMed.
  133. I. Cano, L. M. Martínez-Prieto, B. Chaudret and P. W. N. M. van Leeuwen, Chem. – Eur. J., 2017, 23, 1444–1450 CrossRef CAS PubMed.
  134. I. Cano, L. M. Martinez-Prieto, L. Vendier and P. W. N. M. van Leeuwen, Catal. Sci. Technol., 2018, 8, 221–228 RSC.
  135. M. Liniger, B. Gschwend, M. Neuburger, S. Schaffner and A. Pfaltz, Organometallics, 2010, 29, 5953–5958 CrossRef CAS.
  136. E. Salomo, P. Rojo, P. Hernández-Lladó, A. Riera and X. Verdaguer, J. Org. Chem., 2018, 83, 4618–4627 CrossRef CAS PubMed.
  137. E. Salomó, A. Gallen, G. Sciortino, G. Ujaque, A. Grabulosa, A. Lledós, A. Riera and X. Verdaguer, J. Am. Chem. Soc., 2018, 140, 16967–16970 CrossRef PubMed.
  138. H. Werner and T. N. Khac, Angew. Chem., Int. Ed. Engl., 1977, 16, 324–325 CrossRef.
  139. W. Kläui, W. Eberspach and R. Schwarz, J. Organomet. Chem., 1983, 252, 347–357 CrossRef.
  140. B. Walther, H. Hartung, M. Maschmeier, U. Baumeister and B. Messbauer, Z. Anorg. Allg. Chem., 1988, 566, 121–130 CrossRef CAS.
  141. L.-B. Han, C. Zhang, H. Yazawa and S. Shimada, J. Am. Chem. Soc., 2004, 126, 5080–5081 CrossRef CAS PubMed.
  142. L.-B. Han, Y. Ono and H. Yazawa, Org. Lett., 2005, 7, 2909–2911 CrossRef CAS PubMed.
  143. L. Ackermann and A. Althammer, Chem. Unserer Zeit, 2009, 43, 74–83 CrossRef CAS.
  144. Z. Jin, Y.-J. Li, Y.-Q. Ma, L.-L. Qiu and J.-X. Fang, Chem. – Eur. J., 2012, 18, 446–450 CrossRef CAS PubMed.
  145. B. Bogdanovic, B. Henc, B. Meister, H. Pauling and G. Wilke, Angew. Chem., Int. Ed. Engl., 1972, 11, 1023–1024 CrossRef CAS.
  146. B. Bogdanovic, B. Henc, A. Löser, B. Meister, H. Pauling and G. Wilke, Angew. Chem., Int. Ed. Engl., 1973, 12, 954–964 CrossRef.
  147. G. Wilke, Angew. Chem., Int. Ed. Engl., 1988, 27, 185–206 CrossRef.
  148. F. Speiser, P. Braunstein and L. Saussine, Acc. Chem. Res., 2005, 38, 784–793 CrossRef CAS PubMed.
  149. P. Boulens, E. Pellier, E. Jeanneau, J. N. H. Reek, H. Olivier-Bourbigou and P.-A. R. Breuil, Organometallics, 2015, 34, 1139–1142 CrossRef CAS.
  150. R. Lhermet, E. Moser, E. Jeanneau, H. Olivier-Bourbigou and P.-A. R. Breuil, Chem. – Eur. J., 2017, 23, 7433–7437 CrossRef CAS PubMed.
  151. K. R. Dixon and A. D. Rattray, Can. J. Chem., 1971, 49, 3997–4004 CrossRef CAS.
  152. D. V. Naik, G. J. Palenik, S. Jacobson and A. J. Carty, J. Am. Chem. Soc., 1974, 96, 2286–2288 CrossRef CAS.
  153. E. H. Wong and F. C. Bradley, Inorg. Chem., 1981, 20, 2333–2335 CrossRef CAS.
  154. M. Miura, Angew. Chem., Int. Ed., 2004, 43, 2201–2203 CrossRef CAS PubMed.
  155. G. Y. Li, J. Org. Chem., 2002, 67, 3643–3650 CrossRef CAS PubMed.
  156. G. Y. Li, G. Zheng and A. F. Noonan, J. Org. Chem., 2001, 66, 8677–8681 CrossRef CAS PubMed.
  157. C. Wolf and R. Lerebours, J. Org. Chem., 2003, 68, 7077–7084 CrossRef CAS PubMed.
  158. D. R. Evans, M. Huang, J. C. Fettinger and T. L. Williams, Inorg. Chem., 2002, 41, 5986–6000 CrossRef CAS PubMed.
  159. I. Pryjomska, H. Bartosz-Bechowski, Z. Ciunik, A. M. Trzeciak and J. J. Ziolkowski, Dalton Trans., 2006, 213–220 RSC.
  160. T. Ghaffar, A. Kieszkiewicz, S. C. Nyburg and A. W. Parkins, Acta Crystallogr., Sect. C: Cryst. Struct. Commun., 1994, 50, 697–700 CrossRef.
  161. T. Gebauer, G. Frenzen and K. Dehnicke, Z. Kristallogr., 1995, 210, 539 CAS.
  162. A. Gniewek, I. Pryjomska-Ray, A. M. Trzeciak, J. J. Ziolkowski and T. Lis, Acta Crystallogr., Sect. C: Cryst. Struct. Commun., 2006, 62, m491–m494 CrossRef PubMed.
  163. C. Wolf and K. Ekoue-Kovi, Eur. J. Inorg. Chem., 2006, 1917–1925 CrossRef CAS.
  164. B. Kurscheid, B. Neumann, H.-G. Stammler and B. Hoge, Chem. – Eur. J., 2011, 17, 14935–14941 CrossRef CAS PubMed.
  165. B. Kurscheid, L. Belkoura and B. Hoge, Organometallics, 2012, 31, 1329–1334 CrossRef CAS.
  166. D. Gatineau, D. Moraleda, J.-V. Naubron, T. Bürgi, L. Giordano and G. Buono, Tetrahedron: Asymmetry, 2009, 20, 1912–1917 CrossRef CAS.
  167. J. Bigeault, L. Giordano and G. Buono, Angew. Chem., Int. Ed., 2005, 44, 4753–4757 CrossRef CAS PubMed.
  168. J. Bigeault, I. de Riggi, Y. Gimbert, L. Giordano and G. Buono, Synlett, 2008, 1071–1075 CAS.
  169. H. Clavier and G. Buono, Chem. Rec., 2017, 17, 399–414 CrossRef CAS PubMed.
  170. L. Ackermann, H. K. Potukuchi, A. R. Kapdi and C. Schulzke, Chem. – Eur. J., 2010, 16, 3300–3303 CrossRef CAS PubMed.
  171. A. Vasseur, R. Membrat, D. Palpacelli, M. Giorgi, D. Nuel, L. Giordano and A. Martinez, Chem. Commun., 2018, 54, 10132–10135 RSC.
  172. D.-F. Hu, C.-M. Weng and F.-E. Hong, Organometallics, 2011, 30, 1139–1147 CrossRef CAS.
  173. Y.-C. Chang, C.-H. Chang, L.-W. Wang, Y.-H. Liang, D.-F. Hu, C.-M. Weng, K.-C. Mao and F.-E. Hong, Polyhedron, 2015, 100, 382–391 CrossRef CAS.
  174. Y.-Y. Chang and F.-E. Hong, Tetrahedron, 2013, 69, 2327–2335 CrossRef CAS.
  175. Y.-C. Chang, W.-C. Chang, C.-Y. Hu and F.-E. Hong, Organometallics, 2014, 33, 3523–3534 CrossRef CAS.
  176. W. B. Beaulieu, T. B. Rauchfuss and D. M. Roundhill, Inorg. Chem., 1975, 14, 1732–1734 CrossRef CAS.
  177. P. Bergamini, V. Bertolasi, M. Cattabriga, V. Ferretti, U. Loprieno, N. Mantovani and L. Marvelli, Eur. J. Inorg. Chem., 2003, 918–925 CrossRef CAS.
  178. T. Ghaffar and A. W. Parkins, Tetrahedron Lett., 1995, 36, 8657–8660 CrossRef CAS.
  179. T. J. Ahmed, B. R. Fox, S. M. M. Knapp, R. B. Yelle, J. J. Juliette and D. R. Tyler, Inorg. Chem., 2009, 48, 7828–7837 CrossRef CAS PubMed.
  180. H. Gulyas, I. Rivilla, S. Curreli, Z. Freixa and P. W. N. M. van Leeuwen, Catal. Sci. Technol., 2015, 5, 3822–3828 RSC.
  181. J. Bigeault, L. Giordano, I. de Riggi, Y. Gimbert and G. Buono, Org. Lett., 2007, 9, 3567–3570 CrossRef CAS PubMed.
  182. T. Achard, L. Giordano, A. Tenaglia, Y. Gimbert and G. Buono, Organometallics, 2010, 29, 3936–3950 CrossRef CAS.
  183. R. Membrat, A. Vasseur, A. Martinez, L. Giordano and D. Nuel, Eur. J. Org. Chem., 2018, 5427–5434 CrossRef CAS.
  184. T.-W. Chang, P.-Y. Ho, K.-C. Mao and F.-E. Hong, Dalton Trans., 2015, 44, 17129–17142 RSC.
  185. H. Schmidbaur and A. A. M. Aly, Angew. Chem., Int. Ed. Engl., 1980, 19, 71–72 CrossRef.
  186. C. Hollatz, A. Schier and H. Schmidbaur, J. Am. Chem. Soc., 1997, 119, 8115–8116 CrossRef CAS.
  187. C. Hollatz, A. Schier, J. Riede and H. Schmidbaur, J. Chem. Soc., Dalton Trans., 1999, 111–114 RSC.
  188. C. Hollatz, A. Schier and H. Schmidbaur, Inorg. Chim. Acta, 2000, 300–302, 191–199 CrossRef CAS.
  189. I. Cano, A. M. Chapman, A. Urakawa and P. W. N. M. van Leeuwen, J. Am. Chem. Soc., 2014, 136, 2520–2528 CrossRef CAS PubMed.
  190. I. Cano, M. A. Huertos, A. M. Chapman, G. Buntkowsky, T. Gutmann, P. B. Groszewicz and P. W. N. M. van Leeuwen, J. Am. Chem. Soc., 2015, 137, 7718–7727 CrossRef CAS PubMed.
  191. F. Schröder, C. Tugny, E. Salanouve, H. Clavier, L. Giordano, D. Moraleda, Y. Gimbert, V. Mouriès-Mansuy, J.-P. Goddard and L. Fensterbank, Organometallics, 2014, 33, 4051–4056 CrossRef.
  192. R. Uson, A. Laguna and M. Laguna, Inorg. Synth., 1989, 26, 85–86 CAS.
  193. E. Tomas-Mendivil, L. Menendez-Rodriguez, J. Francos, P. Crochet and V. Cadierno, RSC Adv., 2014, 4, 63466–63474 RSC.
  194. P. Crochet and V. Cadierno, Chem. Commun., 2015, 51, 2495–2505 RSC.
  195. R. García-Álvarez, J. Francos, E. Tomás-Mendivil, P. Crochet and V. Cadierno, J. Organomet. Chem., 2014, 771, 93–104 CrossRef.
  196. B. Lastra-Barreira, J. Francos, P. Crochet and V. Cadierno, Organometallics, 2018, 37, 3465–3474 CrossRef CAS.
  197. X. Tan, W. Zeng, X. Zhang, L. W. Chung and X. Zhang, Chem. Commun., 2018, 54, 535–538 RSC.
  198. E. Rafter, T. Gutmann, F. Low, G. Buntkowsky, K. Philippot, B. Chaudret and P. W. N. M. van Leeuwen, Catal. Sci. Technol., 2013, 3, 595–599 RSC.
  199. N. Almora-Barrios, I. Cano, P. W. N. M. van Leeuwen and N. López, ACS Catal., 2017, 7, 3949–3954 CrossRef CAS.
  200. T. J. Ahmed, S. M. M. Knapp and D. R. Tyler, Coord. Chem. Rev., 2011, 255, 949–974 CrossRef CAS.
  201. R. García-Álvarez, P. Crochet and V. Cadierno, Green Chem., 2013, 15, 46–66 RSC.
  202. T. Ghaffar and A. W. Parkins, J. Mol. Catal. A: Chem., 2000, 160, 249–261 CrossRef CAS.
  203. S. M. M. Knapp, T. J. Sherbow, T. J. Ahmed, I. Thiel, L. N. Zakharov, J. J. Juliette and D. R. Tyler, J. Inorg. Organomet. Polym. Mater., 2014, 24, 145–156 CrossRef CAS.
  204. M. North, A. W. Parkins and A. N. Shariff, Tetrahedron Lett., 2004, 45, 7625–7627 CrossRef CAS.
  205. V. Cadierno, Appl. Sci., 2015, 5, 380–401 CrossRef.
  206. W. F. Yates and R. L. Heider, J. Am. Chem. Soc., 1952, 74, 4153–4155 CrossRef CAS.
  207. S. M. M. Knapp, T. J. Sherbow, J. J. Juliette and D. R. Tyler, Organometallics, 2012, 31, 2941–2944 CrossRef CAS.
  208. R. González-Fernández, P. Crochet and V. Cadierno, Org. Lett., 2016, 18, 6164–6167 CrossRef PubMed.
  209. E. Liardo, R. González-Fernández, N. Ríos-Lombardía, F. Morís, J. García-Álvarez, V. Cadierno, P. Crochet, F. Rebolledo and J. González-Sabín, ChemCatChem, 2018, 10, 4676–4682 CrossRef CAS.
  210. P. W. N. M. Van Leeuwen, C. F. Roobeek, J. H. G. Frijns and A. G. Orpen, Organometallics, 1990, 9, 1211–1222 CrossRef CAS.
  211. P. W. N. M. van Leeuwen and C. F. Roobeek, New J. Chem., 1990, 14, 487–493 CAS.
  212. A. Christiansen, C. Li, M. Garland, D. Selent, R. Ludwig, R. Franke and A. Börner, ChemCatChem, 2010, 2, 1278–1285 CrossRef CAS.
  213. B. Zhang, H. Jiao, D. Michalik, S. Kloß, L. M. Deter, D. Selent, A. Spannenberg, R. Franke and A. Börner, ACS Catal., 2016, 6, 7554–7565 CrossRef CAS.
  214. W. Yang, Y. Wang and J. R. Corte, Org. Lett., 2003, 5, 3131–3134 CrossRef CAS PubMed.
  215. R. R. Poondra, P. M. Fischer and N. J. Turner, J. Org. Chem., 2004, 69, 6920–6922 CrossRef CAS PubMed.
  216. S. P. Khanapure and D. S. Garvey, Tetrahedron Lett., 2004, 45, 5283–5286 CrossRef CAS.
  217. K. L. Billingsley and S. L. Buchwald, Angew. Chem., Int. Ed., 2008, 47, 4695–4698 CrossRef CAS PubMed.
  218. D. X. Yang, S. L. Colletti, K. Wu, M. Song, G. Y. Li and H. C. Shen, Org. Lett., 2009, 11, 381–384 CrossRef CAS PubMed.
  219. F. Hu, B. N. Kumpati and X. Lei, Tetrahedron Lett., 2014, 55, 7215–7218 CrossRef CAS.
  220. A. Sauza, J. A. Morales-Serna, M. García-Molina, R. Gaviño and J. Cárdenas, Synthesis, 2012, 44, 272–282 CAS.
  221. J. A. Morales-Serna, A. Zúñiga-Martínez, M. Salmón, R. Gaviño and J. Cárdenas, Synthesis, 2012, 44, 446–452 CrossRef CAS.
  222. T. M. Shaikh and F.-E. Hong, Beilstein J. Org. Chem., 2013, 9, 1578–1588 CrossRef PubMed.
  223. C. Wolf and R. Lerebours, J. Org. Chem., 2003, 68, 7551–7554 CrossRef CAS PubMed.
  224. C. Wolf and R. Lerebours, Org. Biomol. Chem., 2004, 2, 2161–2164 RSC.
  225. R. E. Islas, J. Cárdenas, R. Gaviño, E. García-Ríos, L. Lomas-Romero and J. A. Morales-Serna, RSC Adv., 2017, 7, 9780–9789 RSC.
  226. C. Wolf, R. Lerebours and E. H. Tanzini, Synthesis, 2003, 2069–2073 CrossRef CAS.
  227. C. Wolf and R. Lerebours, Org. Lett., 2004, 6, 1147–1150 CrossRef CAS PubMed.
  228. L. Ackermann, R. Born, J. H. Spatz and D. Meyer, Angew. Chem., Int. Ed., 2005, 44, 7216–7219 CrossRef CAS PubMed.
  229. C. Wolf and H. Xu, J. Org. Chem., 2008, 73, 162–167 CrossRef CAS PubMed.
  230. D. Ghorai, J. Loup, G. Zanoni and L. Ackermann, Synlett, 2019, 30, 429–432 CrossRef CAS.
  231. H. Xu, K. Ekoue-Kovi and C. Wolf, J. Org. Chem., 2008, 73, 7638–7650 CrossRef CAS PubMed.
  232. R. Lerebours and C. Wolf, J. Am. Chem. Soc., 2006, 128, 13052–13053 CrossRef CAS PubMed.
  233. Z. Zhang, Z. Hu, Z. Yu, P. Lei, H. Chi, Y. Wang and R. He, Tetrahedron Lett., 2007, 48, 2415–2419 CrossRef CAS.
  234. R. Lerebours and C. Wolf, Org. Lett., 2007, 9, 2737–2740 CrossRef CAS PubMed.
  235. K. Ekoue-Kovi, H. Xu and C. Wolf, Tetrahedron Lett., 2008, 49, 5773–5776 CrossRef CAS.
  236. L. Ackermann, R. Vicente and N. Hofmann, Org. Lett., 2009, 11, 4274–4276 CrossRef CAS PubMed.
  237. R. B. Bedford, S. L. Hazelwood, M. E. Limmert, J. M. Brown, S. Ramdeehul, A. R. Cowley, S. J. Coles and M. B. Hursthouse, Organometallics, 2003, 22, 1364–1371 CrossRef CAS.
  238. K. Godula and D. Sames, Science, 2006, 312, 67–72 CrossRef CAS PubMed.
  239. R. G. Bergman, Nature, 2007, 446, 391–393 CrossRef CAS PubMed.
  240. B. G. Hashiguchi, S. M. Bischof, M. M. Konnick and R. A. Periana, Acc. Chem. Res., 2012, 45, 885–898 CrossRef CAS PubMed.
  241. T. Gensch, M. N. Hopkinson, F. Glorius and J. Wencel-Delord, Chem. Soc. Rev., 2016, 45, 2900–2936 RSC.
  242. J. F. Hartwig and M. A. Larsen, ACS Cent. Sci., 2016, 2, 281–292 CrossRef CAS PubMed.
  243. M. M. Díaz-Requejo and P. J. Pérez, Chem. Rev., 2008, 108, 3379–3394 CrossRef PubMed.
  244. L. Ackermann, Chem. Rev., 2011, 111, 1315–1345 CrossRef CAS PubMed.
  245. P. B. Arockiam, C. Bruneau and P. H. Dixneuf, Chem. Rev., 2012, 112, 5879–5918 CrossRef CAS PubMed.
  246. L. Ackermann, Org. Process Res. Dev., 2015, 19, 260–269 CrossRef CAS.
  247. L. Ackermann, Synlett, 2007, 507–526 CrossRef CAS.
  248. L. Ackermann, Isr. J. Chem., 2010, 50, 652–663 CrossRef CAS.
  249. L. Ackermann and A. V. Lygin, Org. Lett., 2011, 13, 3332–3335 CrossRef CAS PubMed.
  250. L. Ackermann, R. Vicente and A. Althammer, Org. Lett., 2008, 10, 2299–2302 CrossRef CAS PubMed.
  251. L. Ackermann, E. Diers and A. Manvar, Org. Lett., 2012, 14, 1154–1157 CrossRef CAS PubMed.
  252. L. Ackermann, A. Althammer and R. Born, Angew. Chem., Int. Ed., 2006, 45, 2619–2622 CrossRef CAS PubMed.
  253. A. Tenaglia, L. Giordano and G. Buono, Org. Lett., 2006, 8, 4315–4318 CrossRef CAS PubMed.
  254. A. Tenaglia, K. Le Jeune, L. Giordano and G. Buono, Org. Lett., 2011, 13, 636–639 CrossRef CAS PubMed.
  255. P. Nava, H. Clavier, Y. Gimbert, L. Giordano, G. Buono and S. Humbel, ChemCatChem, 2015, 7, 3848–3854 CrossRef CAS.
  256. K. Le Jeune, S. Chevallier-Michaud, D. Gatineau, L. Giordano, A. Tenaglia and H. Clavier, J. Org. Chem., 2015, 80, 8821–8829 CrossRef CAS PubMed.
  257. H. Clavier, A. Lepronier, N. Bengobesse-Mintsa, D. Gatineau, H. Pellissier, L. Giordano, A. Tenaglia and G. Buono, Adv. Synth. Catal., 2013, 355, 403–408 CAS.
  258. R. Thota, D. Lesage, Y. Gimbert, L. Giordano, S. Humbel, A. Milet, G. Buono and J. Tabet, Organometallics, 2009, 28, 2735–2743 CrossRef CAS.
  259. A. Lepronier, T. Achard, L. Giordano, A. Tenaglia, G. Buono and H. Clavier, Adv. Synth. Catal., 2016, 358, 631–642 CrossRef CAS.
  260. M. Karanik, D. Lesage, Y. Gimbert, P. Nava, S. Humbel, L. Giordano, G. Buono and J.-C. Tabet, Organometallics, 2011, 30, 4814–4821 CrossRef CAS.
  261. T. Achard, A. Lepronier, Y. Gimbert, H. Clavier, L. Giordano, A. Tenaglia and G. Buono, Angew. Chem., Int. Ed., 2011, 50, 3552–3556 CrossRef CAS PubMed.
  262. K. Xu, N. Thieme and B. Breit, Angew. Chem., Int. Ed., 2014, 53, 7268–7271 CrossRef CAS PubMed.
  263. A. M. Haydl, K. Xu and B. Breit, Angew. Chem., Int. Ed., 2015, 54, 7149–7153 CrossRef CAS PubMed.
  264. K. Xu, W. Raimondi, T. Bury and B. Breit, Chem. Commun., 2015, 51, 10861–10863 RSC.
  265. A. M. Haydl, L. J. Hilpert and B. Breit, Chem. – Eur. J., 2016, 22, 6547–6551 CrossRef CAS PubMed.
  266. J. F. Buergler and A. Togni, Chem. Commun., 2011, 47, 1896–1898 RSC.
  267. J. F. Buergler, K. Niedermann and A. Togni, Chem. – Eur. J., 2012, 18, 632–640 CrossRef CAS PubMed.
  268. R. Schwenk and A. Togni, Dalton Trans., 2015, 44, 19566–19575 RSC.
  269. K. Dong, Z. Wang and K. Ding, J. Am. Chem. Soc., 2012, 134, 12474–12477 CrossRef CAS PubMed.
  270. Y. Li, K. Dong, Z. Wang and K. Ding, Angew. Chem., Int. Ed., 2013, 52, 6748–6752 CrossRef CAS PubMed.
  271. K. Dong, Y. Li, Z. Wang and K. Ding, Angew. Chem., Int. Ed., 2013, 52, 14191–14195 CrossRef CAS PubMed.
  272. Y. Li, Z. Wang and K. Ding, Chem. – Eur. J., 2015, 21, 16387–16390 CrossRef CAS PubMed.
  273. Y. Fan, M. Cong and L. Peng, Chem. – Eur. J., 2014, 20, 2698–2702 CrossRef CAS PubMed.
  274. X. Yin, C. Chen, X. Li, X.-Q. Dong and X. Zhang, Org. Lett., 2017, 19, 4375–4378 CrossRef CAS PubMed.
  275. H. Wang, B. Wang and B. Li, J. Org. Chem., 2017, 82, 9560–9569 CrossRef CAS PubMed.
  276. I. Cano, M. J. L. Tschan, L. M. Martinez-Prieto, K. Philippot, B. Chaudret and P. W. N. M. Van Leeuwen, Catal. Sci. Technol., 2016, 6, 3758–3766 RSC.
  277. X. Jiang, A. J. Minnaard, B. L. Feringa and J. G. de Vries, J. Org. Chem., 2004, 69, 2327–2331 CrossRef CAS PubMed.
  278. T. Nemoto, T. Masuda, Y. Akimoto, T. Fukuyama and Y. Hamada, Org. Lett., 2005, 7, 4447–4450 CrossRef CAS PubMed.
  279. T. Nemoto, T. Sakamoto, T. Matsumoto and Y. Hamada, Tetrahedron Lett., 2006, 47, 8737–8740 CrossRef CAS.
  280. T. Nemoto, T. Fukuyama, E. Yamamoto, S. Tamura, T. Fukuda, T. Matsumoto, Y. Akimoto and Y. Hamada, Org. Lett., 2007, 9, 927–930 CrossRef CAS PubMed.
  281. T. Nemoto, T. Sakamoto, T. Fukuyama and Y. Hamada, Tetrahedron Lett., 2007, 48, 4977–4981 CrossRef CAS.
  282. L. Jin, T. Nemoto, H. Nakamura and Y. Hamada, Tetrahedron: Asymmetry, 2008, 19, 1106–1113 CrossRef CAS.
  283. T. Nemoto, M. Kanematsu, S. Tamura and Y. Hamada, Adv. Synth. Catal., 2009, 351, 1773–1778 CrossRef CAS.
  284. T. Nemoto, E. Yamamoto, R. Franzén, T. Fukuyama, R. Wu, T. Fukamachi, H. Kobayashi and Y. Hamada, Org. Lett., 2010, 12, 872–875 CrossRef CAS PubMed.
  285. K. Kakugawa, T. Nemoto, Y. Kohno and Y. Hamada, Synthesis, 2011, 2540–2548 CAS.
  286. T. Nemoto, M. Yamaguchi, K. Kakugawa, S. Harada and Y. Hamada, Adv. Synth. Catal., 2015, 357, 2547–2555 CrossRef CAS.
  287. P. A. Donets and N. Cramer, J. Am. Chem. Soc., 2013, 135, 11772–11775 CrossRef CAS PubMed.

Footnote

Dedicated to Prof. Guillermo Muller on the occasion of his retirement.

This journal is © The Royal Society of Chemistry 2019