General ppm-level Pd-catalysed asymmetric diarylalkyne hydrosilylation to access structurally diverse Si-stereogenic vinylsilanes

Abstract

Alkyne hydrosilylation represents a straightforward method to synthesize vinylsilanes, which have found numerous applications in synthetic chemistry and materials science. However, it is challenging to produce silicon-stereogenic hydrosilanes through catalytic asymmetric hydrosilylation of prochiral dihydrosilanes with internal diarylalkynes. Herein, we present a highly efficient palladium-catalyzed asymmetric hydrosilylation of general internal diarylalkynes with different symmetric or asymmetric aryl groups, each proceeding through the desymmetrization of prochiral dihydrosilanes. The efficiency of this approach is systematically demonstrated for the first time in the synthesis of chiral-at-silicon organosilicon compounds bearing various aromatic groups with high enantioselectivity (up to 97% ee), which would be challenging to produce by means of traditional synthetic methods. Unique efficiency and reaction results for the chiral ligand-controllable hydrosilylation reaction of alkynes were also described using a ppm-level Pd catalyst. To date, this is the most efficient palladium-based catalyst system for the catalytic asymmetric hydrosilylation of alkynes.

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Introduction

Catalytic hydrosilylation involving the addition of the silicon-hydrogen bond of hydrosilanes to an unsaturated bond of alkenes or alkynes is one of the most important Si–C bond-forming reactions in organosilicon chemistry.¹ Since the first hydrosilylation of alkenes was reported in 1947,^2a new catalytic strategies for hydrosilylation of various unsaturated compounds, especially alkyne hydrosilylation, have been realized by structurally diverse transition-metal complexes, such as platinum,² palladium,³ rhodium,⁴ cobalt,⁵ iron,⁶ zinc,⁷ rare-earth metals,⁸ and other molecular catalysts,⁹ presenting a straightforward strategy to afford vinylsilanes and other organosilicon products with a high atom economy. Furthermore, the catalytic hydrosilylation of alkynes and their synthetic application in the synthesis of functionalized vinylsilanes have attracted increasing attention.¹⁰ Nevertheless, the development of enantioselective alkyne hydrosilylation for the synthesis of optically pure silicon-stereogenic vinylsilanes is still a challenging task since an effective ligand and chiral catalyst systems are limited and unpredictable owing to the difficulty in desymmetric activation of one of the Si–H bonds on dihydrosilanes.

During the past years, different ligands and transition-metal catalyst systems have been developed for the enantioselective hydrosilylation of alkynes to afford chiral silanes or other silacycles.^11–14 For example, Tomooka and co-workers reported a pioneering example of asymmetric hydrosilylation of alkynes in 2012,¹² in which they found that the enantioselective hydrosilylation of dihydrosilanes with symmetric internal dialkylalkynes but not aromatic alkynes could be completed by [Pt(dba)₃] with the aid of TADDOL-derived phosphonite ligands (Fig. 1a). Although the enantioselective induction of the platinum catalyst system was not good enough (up to 86% ee), this method provided a facile process for the desymmetrization of dihydrosilanes as starting materials to afford Si-stereogenic alkenylhydrosilanes. Subsequently, Lu and co-workers developed co-catalyzed one-pot hydrosilylation/hydrogenation of aromatic terminal alkynes to give structurally diverse chiral silanes (>99% ee) (Fig. 1b).¹³ In 2018, Huang and co-workers reported an efficient method for catalytic asymmetric hydrosilylation of prochiral dihydrosilanes with alkyne, which was completed by a new cobalt/PyBox complex to obtain Si-stereogenic vinylhydrosilanes with excellent enantioselectivities (up to 91% ee) (Fig. 1b).¹⁴ Although this is an elegant example of an enantioselective hydrosilylation reaction with terminal alkynes that occurs with high regio- and enantio-selectivity, this catalyst system is not suitable for the enantioselective hydrosilylation of internal diarylalkynes with symmetric aromatic groups because of its inferior enantioselectivity. For example, when diphenylacetylene was used in this reaction, only 63% ee of the corresponding silicon-stereogenic vinylsilane was achieved. Therefore, the development of new catalyst systems for the enantioselective hydrosilylation of simple and internal diarylalkynes with dihydrosilanes is still an underexplored and continuous task.¹⁵

Fig. 1 Asymmetric hydrosilylation of alkyne reactions. (a) Pt-catalyzed asymmetric hydrosilylation of aliphatic alkynes. (b) Co-catalyzed asymmetric hydrosilylation of aromatic alkynes. (c) Development of Pd-catalyzed asymmetric hydrosilylation of aromatic alkynes (this work).

In this work, we realize the aim of the efficient synthesis of silicon-stereogenic vinylsilanes through the enantioselective hydrosilylation of internal diarylalkynes and describe a powerful ligand-enabled desymmetrization platform using a chiral P-ligand to control the enantioselective palladium-catalyzed hydrosilylation of dihydrosilanes with general aromatic internal alkynes (Fig. 1c), resulting in high enantioselectivity through desymmetric Si–H bond activation and new Si–C bond formation. By exploiting the palladium catalysis enhanced by the chiral P-ligand, a broad scope of aromatic or heterocyclic internal alkynes can be efficiently converted into silicon-stereogenic organosilicon compounds with compatible functional groups.

Results and discussion

Initially, we investigated the enantioselective palladium-catalyzed hydrosilylation of diphenylacetylene, a symmetric and aromatic internal alkyne, 1a, with hydrosilane 2a. The model reaction was performed using Pd₂(dba)₃ as a precatalyst and in the presence of a series of chiral P-ligands to establish the optimized reaction conditions. It was found that the chiral P-ligands played varied roles in this reaction, for example, when BINAP (L1) was used as the chiral ligand in dichloromethane (DCM) at 0 °C, poor enantioselectivity (10% ee) and low yield (10%) were obtained. Also, the TADDOL-derived phosphoramidite ligand (L2) exhibited no enantioselective induction in this reaction (entries 1 and 2, Table 1). Thus, inspired by previous findings on the palladium-catalyzed hydrosilylation of 1,3-diynes and ynones,¹⁶ we focused on investigating BINOL-derived phosphoramidite ligands for this reaction with cyclohexane as the solvent at 0 °C. As expected, some exciting findings in the Pd-catalyzed enantioselective hydrosilylation of diphenylacetylene 1a were observed, as shown in Table 1. By examining a set of chiral BINOL-derived P-ligands and their analogues, we were pleased to find that most of the BINOL-derived P-ligands enabled the enantioselective synthesis of silicon-stereogenic vinylsilane 3a, albeit with different enantioselective induction and conversion of dihydrosilane 2a. For example, BINOL-derived phosphoramidite ligand L8 exhibited inferior enantioselectivity in comparison to that of L3–L7. Further investigation revealed that BINOL-derived phosphoramidite ligand L9 would be an ideal P-ligand, which was obtained by precise modification of the phenyl substituents on the binaphthyl backbone or nitrogen atom, for the enhancement of enantioselective induction in this asymmetric Pd-catalyzed hydrosilylation (entry 9, 90% yield and 92% ee).

Table 1 Pd-catalyzed enantioselective hydrosilylation of diphenylacetylene 1a with dihydrosilane 2a in the presence of different chiral ligands^a

Entry	Ligand	Solvent	Yield^b (%)	ee^c (%)
a Reaction conditions: 1a (0.2 mmol) and 2a (0.2 mmol), with Pd2(dba)3·CHCl3 (2 mol%), chiral P-ligand (8 mol%), solvent (2 mL), at 0 °C for 6 h and the data of yield is determined by NMR analysis. b Isolated yield. c The ee values were determined through HPLC using chiral columns.
1	L1	DCM	10	10
2	L2	DCM	30	0
3	L3	c-Hex	85	82
4	L4	c-Hex	65	85
5	L5	c-Hex	75	72
6	L6	c-Hex	88	87
7	L7	c-Hex	89	83
8	L8	c-Hex	88	87
9	L9	c-Hex	90	92

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With the BINOL-derived phosphoramidite ligand L9 in hand, and after successfully determining the optimized reaction conditions, subsequently we investigated the substrate scope of the asymmetric Pd-catalyzed hydrosilylation of aromatic internal alkynes 1 and dihydrosilanes 2. Firstly, a series of symmetric aromatic alkynes bearing various electron-withdrawing, electron-neutral, and electron-donating groups on the phenyl ring was tested (Scheme 1). We found that the Pd-catalyzed asymmetric hydrosilylation of aromatic internal alkynes proceeded smoothly to give the desired silicon-stereogenic vinylsilanes 3a–3q with excellent enantioselectivity (up to 94% ee). Also, the enantioselectivity was generally high between 90% ee and 94% ee when aromatic alkynes with substituted groups (Me, ⁱPr, Et, F, CF₃, Cl, Br, etc.) on the para-position were used in this reaction, which revealed that the reactivity of the aromatic internal alkynes was not affected by the electrical properties of the substituents on the internal alkynes. We also checked other aromatic internal alkynes with substituted groups on the meta-position of the phenyl ring, in which the corresponding products could be achieved in good yields and enantiomeric excess. For example, product 3k bearing a meta-substituted fluorine was obtained in 90% yield and 91% ee. However, methoxy-substituted aromatic internal alkynes resulted in inferior enantioselectivity in this reaction (80% ee). Notably, this reaction could not tolerate ortho-substituted aromatic alkynes, which always resulted in poor transformations. For example, when 1,2-di-o-tolylethyne was used the substrate, poor yield of the desired product 3u was obtained in this reaction. Fortunately, the use of 1,2-bis(2-fluorophenyl)ethyne as the substrate led to the formation of silicon-stereogenic vinylsilane 3l with excellent yield and ee (98% yield, 92% ee). In addition, we also evaluated the asymmetric Pd-catalyzed hydrosilylation of symmetric internal alkynes with heterocyclic rings (1,2-di(thiophen-2-yl)ethyne) and found that good yield and enantioselectivity could be achieved in this case (3o, 88% yield and 84% ee). This result indicates that the sulfur atom on the thiophene ring did not have a significant effect on the chiral palladium catalyst system, leading to a slight decrease in enantioselectivity. Similarly, 1,2-di(pyridin-3-yl)ethyne was also an effective substrate in the Pd-catalyzed hydrosilylation, giving the corresponding silicon-stereogenic vinylsilane 3q with 87% ee. Similar to previous work,^16,17 we also found that steric repulsion on aromatic dihydrosilanes is crucial for the enhancement in enantioselectivity in this reaction. For example, when phenyl(o-tolyl)silane was used as the substrate in this hydrosilylation, only 34% ee of the corresponding product 3r was achieved. Unfortunately, the hydrosilylation of diphenylacetylene with methyl(phenyl)silane or tert-butyl(phenyl)silane resulted in product 3s or 3t without the exact ee value because it could not be determined by chiral HPLC analysis. Although there is no report on the catalytic hydrosilylation of 1,2-bis(4-nitrophenyl)ethyne to date, we checked this reaction with dihydrosilane 2a and found only trace product 3v was detected.

Scheme 1 Substrate scope of enantioselective Pd-catalyzed hydrosilylation of aromatic internal alkynes to access silicon-stereogenic vinylsilanes.

Subsequently, the gram-scale synthesis of silicon-stereogenic vinylsilane 3a was investigated to realize the synthetic practicality (Scheme 2). It should be noted that this year, Profs. Luescher, Gallou and Lipshutz jointly published an important perspective about the impact of earth-abundant metals as a replacement for palladium catalysts in cross-coupling reactions, which provides comprehensive and objective support for the synthetic value of palladium catalyst systems.¹⁸ Thus, the price of palladium catalysts should not be the main consideration in the development of atom-economic catalytic transformations. In this case, to realize the powerful potential of the Pd-catalyzed hydrosilylation of aromatic alkynes, it is necessary to significantly reduce the catalytic amount of Pd catalyst in this reaction. We evaluated the model reaction of 1a with 2a on a 20 mmol scale to support the synthetic potential of this enantioselective Pd-catalyzed hydrosilylation. As shown in eqn (1) of Scheme 2, the use of a low loading of Pd catalyst (0.03 mol% of Pd₂(dba)₃) resulted in the corresponding silicon-stereogenic vinylsilane in the same level of excellent enantioselectivity (87% ee), albeit the yield of 3a decreased slightly to 78%. In this case, the yield of the desired product was obtained to reach a record-high turnover number (TON, 1300) in catalytic hydrosilylation. To the best of our knowledge, besides the Pd-catalyzed cross-coupling reaction that enabled the use of ppm-level palladium catalyst systems in the past,¹⁹ this is the first example of using ppm-level palladium catalysts to achieve homogenous catalytic hydrosilylation reactions of alkynes²⁰ and realize good enantioselective control in this process.

Scheme 2 Gram-scale synthesis of silicon-stereogenic vinylsilane 3a with the ppm level of palladium catalyst, the determination of new reaction systems that are insensitive to water or air, and a preliminary result for enantioselective Pd-catalyzed hydrosilylation of polyalkyne and asymmetric diarylalkyne.

Moreover, evidence that the palladium catalysts are not easily deactivated was also obtained from other control experiments (eqn (2) of Scheme 2). For example, the Pd-catalyzed hydrosilylation reaction could be carried out in air without protection of the reaction system under a special nitrogen or argon atmosphere. Also, the addition of a trace amount of water (1 eq.) to the solvent had almost no impact on the catalytic performance, giving the corresponding silicon-stereogenic vinylsilane 3a without any loss of yield and enantioselectivity (90% yield, 92% ee). When this reaction was carried out in the presence of 1 eq. H₂O under an air atmosphere, the desired product was obtained with slightly inferior results of 85% yield and 90% ee. In addition, silicon-stereogenic silane 3a could be transformed into Si-stereogenic silanol 3aa and methyl-substituted derivatives 3ab with retention of the configuration of the chirality-at-silicon (eqn (3)).^16b,21 These experimental results are sufficient to support that the enantioselective Pd-catalyzed hydrosilylation reaction was unaffected by moisture and oxygen, which may exhibit powerful potential in industrial application and has synthetic value in large-scale preparation.

Next, the enantioselective hydrosilylation of aromatic alkynes inspired us to expand it as a benchtop protocol for the synthesis of a chiral organosilicon polymer bearing a silicon-stereogenic pendant. Although the newly synthesized polyalkyne 4 was prepared as an insoluble and solid material in cyclohexane, we also established the possibility for the catalytic synthesis of a chiral organosilicon polymer bearing silicon-stereogenic centers. In this content, we checked the palladium-catalyzed hydrosilylation of polyalkyne 4 (M_w > 110 000) with the aid of chiral P-ligand L9 (eqn (4) of Scheme 2). As expected, a high yield of the corresponding poly(vinylsilane) 5 with an Si-stereogenic pendant, as confirmed by IR, SEM, TGA, and circular dichroism (CD) spectra, could be achieved (see eqn (4) and (5) of Scheme 2 and Fig. S1–S4 of the ESI†). For example, during the circular dichroism (CD) analysis, chiral poly(vinylsilane) 5 with a reverse chiral-at-silicon configuration for a pendant Si-stereogenic center exhibited different and corresponding to positive and negative Cotton effect, which realized the CD spectrum of chiral poly(vinylsilane) caused by the fragments of silicon-stereogenic silanes.

Notably, the development of the catalytic asymmetric hydrosilylation reaction of asymmetric internal diarylalkynes is one of the most challenging reaction systems in synthetic chemistry, given that the substituents at both ends of aromatic internal alkynes rarely have significant differences in their electrical properties. Thus, two possible products are always produced because of the poor regioselectivity in hydrosilylation reaction. Based on the exploration of the scope of the Pd-catalyzed hydrosilylation with symmetric and aromatic internal alkynes, the poor reactivity of ortho-substituted aromatic internal alkynes provided useful information to design effective asymmetric internal alkynes for regioselective hydrosilylation. To test the hypothesis that an asymmetric internal alkyne bearing an ortho-substituted aromatic ring may be a suitable substrate for the regioselective formation of an Si–C bond, which occurs on the side of the internal alkyne with lower steric hindrance, we checked the Pd-catalyzed hydrosilylation of asymmetric and aromatic internal alkyne 6 with dihydrosilane 2a. Interestingly, the desired product 7 was obtained in only moderate yield and regioselectivity (40% yield of mixed isomers with 70 : 30 rr). This result indicates that it is difficult to achieve the regioselective Si–C bond-forming hydrosilylation of asymmetric and aromatic alkynes through the precise tuning of the steric hindrance effect. Therefore, it is expected to achieve high regioselective hydrosilylation of asymmetric internal alkynes by controlling the different electronic effects at both ends of alkynes.

As is well known, ferrocene displays high stability, exceptionally well-defined redox features, and interesting geometries with two cyclopentadienyl rings.²² In particular, the special electronic properties of ferrocene and its derivatives make them ideal candidates in the field of organometallic chemistry and materials science.^22,23 Accordingly, there are significant differences in the aromaticity or electronic effects between ferrocene and benzene rings.²⁴ Thus, to expand the scope of this protocol, we investigated the catalytic asymmetric hydrosilylation of asymmetric internal alkynes with a ferrocenyl group and aromatic ring at both ends of the internal alkyne (ferrocenyl phenylacetylenes), respectively (Scheme 3). Interestingly, when asymmetric ferrocenyl phenylacetylene 8a was employed as the model substrate to react with dihydrosilane 2a under the standard conditions, only one isomer of vinylsilane (9a) was achieved with excellent regioselectivity (>20 : 1) and good enantioselectivity (97% ee). Then, we conducted substrate-scope studies to determine the range of substituted ferrocenyl arylacetylenes that can be used in the enantioselective hydrosilylation reaction with dihydrosilane, in which a series of ferrocenyl-substituted silicon-stereogenic vinylsilanes 9 was obtained in good yields (up to 94%) and excellent regio- and enantio-selectivity (up to 20 : 1 rr and 90–97% ee). As shown in Scheme 3, our results showed that the hydrosilylation reaction was effective with a variety of asymmetric ferrocenyl arylacetylenes, regardless of the substituents on the benzene ring, such as Me, t-Bu, Cl, Br, F, CF₃, and OMe. Additionally, the reaction proceeded well with ferrocenyl arylacetylenes bearing a heterocyclic substituent in good yield and enantioselectivity (for example, 92% yield and 90% ee for 9m, 65% yield and 90% ee for product 9n, respectively). The excellent regioselectivity and enantioselectivity of the ferrocenyl arylacetylenes can be attributed to the different electronic properties on the two alkynyl carbons and steric hindrance of dihydrosilane, which provided suitable coordination for Si–Pd–H insertion into the alkynyl substrate.

Scheme 3 Synthesis of ferrocene-derived chiral Si-stereogenic vinylsilanes 9 from enantioselective Pd-catalyzed hydrosilylation of asymmetric ferrocene-based aromatic alkynes.

To gain insight into the reaction mechanism of the Pd-catalyzed hydrosilylation of diarylalkyne with dihydrosilane, we performed a kinetic study to reveal the effect of product 3a on the reaction. Similar to previous work,^16b the Pd-catalyzed hydrosilylation could be accelerated by the addition of a catalytic amount of product 3a (see Fig. 2a). Although almost no product was obtained in the absence of the chiral P-ligand when 3a was used as an additive in this Pd-catalyzed hydrosilylation, these experimental results also support the possibility of product-involved autocatalysis because of the existence of catalyst aggregation in this reaction. Moreover, the non-linear effect (NLE)²⁵ was investigated in the presence of ligand L9 (Fig. 2b), and it was found to be a positive model of NLE under the optimized reaction conditions. Notably, this reaction could be successfully carried out in an air atmosphere or aqueous solvent to achieve the desired products with a high ee value (90%–92% ee, see Scheme 2). These experimental results revealed that the catalyst aggregation would be possibly formed by the product-involved molecular interaction (aromatic interaction) between the catalyst and substrate in this reaction.

Fig. 2 Controlled experiment for the proposed mechanism for Pd-catalyzed hydrosilylation of aromatic alkynes. (a) Kinetic analysis for the determination of the additive effect of product 3a on the Pd-catalyzed hydrosilylation of 1a and 2a. (b) Non-linear effect (NLE) study for Pd-catalyzed hydrosilylation of 1a with 2a. (c) Proposed reaction mechanism.

Based on the previously reported mechanism of palladium-catalyzed hydrosilylation of alkynes,¹⁶ we proposed the reaction mechanism in Fig. 2c. Initially, the alkyne-coordinated Pd species A is generated from the Pd₂(dba)₃/L9 complex with ligand exchange and substrate coordination with diarylalkyne. Then, the Si–H activation of intermediate B and subsequent hydrometallation accelerated by the Pd catalyst occur to give intermediate C through the α-selective alkyne insertion of a hydrogen atom. Finally, the reductive elimination of Pd species D gives the desired silicon-stereogenic vinylsilanes, and the corresponding palladium species is re-generated for the next catalytic cycles.

Conclusions

In summary, we reported the first Pd-catalyzed asymmetric hydrosilylation of diarylalkynes with dihydrosilanes using a chiral phosphine ligand. The catalytic asymmetric hydrosilylation of symmetric diarylalkynes and asymmetric ferrocenyl arylacetylenes was characterized by a wide substrate scope and excellent regioselectivity under mild conditions, in which the enantioselective synthesis of the corresponding silicon-stereogenic vinylsilanes with high enantioselectivities (up to 97% ee) was achieved. In addition, this method could be expanded to the facile synthesis of chiral polyvinylsilane bearing a silicon-stereogenic pendant. Furthermore, we found that a ppm-level of Pd catalyst could be used for the asymmetric hydrosilylation of diarylalkynes with dihydrosilanes. With the BINOL-derived phosphoramidite ligand, a palladium catalyst loading as low as 169 ppm could efficiently catalyze this hydrosilylation with a high TON of 1300.

Author contributions

L. W. X. conceived the concept. H. X. developed the enantioselective Pd-catalyzed hydrosilylation of aromatic alkynes. H. X. Y. carried out some hydrosilylation reaction, A. J. W. carried out the DFT calculation and spectral analysis of absolute configuration of new products. F. Y. L., F. Y., L. L. and F. Y. K. assisted with structural analysis of the new silicon-stereogenic product and theoretical analysis of reaction pathway. All authors discussed the results and participated in writing the manuscript. L. W. X. supervised the project and wrote this paper.

Data availability

All experimental procedures, details of the optimization, and additional reaction data can be found in the ESI.†

Conflicts of interest

The authors declare no competing interests.

Acknowledgements

This work was supported by the grants of National Natural Science Foundation of China (NSFC No. 22371060, 22072035, and 22361162606), NSFC/RGC Joint Research Scheme (N_CUHK426/23), Special Support Program for High-level Talents of Zhejiang Province (2021R51005), Zhejiang Provincial Natural Science Foundation of China (No. LZ23B020002, LR22B020002 and LY22B020006) are gratefully acknowledged. The authors also thank Dr Y. M. Cui, Dr K. Z. Jiang, Dr K. F. Yang and Dr Z. Xu for their assistance on the spectral analysis.

References

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Footnotes

† Electronic supplementary information (ESI) available. CCDC 2380481. For ESI and crystallographic data in CIF or other electronic format see DOI: https://doi.org/10.1039/d4qo01698j

‡ These authors contributed equally.

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