Substitution of allylic acetates with sodium para-toluenesulfinate in aqueous media using allylpalladium chloride dimer and a water-soluble ligand as the catalytic system; electrospray ionisation mass spectrometry analysis

Abstract

The allylic substitution of allylic acetates by sodium para-toluenesulfinate in aqueous media was catalyzed by [(η³-allyl)PdCl]₂ associated with [(HOCH₂CH₂NHCOCH₂)₂NCH₂]₂. High yields could be obtained but the recycling of the catalytic system proved to be weakly effective. ESI-MS analysis has led to the suggestion of a possible catalytic cycle involving a Pd^IV intermediate.

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1. Introduction

Nowadays, chemistry that takes place in water receives considerable attention for environmental, economic and safety reasons, and also for showing unique reactivities and selectivities that are not seen in organic solvents.¹ In recent years, we have been involved in this research topic.^2–5 In particular, we have disclosed the water-promoted allylic substitution of (E)-1-acetoxy-1,3-diphenylpropene (1) by various nucleophiles at 50 °C in the absence of palladium complexes³ (eqn. (1)) and, with acetylacetonate anion as the nucleophilic species, the unusual role of PdCl₂(MeCN)₂/L_H in the formation of PhCH(CH(COMe)₂)CH=CHPh⁴ (eqn. (2) (L_H = [(HOCH₂CH₂NHCOCH₂)₂NCH₂]₂). The allylic substitution of (E)-3-phenyl-1-para-tolylallyl acetate by C-, O-, S- and N-nucleophiles also occurred effectively in the absence of a metal catalyst³ but the reactivity of (E)-4-phenylbut-3-en-2-yl acetate (2) was much lower.⁶ The results obtained from 2 and a range of other allylic acetates are described here, with an electrospray ionisation mass spectrometry (ESI-MS) investigation suggesting mechanistic possibilities.

2. Results and discussion

Treating 2 with 1.1 equiv. of sodium para-toluenesulfinate (3) in a 1 : 1 mixture of H₂O and DMF at 100 °C for 24 h led to a 55% conversion of 2, with the appearance of 4-phenylbut-3-en-2-ol (4) (33%), and adducts 5 and 6 (20%, 5/6 ratio = 1 : 1) (eqn. (3)). With 2 equiv. of 3, complete conversion of 2 took place and the yield of the adducts increased to 41%. Decreasing the reaction temperature afforded lower conversions, as exemplified in Fig. 1; no reaction occurring below 60 °C. At 100 °C, switching from 3 to morpholine or CH₂(COMe)₂/K₂CO₃ as the nucleophilic species led only to the formation of 4.

Fig. 1 Influence of reaction temperature on the conversion of 2 in the absence of a Pd catalyst. Experimental conditions: 2 (0.8 mmol), 3 (2 equiv.), H₂O (1 mL), DMF (1 mL), 3 h.

The above low selectivities and reactivities led us to examine the influence of a Pd catalyst using our previously reported aqueous conditions.^3,4 In contrast to the results produced with 1 as the substrate, the nucleophilic addition of 3 to 2 did not occur at 50 °C using PdCl₂(MeCN)₂/L_H as the catalyst and H₂O/MeOH as the solvent. Gratifyingly though, a mixture of [(η³-allyl)PdCl]₂ (7) and L_H proved to be an efficient and selective catalytic system in H₂O/MeOH, even at 50 °C, affording a 98% yield of 5 in 3 h (Table 1, entry 1). Similar yields were obtained using DMF, MeCN and HMPA as the co-solvent, while DMSO and THF were less effective (Fig. 2). A high (98%) selectivity was also observed using only water as solvent but the conversion dropped to 68%. As above, the substitution of 2 by morpholine, or the sodium salt of acetylacetone or dimethylmalonate did not occur under these Pd-catalyzed conditions.

Table 1 Substitution of allylic acetates with 3 in H₂O/MeOH in the presence of [(η³-allyl)PdCl]₂/L_H. Experimental conditions: Substrate (1.0 mmol), 3 (2 equiv.), 7 (0.01 equiv.), L_H (0.02 equiv.), H₂O (1 mL), MeOH (1 mL), 50 °C

Entry	Substrate	Time/h	Product	Yield (%)
1		3		98
2		3		99
3		3		99
4		24		99
5		48		22
6		24		59
7		22		55

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Fig. 2 Reaction of 2 with 3 using [(η³-allyl)PdCl]₂/L_H as the catalyst in various aqueous mixtures. Experimental conditions: 2 (1 mmol), 3 (2 equiv.), 7 (0.01 equiv.), L_H (0.02 equiv.), H₂O (1 mL), co-solvent (1 mL), 50 °C, 3 h.

Following these observations, a range of allylic acetates were subjected to a 7/L_H-catalyzed reaction with 3 in H₂O/MeOH. As shown by the results collected in Table 1, the substitution occurred in all cases on the less crowded carbon. The fact that isomeric allylic acetates 11 and 13 afford the same product (12; Table 1, entries 4 and 5) indicates that 11 and 13 operate through the same allylic intermediate. Substitution of the central atom of the allylic moiety strongly decreased the reaction rate (Table 1, entries 1 and 4). According to the high difference of reactivity between 11 and 13 (Table 1, entries 4 and 5), it also appears that the facility to form the reactive intermediate greatly depends on the structure of the substrate.

A point of interest in metal-catalyzed reactions in aqueous media is the possibility of recycling. To clarify this issue, a reaction carried out as indicated in Table 1, entry 1, was extracted with CH₂Cl₂. After addition of MeOH, 2 and 3 to the aqueous phase, followed by heating at 50 °C, 5 h was required for the complete consumption of 2, providing 5 in 98% yield. A second recycling was much more sluggish, affording no more than 40% conversion in 24 h.

To obtain information about the mechanism of these reactions, an ESI-MS analysis was undertaken.⁷ Each component of the catalytic system was first analyzed. Because Pd and Cl display 6 and 2 isotopes, respectively, the ions containing these isotopes should be mass-detected as clusters of isotopomeric ions whose center depends on the most abundant isotope (106 for Pd and 35 for Cl).

The ESI(+)-MS of 7 in a MeOH/H₂O/THF (5 : 5 : 1) mixture showed three main peaks at m/z = 389, 461 and 513, corresponding to [(PdC₃H₅Cl)₂ + Na]⁺, [(PdC₃H₅Cl)₂ + THF + Na]⁺ and the trinuclear metal cluster [(PdC₃H₅Cl)₃ – Cl]⁺, respectively (Fig. S1, ESI†). Two cationic forms of L_H at m/z = 465 ([L_H + H]⁺) and 487 ([L_H + Na]⁺) were observed from the ESI(+)-MS of L_H in MeOH/H₂O (Fig. S2, ESI†). The ESI(+)-MS of an 1 : 2 mixture of 7 and L_H in MeOH/H₂O revealed a peak at m/z = 611 corresponding to [L_HPdC₃H₅]⁺ (Fig. S3, ESI†). The addition of allylic acetate 2 (2.0 equiv. with respect to 7) to this 7 + L_H mixture did not afford new peaks. The ESI(+)-MS spectra only evolved after the addition of sulfinate 3 (4 equiv./7); after one hour at 50 °C, besides the previous [L_HPdC₃H₅]⁺ peak, peaks at m/z = 701 and 879 were obtained. Peak m/z = 701 is attributable to [L_HPd(2) – OAc]⁺ while m/z = 879 is consistent with [L_HPd(2)SO₂p-Tol – OAc + Na]⁺ and [L_HPd(5) + Na]⁺ (Fig. 3 and Fig. 4). After 24 h, the mixture was extracted with CH₂Cl₂ and the ESI(+)-MS spectrum of the aqueous phase recorded. This spectrum showed the peak at m/z = 611 already observed, and two new clusters centered at m/z = 747 and 769, attributable to [L_HPdSO₂p-Tol – H + Na]⁺ and [L_HPdSO₂p-Tol – 2H + 2Na]⁺ (Fig. 5; Fig. S4, ESI†).

Fig. 3 ESI(+)-MS spectrum of the crude mixture obtained under the following conditions: 7, L_H (2 equiv.), 2 (2 equiv.), 3 (4 equiv.), H₂O/MeOH (1 : 1), 50 °C, 1 h.

Fig. 4 Expanded ESI(+)-MS spectra of detected species from Fig. 3 and corresponding theoretical spectra.

Fig. 5 ESI(+)-MS spectrum of the aqueous phase after extraction of the crude mixture with CH₂Cl₂. Experimental conditions: As in Fig. 3 for 24 h followed by extraction with CH₂Cl₂.

Further ESI-MS experiments were carried out using either CH₂(COMe)₂ + K₂CO₃ or NaCH(COMe)₂. In the first case (Fig. S5, ESI†), we only observed [L_HPdC₃H₅]⁺ and [L_HPdC₃H₅ – H + K]⁺ clusters. The use of NaCH(COMe)₂ led similarly to [L_HPdC₃H₅]⁺ and [L_HPdC₃H₅– H + Na]⁺ clusters but, in addition, to traces of [L_HPd(2) – OAc]⁺ (Fig. S6, ESI†). These ESI-MS analyses, which are in agreement with the reactivity of the sulfinate anion and the reluctant addition of acetylacetonate anion highlighted in the preparative experiments, have lead us to make mechanistic proposals for the allylic substitution of 2 by 3 under these particular Pd-catalyzed conditions.

Mixing 7 with L_H led to a monomeric η³- or η¹-allylpalladium complex, having L_H as a bidentate ligand, namely L_HPd(C₃H₅)Cl, revealed by the cluster [L_HPdC₃H₅]⁺. This complex is either cationic with an η³-allyl moiety, or neutral with an η¹-allyl moiety (Scheme 1). Let us now to consider two possible mechanisms, A and B, that could explain the reactivity of these species in the presence of 3 and 7.

Scheme 1

Mechanism A

Mechanism A is similar to the catalytic cycle usually described for Tsuji–Trost reactions using a mixture of 7 and a phosphine as the catalyst (Scheme 2). Pd⁰ complex A₁ arises from the nucleophilic addition of 3 to L_HPd(C₃H₅)Cl. An exchange of ligand with 2 leads to A₂ and then to cationic Pd^II complex A₃. The reaction of 3 with A₃ affords Pd⁰ complex A₄, which evolves to A₂, closing the catalytic cycle.

Scheme 2

Intermediates A₂ and A₃ are consistent with the cluster at m/z = 701, attributed to [L_HPd(2) – OAc]⁺, while A₄ is in agreement with the cluster at m/z = 879. In contrast, mechanism A is not in accordance with the ESI-MS spectrum recorded at the end of the reaction, which highlights a cluster at m/z = 611 corresponding to the complex L_HPd(C₃H₅)X (X = Cl, OAc or SO₂p-Tol). To accept mechanism A, it is a required consideration that only a slight portion of L_HPd(C₃H₅)Cl reacts with 3. L_HPd(C₃H₅)Cl would act as a reservoir of Pd^II, which is reduced to Pd⁰ to enter the catalytic cycle. However the need for an explanation remains as to why 3 would react with A₃ rather than with L_HPd(C₃H₅)Cl.

Since 2 equiv. per Pd of polydentate ligand L_H are used, according to the research groups of Shaw and Jutand,^8–11 the most likely form of the allyl moiety of a species such as L_HPd(C₃H₅)Cl is the η¹-allyl one.¹² Consequently, this species would be the neutral complex L_HPdCl(η¹-allyl) (Scheme 1). As a cationic palladium complex is more reactive towards nucleophilic addition than a neutral one,¹⁰ this could explain the reaction of 3 with the cationic complex A₃ rather than with L_HPdCl(η¹-allyl).

Mechanism B

For mechanism B, we take into consideration (i) the most likely structure of L_HPd(C₃H₅)Cl,^8–11i.e. the neutral complex L_HPdCl(η¹-allyl), (ii) the reported substitution at Pd of chloropalladium complexes with PhSO₂Na,¹³ and (iii) Kurosawa’s proposal of a reductive elimination leading to an R-allyl from Pd^IV(allyl)₂R(X)PPh₃.¹⁴

An exchange of anion between L_HPdCl(η¹-allyl) and 3 affords B₁ (Scheme 3). Such a step could be considered a transmetallation. The reaction of B₁ with 2 leads to cationic Pd^IV complex B₂.¹⁵ Kurosawa et al. have shown that reductive elimination was more efficient from an η³-allyl than from an η¹-allylpalladium complex.^17–19 Consequently, reductive elimination from B₂ produces 5 rather than allylSO₂p-Tol. Such an elimination results in B₃, which interacts with 3 to close the catalytic cycle.

Scheme 3

According to mechanism B, the real catalyst is L_HPd(SO₂p-Tol)(η¹-allyl) (B₁). B₁ is consistent with the cluster at m/z = 611, corresponding to [L_HPdC₃H₅]⁺, which has been observed in both the crude mixture and the aqueous extraction phase. Pd^IV complexes have been isolated²⁰ but the formation of a Pd^IV complex from a Pd^II complex and an allylic acetate, i.e. in Scheme 3 the formation of (η¹-allyl)(η³-allyl)Pd^IV species B₂ from 2 and (η¹-allyl)Pd^II intermediate B₁, is an unusual step. Nevertheless, Kurosawa’s team have (i) already suspected a Pd^IV intermediate from the oxidative addition of allyl acetate to an (η³-allyl)Pd^II complex¹⁴ and (ii) shown that (η¹-organo)metal^II complexes are more similar to metal⁰ than metal^II complexes.²¹ In addition, the formation of a cationic (η³-allyl)Pd^IV intermediate has recently been proposed for the reaction of a vinylpalladium chloride with an allylic alcohol in an aqueous medium.²²

The clusters at m/z = 701, 747, 769 and 879 are not directly consistent with the intermediates of mechanism B but can be connected to B₂ through the reactions depicted in Scheme 4. Indeed, the cluster at m/z = 701 corresponds to [L_HPd(PhC₃H₃Me)]⁺ while those centered at m/z = 747 and 769 occur from [L_HPdSO₂p-Tol]⁺. These species would arise from reductive eliminations (paths (a)²³ and (b)²⁴) less favored than the one depicted in Scheme 3. The cluster at m/z = 879 can be produced not only from L_HPd(5) (intermediate A₄ in Scheme 2) but also from B₂via the nucleophilic addition of acetate anion to the η¹-allyl ligand (path (c)), i.e. a reaction already documented for a cationic (η¹-allyl)palladium complex.^25–27 Steps (a), (b) and (c) would produce minute amounts of organic compounds since the C₃H₅ moiety comes from the catalyst. Furthermore, step (c), which leads to allyl acetate, would be reversible.

Scheme 4

A possible explanation for the unreactivity of morpholine, and the sodium salts of acetylacetone and dimethylmalonate, involves the influence of steric effects.^26,33 L_H is a crowded ligand and, according to mechanism B, the sulfinate group is a ligand of palladium before being linked to the allyl moiety. Consequently, the size of the nucleophilic species could play a decisive role. Mechanisms A and B remain, nevertheless, hypothetical but we have, however, to point out that the extracted aqueous phase contains L_HPdX(allyl). Such a species, namely B₁, is directly involved in mechanism B, and we have obtained 5 in 98% yield from a preparative experiment using a recycled aqueous phase.

3. Conclusion

A mixture of both [(η³-allyl)PdCl]₂ and a water soluble ligand catalyzes the allylic substitution of allylic acetates by sodium para-toluenesulfinate in a variety of aqueous media, but the system quickly looses its catalytic properties upon recycling. Substitution of the allylic moiety of the substrate has a significant influence on the efficiency of the reaction. ESI-MS analyses have led to the suggestion of a possible catalytic cycle involving a Pd^IV intermediate.

4. Experimental

3.1. General procedure for Pd-catalyzed substitution

To a mixture of [(η³-allyl)PdCl]₂ (3.6 mg, 0.01 mmol) and [(HOCH₂CH₂NHCOCH₂)₂NCH₂]₂ (9.3 mg, 0.02 mmol) in MeOH (1 mL), stirred at room temperature for 1 h, was added the substrate (1.0 mmol), sodium para-toluenesulfinate (388 mg, 2 mmol) and H₂O (1 mL). After heating at 50 °C for the time indicated in Table 1, the organic compounds were extracted with CH₂Cl₂. The organic layer was dried over MgSO₄ and concentrated to give the crude product. This product was purified by chromatography over silica gel eluted with a 10 : 90 mixture of AcOEt and petroleum ether.

3.2. ESI-MS analysis

ESI-MS analyses were recorded on a Navigator quadrupole mass spectrometer (Finnigan AQA ThermoQuest) equipped with an electrospray ion source. The instrument was operated either in the positive ESI(+) or negative ESI(–) ion mode at a probe tip voltage of 3 kV. Samples were introduced into the mass spectrometer ion source directly through a Rheodyne injector with a 20 µL sample loop. The mobile phase flow (100 µL min^–1 of 70 : 30 v/v MeOH/H₂O) was delivered by a P2000 HPLC pump (ThermoQuest) to the vaporization nozzle of the electrospray ion source (165 °C for ESI(+) and 140 °C for ESI(–)) and nitrogen was employed as both a drying and nebulizing gas. Skimmer cone voltages were varied between 10 and 100 eV. Spectra were typically an average of 15–20 scans. Theoretical isotope patterns were calculated using the Isoform program and were used to aid assignment. Association of the substrate with ions was common. [M + H]⁺ and [M + Na]⁺ were often observed, the latter appeared from reactions with traces of cations present, even in HPLC-grade solvent.⁷

Acknowledgements

We are indebted to “Région Champagne-Ardenne” for a PhD studentship to C. C. Financial support from CNRS, PAI (Picasso), the MEC of Spain (project CTQ2005-04968) and the CIRIT-Generalitat de Catalunya (project 2005SGR00305) is gratefully acknowledged.

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Footnote

† Electronic supplementary information (ESI) available: Observed electrospray mass spectra and calculated isotopic distribution of the detected species (Fig. S1–S5). See DOI: 10.1039/b613562e

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