Catalytic hydroboration of aldehydes and ketones with an electron-rich acyclic metallasilylene

The application of main group metal complexes in catalytic reactions is of increasing interest. Here we show that the electron-rich, acyclic metallasilylene L′(Cl)GaSiL C (L′ = HC[C(Me)NDipp]2, Dipp = 2,6-iPr2C6H3; L = PhC(NtBu)2) acts as a precatalyst in the hydroboration of aldehydes with HBPin. Mechanistic studies with iso-valeraldehyde show that silylene C first reacts with the aldehyde with [2 + 1] cycloaddition in an oxidative addition to the oxasilirane 1, followed by formation of the alkoxysilylene LSiOCH[Ga(Cl)L′]CH2CHMe2 (2), whose formation formally results from a reductive elimination reaction at the Si center. Alkoxysilylene 2 represents the active hydroboration catalyst and shows the highest catalytic activity with n-hexanal (reaction time: 40 min, yield: >99%, TOF = 150 h−1) at room temperature with a catalytic load of only 1 mol%. Furthermore, the hydroboration reaction catalysed by alkoxysilylene 2 is a living reaction with good chemoselectivity. Quantum chemical calculations not only provide mechanistic insights into the formation of alkoxysilylene 2 but also show that two completely different hydroboration mechanisms are possible.


Introduction
The addition of a boron-hydrogen bond to an unsaturated organic group, the so-called hydroboration reaction, has received increasing interest since its initial report by H. C. Brown in 1956. 1 Substantial progress has been made with the development of new boranes such as pinacolborane (HBPin), catecholborane (HBcat), 2 and Piers' borane (HB(C 6 F 5 ) 2 ), 3 respectively, and a large number of active early and late transition metal catalysts have been established since the rst reports of transition metal-catalysed hydroboration of alkynes and alkenes. 4Moreover, the concept of hidden boron catalysis in the hydroboration of alkynes and alkenes has recently been discussed. 5In marked contrast, the number of metal-free catalysts, 6 which were rst reported in 2012, 7 as well as main group metal catalysts including s-block 8 and p-block elements 9 is still limited.
The catalytic hydroboration of aldehydes oen uses transition metal catalysts, which promote the addition of pinacolborane to aryl and alkyl aldehydes, 10 but aldehyde hydroboration can be also performed without any catalyst. 11][18][19][20] Divalent tetrylenes are also of general interest in catalytic hydroboration transformations 21 due to their Lewis ambiphilic behavior, which results from the presence of an electron lonepair and a vacant p-orbital, thus allowing transition metal-like reactivity such as oxidative addition reactions.While neutral and cationic germylenes, [22][23][24][25][26][27] stannylenes, [27][28][29] and plumbylenes 30 have been successfully used in hydroboration catalysis, the catalytic activity of silylenes has been much less explored.Silylenes showed promising potential in (stoichiometric) small molecule activation reactions, 31,32 including CO activation, [33][34][35] but their catalytic activity is oen limited by the use of sterically demanding substituents Scheme 1 Silylenes applied in catalytic hydroboration reactions of carbonyl compounds.

Synthetic procedures
We rst tested the catalytic activity of gallasilylene C in the hydroboration of various aldehydes with HBPin at room temperature.C did not react with HBPin in the absence of any aldehydes according to in situ 1 H and 11 B NMR studies (Fig. S26 and S27 †), whereas amidinate-substituted silylenes were reported to undergo cooperative B-H bond activation of HBPin 43 In contrast, C was found to hydroborate iso-valeraldehyde, benzaldehyde, 4-hexylbenzaldehyde and 3-bromobenzaldehyde, which was accompanied by an immediate color change of the solution from yellow to colorless, whereas no reaction occurred with n-heptanal, 4-nitrobenzaldehyde, and 4-hydroxybenzaldehyde under identical reaction conditions.Obviously, the catalytic activity of gallasilylene C strongly depends on the electronic/steric properties of the aldehyde.
To obtain more detailed information about the initial step of the reactions and the true nature of the catalytically active species, we studied the equimolar reaction of compound C with iso-valeraldehyde at low temperature (−80 °C).The reaction proceeded by [2 + 1] cycloaddition (oxidative addition) of the aldehyde to the silylene center to give the oxasilirane 1, followed by Si-C bond cleavage and Ga-C bond formation upon warming to room temperature and formation of the alkoxysilylene LSiOCH[Ga(Cl)L 0 ]CH 2 CHMe 2 2 (Scheme 2).The conversion of oxasilirane 1, in which the Si atom adopts the formal oxidation state of +IV, into the alkoxysilylene 2 formally represents a reductive elimination reaction at the Si center.Even more remarkable is that this reaction starts at -5 °C according to in situ 1 H NMR studies.Moreover, comparable to our ndings with gallasilylene C, alkoxysilylene 2 also did not react with HBPin in the absence of any aldehydes according to in situ 1 H and 11 B NMR studies (Fig. S28 and S29 †).

Crystallographic studies
Single crystals suitable for X-ray diffraction were obtained from saturated solutions in benzene aer storage at 4 °C (1) for only 10 min and in n-hexane aer storage at 25 °C (2), respectively.1 and 2 crystallize in the triclinic space group P 1 with one molecule (accompanied by solvent) in the asymmetric unit (Fig. 2).The SiNCN units in oxasilirane 1 and alkoxysilylene 2 adopt almost Scheme 2 Reaction of silylene C with one equivalent of iso-valeraldehyde (Dipp = 2,6-i Pr 2 C 6 H 3 ).
The calculated Gibbs energies for the formation of the catalyst S2 starting from C and propanal are shown in Fig. 1.The rst step is the formation of oxasilirane S1, which must pass an activation barrier of 15.0 kcal mol −1 .The ring opening leads to the energetically more favorable silylene S2.The activation energy for this step (24.4 kcal mol −1 ) is much higher than that of the rst step, allowing the isolation of S1 (or oxasilirane 1 if iso-valeraldehyde is used).Overall, this results in an insertion of the aldehyde into the Si-Ga bond of compound C.An alternative opening of the three-membered ring in S1, in which the oxygen is bound to the gallium atom, is also feasible.The activation energy for such an opening is even lower, but the product formed is destabilised compared to S1 and S2 (Fig. S36 and S37 †).

Catalytic studies
The isolated alkoxysilylene 2 was then used in the catalytic hydroboration of aldehydes and ketones with HBPin at room temperature and 1 mol% catalyst loading (Table 1).Moderate catalytic activities were observed with benzaldehyde (TOF 3.9 h −1 ), while electron withdrawing groups in the para-position were found to have a benecial catalytic effect most likely due to an increase of the electrophilic nature of the carbonyl carbon atom of these aromatic aldehydes (TOF: 4-nitrobenzaldehyde 6.25 h −1 ; methyl-4-formylbenzoate 5.6 h −1 ).However, much higher catalytic activities were observed in the hydroboration of linear aliphatic aldehydes.The highest activities were found in the hydroboration of n-hexanal (TOF: 150 h −1 ) and n-propanal (TOF: 133 h −1 ), while the hydroboration of n-octanal is slower (TOF: 29 h −1 ).Studies with n-pentanal (TOF: 44 h −1 ) and iso-valeraldehyde (TOF: 11.1 h −1 ) show that the reaction time decreases with increasing steric demand of the aldehyde, however, the steady trend was not observed in the linear aliphatic aldehydes.In comparison, the hydroboration of ketones required longer reaction times under identical reaction conditions (TOF: 0.33-0.47h −1 ), which is consistent with the lower reactivity of ketones in catalytic hydroboration reactions.

Table 1 Aldehydes and ketones applied in catalytic hydroboration reactions with silylene 2 and HBPin
Compared to the silyliumylidene cation A, for which TOFvalues in the range of 179 to 115.8 h −1 were reported, alkoxysilylene 2 was found to catalyse the hydroboration of aldehydes in a similar range of TOF values (TOF: 3.9-150 h −1 ), but with much lower catalyst loading (1 mol%) compared to A (10 mol%). 37In addition, the alkoxysilylene 2 showed a much higher catalytic activity in the hydroboration of aldehydes compared to the amidinate-substituted silylene B (TOF: 0.2-19.8h −1 ) which was only active at high temperature (90 °C) and high catalyst loading (5 mol%). 40However, the alkoxysilylene 2 is catalytically less active compared to the heavier tetrylenes, i.e., the hydridogermylene [Ar*( i Pr 3 Si)NGeH] (TOF: 179-6000 h −1 , catalyst loading 0.05-1 mol%) and hydridostannylene Ar*( i Pr 3 -Si)NSnH (TOF: 400-13 300 h −1 , catalyst loading 0.05 mol%), respectively. 39o identify the active catalyst, the catalytic reaction of benzaldehyde, HBPin and the alkoxysilylene 2 (10 mol%) was performed at room temperature in the presence of two equivalents of TMEDA.Full conversion was observed aer 16 h according to in situ 1 H NMR spectroscopic studies (Fig. S22 †), which proved that the catalytic reaction occurred without the formation of borane BH 3 , proving that the alkoxysilylene 2 is the active catalyst in the reaction. 5To rule out a catalyst-free hydroboration reaction, 11 we also reacted n-propanal with HBPin in the absence of catalyst 2. Only 76% (TOF: 1.58 h −1 ) of the aldehyde were converted into the corresponding borate ester aer 48 h, whereas the reaction in the presence of alkoxysilylene 2 (1 mol%) was nished aer 45 min (TOF: 133 h −1 ).Since we also did not observe any induction period in the catalysed reaction, we assume that 2 is the active catalyst.
We also became interested in its chemoselectivity and reacted 2 (1 mol%, r.t.) with a mixture of equimolar amounts of acetophenone, iso-valeraldehyde and HBPin.The resulting 1 H NMR spectrum showed almost complete conversion of HBPin aer 24 h.94% conversion of the aldehyde and only 5% conversion of the ketone to their corresponding borane esters (Fig. S24 †) clearly demonstrated the good chemoselectivity of catalyst 2 for aldehydes over ketones.In addition, alkoxysilylene 2 was shown to be a living hydroboration catalyst by adding an additional equivalent of both HBPin and benzaldehyde to a reaction mixture of 2, HBPin and iso-valeraldehyde (1 mol%, r.t.) aer full conversion.The estimated signals of both products were detected in the 1 H NMR spectrum aer 24 h (Fig. S23 †), proving full conversion of both aldehydes.

Computational studiesreaction mechanism
To nd out the mechanism for the catalytic hydroboration reaction with S2 (or 2 if iso-valeraldehyde is used) we considered three different scenarios in our DFT studies.In the rst case, the inuence of the catalyst on the transition state of the uncatalysed reaction was calculated (Fig. S38 †).In the other two cases, stepwise reactions of the components with the active species S2 were studied (Fig. 3a and b).In the rst case, there is no lowering of the activation barrier.In fact, it is even higher (44.1 kcal mol −1 ) than that for the uncatalysed reaction (37.6 kcal mol; Fig. S38 †) due to the entropy loss.
In the second case, the aldehyde is rst added to the silylene center to form the oxasilirane Int-2 (Fig. 3a), which together with HBPin forms the complex Int-1, leading to the silane Int-3.The latter can then transfer the hydride to another aldehyde in the rate-determining step (26.7 kcal mol −1 ).In the corresponding seven-membered transition state TS-7, the oxygen atom of the carbonyl group is coordinated to the boron atom.The resulting intermediate Int-4 reacts to the product PinB-OPr and the catalytically active species Int-2 without any major activation barriers (Fig. 3a).
In the third case, the rst step involves the addition of HBPin to form the complex Int-6 (Fig. 3b).The hydride on the boron atom now has a higher electron density and can therefore be transferred more efficiently to the carbon atom of the carbonyl group.In the corresponding ve-membered transition state TS-10, the oxygen atom of the carbonyl group is bonded to the silicon atom.The activation barrier starting from the reactants amounts to 27.3 kcal mol −1 .In the next step, the alkoxy group is transferred to the boron atom, forming the complex Int-8.The latter decomposes into the product PinB-OPr and the catalyst S2 (Fig. 3b).
A comparison of the latter cases reveals two completely different types of reaction.While in one case the hydride is transferred from the boron atom to the carbon atom, in the other case it is transferred from the silicon atom.While in one case S2 is the catalytically active species, in the other case Int-2 is the active catalyst.However, it is interesting to note, that in both cases the hydride transfer is the rate-determining step and the activation barriers are similar (27.3 kcal mol −1 versus 26.7 kcal mol −1 ).
Considering that the reaction takes place at room temperature, the calculated values are too high.This is due to the overestimation of the calculated entropies (for more details see calculation details).Therefore, the G 70% values were used for comparison with the experimental data (Fig. S40 and S41 †).If the TOF values for the two mechanisms are estimated at room temperature, a TOF value of 0.9 h −1 is obtained for the rst mechanism (via TS-7) and a value of 217 h −1 for the second one (via TS-10).The latter agrees well with the experimentally determined TOF values (133 h −1 ), which indicates that the second mechanism is dominant.
Unfortunately, we did not observe any reaction intermediate formed in these reactions by means of temperature-dependent in situ 1 H NMR spectroscopy.Even when the active silylene 2 was reacted with an equivalent of the aldehyde, we did not observe the formation of compound Int-2.In addition, we also did not observe the formation of Int-3 or Int-7 in reactions of 2 with an equivalent of the aldehyde and HBPin in the temperature range from -70 °C to room temperature.

Materials and methods
All manipulations were performed in a puried argon atmosphere using standard Schlenk and glovebox techniques.Toluene was dried using an mBraun Solvent Purication System (SPS).Deuterated benzene was dried over activated molecular sieves (4 Å) and degassed prior to use.The anhydrous nature of the solvents was veried by Karl Fischer titration.L 0 (Cl)GaSiL C was prepared according to the literature. 42Microanalyses were performed at the Microanalysis Laboratory of the University of Duisburg-Essen.Melting points were measured using a Thermo Scientic 9300 apparatus.

Synthesis
General procedure for synthesis of L 0 Si[OCHCH 2 CHMe 2 ] Ga(Cl)L (1).A solution of iso-valeraldehyde (25.84 mg, 0.3 mmol) in 1 mL of toluene was added dropwise to a cooled (−80 °C) solution of L(Cl)GaSiL 0 (234 mg, 0.3 mmol) in 2 mL of toluene.The solution was stirred at −80 °C for 5 minutes, concentrated to 1 mL and stored at −18 °C for 12 h to give oxasilirane 1 in the form of colorless crystals.

Catalytic studies
General procedure for the catalytic hydroboration of aldehydes and ketones.HBPin (28.2 mg, 0.22 mmol, 1.1 equiv.)and the substrate (aldehyde and ketone: 0.20 mmol) were placed in a J-Young NMR tube, dissolved in 0.3 mL of C 6 D 6 and 0.2 mL of a 0.01 M solution of 2 (1.73 mg, catalytic loading: 1 mol%) was added.The solution was vortexed to form a homogeneous solution.The progress of the reaction was monitored by 1 H NMR spectroscopy and a suitable internal standard was used to determine the yield.[60] Mechanistic studies Determination of kinetics.HBPin (25.6 mg, 0.2 mmol, 1 equiv.)and iso-valeraldehyde (17.2 mg, 0.2 mmol) were added to a J-Young NMR tube and dissolved in 0.3 mL of C 6 D 6 .0.2 mL of a 0.01 M solution of 2 (1.73 mg, catalytic loading: 1 mol%) was injected via syringe and the resulting solution was shaken to form a homogeneous solution.The progress of the reaction was monitored by 1 H NMR spectroscopy at 80 °C by taking a spectrum every 30 s.
Living reaction.HBPin (28.2 mg, 0.22 mmol, 1.1 equiv.)and iso-valeraldehyde (17.2 mg, 0.2 mmol) were added to a J-Young NMR tube and dissolved in 0.3 mL of C 6 D 6 .0.2 mL of a 0.01 M solution of 2 (1.73 mg, catalytic loading: 1 mol%) were added and the solution shaken to form a homogeneous solution.The progress of the reaction was monitored by 1 H NMR spectroscopy.Aer complete conversion, a second fraction of HBPin (28.2 mg, 0.22 mmol, 1.1 equiv.)and benzaldehyde (21.2 mg, 0.2 mmol) were added.Aer 24 h, the progress of the reaction was monitored by 1 H NMR spectroscopy, which again showed complete conversion of both substrates.
Chemoselectivity studies.HBPin (12.8 mg, 0.1 mmol, 1 equiv.),iso-valeraldehyde (8.6 mg, 0.1 mmol) and acetophenone (12.0 mg, 0.1 mmol) were added to a J-Young NMR tube and dissolved in 0.4 mL of C 6 D 6 .Then 0.1 mL of a 0.01 M solution of 2 (0.87 mg, catalytic loading: 1 mol%) was added and the mixture was shaken to form a homogeneous solution.The progress of the reaction was monitored by 1 H NMR spectroscopy and a suitable internal standard was used to determine the yield.
TMEDA reaction.TMEDA (0.2 mmol, 23.2 mg), HBPin (0.1 mmol, 12.8 mg) and benzaldehyde (0.1 mmol, 10.2 mg) were added to a J-Young NMR tube and dissolved in 0.4 mL of C 6 D 6 .A solution of compound 2 (0.01 mmol, 8.68 mg) in 0.1 mL of C 6 D 6 was added and the resulting solution was then shaken to form a homogeneous solution.The expected product was obtained (conversion >99%).

Crystallography
The crystals were mounted on nylon loops in inert oil.The data of 1 were collected on a Bruker AXS D8 Venture diffractometer with Photon II detector (monochromated Cu Ka radiation, l = 1.54178Å, microfocus source) at 100(2) K, while those of 2 were collected on a Bruker AXS D8 Kappa diffractometer with APEX2 detector (monochromated Mo Ka radiation, l = 0.71073 Å) at 100(2) K.The structures were solved by Direct Methods (SHELXS-2013) 61 and anisotropically rened by full-matrix leastsquares on F 2 (SHELXL-2017). 62,63Absorption corrections were performed semi-empirically from equivalent reections on basis of multi-scans (Bruker AXS APEX3).Hydrogen atoms were rened using a riding model or rigid methyl groups.In oxasilirane 1, the ligands of the silicon atom show a correlated disorder over two sites.All corresponding bond lengths were restrained to be equal (SADI) and the phenyl ring of the amidinate was restrained to be planar (FLAT).Global RIGU and SIMU restraints were applied to the displacement parameters of the disordered atoms.Additional specic SIMU restraints or common displacement parameters (EADP) were used for atoms in close proximity.The solvent molecules were restrained to a regular hexagon (SADI, FLAT) and RIGU and SIMU restraints were applied to their displacement parameters.One of the solvent molecules is disordered over a centre of inversion.The local symmetry was ignored in the renement (negative PART).Due to the vast disorder and the consequent use of restraints, the quantitative results may be unreliable and biased and should be interpreted with caution.The solvent molecule in silylene 2 is disordered over two sites.Its bond lengths and angles were restrained to be equal (SADI) and RIGU restraints were applied to its atoms' displacement parameters.The displacement parameters of one isopropyl group suggested disorder but attempts to resolve it yielded a very low occupancy for the minor component and non-positive denites for the displacement parameter.Restraints did not succeed to overcome this, so the model was ultimately discarded.

Computational details
All calculations were performed by using the program package Gaussian 16. 64 The geometrical parameters of the stationary points were optimized by means of the density functional methods PBE0, 54 PBE 65,66 and M06-2X 67 with the empirical dispersion D3 68 and D3BJ. 55The basis sets def2-SVP 69,70 and 6-31G(d) 71,72 were employed.For all stationary points no symmetry restriction was applied.Frequency calculations were carried out at each of the structures to verify the nature of the stationary point.It turned out that all transition states have exactly one imaginary frequency, whereas all other structures have none.4][75][76] Solvent effects were taken into account by using the solvent model SMD 77 (benzene as solvent) for single point calculations.][80] A well-known problem during the calculation of the Gibbs energies is the overestimation of the calculated entropies.The entropies are always computed on the assumption of an ideal gas.This leads to a large deviation for the translation and conformational term of the entropy if the reaction takes place in solution. 81,82This is particularly dramatic for bi-and trimolecular reactions.3][84][85] In our case, we have therefore used the upper limit, i.e. 70% (instead of 100%) of the calculated entropy.The thus obtained the Gibbs energies G70% were used for comparison with experimental data.

Conclusions
Gallasilylene C serves as a precatalyst in the catalytic hydroboration of aldehydes with HBPin.The active catalyst is an alkoxysilylene as demonstrated by the reaction of C with isovaleraldehyde, which occurs with oxidative addition to give the oxasilirane 1 followed by rearrangement to alkoxysilylene 2. The hydroboration catalysis using alkoxysilylene 2 shows a living character and a good chemoselectivity in the hydroboration of aldehydes over ketones.Quantum chemical calculations provided mechanistic insights into the energetics of the formation of oxasilirane 1 and alkoxysilylene 2 and also provided two possible reaction mechanism for the catalytic hydroboration.

Fig. 1
Fig. 1 Gibbs energies for the formation of the catalyst S2 starting from C and n-propanal calculated by means of PBE0-D3BJ.The values are given in kcal mol −1 .

Fig. 3
Fig. 3 Mechanism for the hydroboration of n-propanal with Int-2 (a) and S2 (b) as catalytically active species calculated by means of PBE0-D3BJ.The values are given in kcal mol −1 .