Ligand -controlled nickel-catalyzed C–O bond cleavage of silyl enol ether for the divergent synthesis of aryl alkenes and silicon-containing product

Abstract

Silyl enol ethers are easily prepared from available ketones that are widely used as α-carbon nucleophiles in organic synthesis. However, it is challenging to use this motif as electrophiles via the activation of the alkenyl C(sp²)–O bond. Here, we report a ligand-controlled nickel-catalyzed C(sp²)–O bond activation of the silyl enol ether for the divergent formation of styrenes, benzyl silane and alkenyl silanes, respectively. In the presence of the PCy₃ ligand, the (PCy₃)Ni–H formed in situ preferably inserts into the more electron-rich C–C double bond of the silyl enol ether so that the product stays in the olefin stage (23 examples). When electron-rich ICy·HCl is used as the ligand, the Ni–H species could be inserted into silyl enol ether and the styrene derivatives formed in situ are further converted into benzyl silane products (29 examples). In addition, the Ni(0)/PCy₃/hydride acceptor (1-octene) system can form the active Ni–[Si] intermediate in situ, which can react with the styrenes intermediate to construct alkenyl silanes (3 examples). The utility of the methodology is demonstrated by gram-scale reaction and late-stage modification of complex molecules, whereby a diverse set of functional groups can be tolerated. DFT calculations explain the divergent synthesis: the (PCy₃)Ni(I)–H species prefers to insert the electron-rich C–C double bond of silyl enol ether and the (NHC)Ni(I)–H species can insert not only the electron-rich C–C double bond but also continue to insert electron-poor styrenes.

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Introduction

Transition-metal catalyzed C–O bond activation and cross-coupling reactions have emerged as one of the most powerful tools to construct complex molecules,^1–10 where the activated C–O electrophiles are used routinely.^11–19 The activation of the C(alkenyl)–O bond is important in constructing alkene motifs that represent an important class of valuable synthons and are the basic structures in numerous bioactive molecules. Most studies in this field describe the coupling of enol triflates with enol tosylates due to their intrinsic properties: the high propensity of –OTf or –OTs residues to act as leaving groups.^20–43 In addition, there are a number of protocols demonstrating the coupling of enol acetates.^44–49 However, the disadvantages, such as complex synthesis and high cost of operation cannot be ignored (Scheme 1A). Thus, the development of predigesting synthetic operations and low-cost alternative cross-coupling partners would increase the ever-growing synthetic toolkit.

Scheme 1 C–O Bond activation of the silyl enol ethers: (A) comparison of (alkenyl)-O electrophiles. (B) Four reaction models of silyl enol ethers. (C) Ni-catalyzed hydrogenation and hydrosilylation of silyl enol ether via C–O bond activity.

Silyl enol ethers are important organic reactants that can be prepared from wildly available ketone derivatives with price-friendly chlorosilane in one step. Generally, the silyl enol ethers are most frequently used as the carbonyl α-carbon nucleophiles in various organic reactions, including Mukaiyama aldol reactions,⁵⁰ Rubottom oxidation,^51,52 Saegusa oxidation,^53,54 alkylation,^55–57 arylation^58–63 and ethenylation^64–69 (Scheme 1). Recently, Zhao et al. reported the hydroboration of silyl enol ethers, where the silyl enol ethers participate in reactions as electron-rich C–C double bond.^70–73 The Chang group reported copper-catalyzed dehydrogenative arylation via the allylic C–H bond activation of silyl enol ethers.⁷⁴ It is surprising that the functioning of silyl enol ethers as C(alkenyl)–O electrophiles has received considerably less attention (Scheme 1B-4).^75–83 This is probably attributed to the remarkably high activation barrier required for C(alkenyl)–O bond cleavage and the lower leaving tendency of siloxy residues. Ni(0) was utilized to activate the C(alkenyl)–O bonds of silyl enol ethers;^75–79 however, these reactions were limited to reactive Grignard reagents via ate-complex intermediate,^84–89 resulting in low functional group tolerance and limited scope (Scheme 1B-4). Furthermore, a metal-free method is developed to cleave the C(alkenyl)–O bond of silyl enol ethers in the presence of Et₃SiH and a catalytic amount of B(C₆F₅)₃,⁸⁰ in spite of the fact that there is poor chemical selectivity in those transformations. The main reasons behind the challenge to perform such transformations are the high C–O bond strength, unstable O–[Si] bond towards the base, and steric hindrance of silyl enol ethers. Due to the wide availability and structural richness of enol silyl ethers, it is of great importance to develop a new catalytic system to convert their C–O bond into other functional groups. Based on Martin and Chatani's seminal works^{78,79,90–99} and our previous work,¹⁰⁰ herein, we report the ligand-controlled nickel-catalyzed C(sp²)–O bond activation of silyl enol ether. We reveal that the PCy₃ ligand-coordinated Ni–H prefers to insert the more electron-rich C–C double bond of the silyl enol ether, other than the styrene products, to produce olefins. However, using the electron-rich ICy·HCl ligand can encourage Ni–H to be further inserted into the relatively electron-deficient aryl alkenes that are formed in situ and convert the styrenes into benzyl silanes. In addition, Ni(0)/PCy₃/hydride acceptor (1-octene) system can form the active Ni–[Si] intermediate in situ, which could react with the styrene intermediates to synthesize alkenyl silanes (Scheme 1C).

Results and discussion

Our study began by optimization of the reaction conditions between trimethyl((1-(p-tolyl)vinyl)oxy)silane (1a) and Et₃SiH (2a) (Table 1). First, we examined various phosphine ligands (Table 1, entries 1–4).^{90,91,101–106} Interestingly, as the ligand, PCy₃ (L1) promoted this reaction to a great extent, and the desired 4-methylstyrene 3a was obtained in 60% yield (Table 1, entry 1). However, other phosphine ligands were not suitable for this reaction, indicating that this protocol was extremely ligand-dependent (Table 1, entries 2–4). Improved results were obtained, when the reaction was run at a higher temperature; the desired product 3a was delivered in 82% yield at 100 °C (Table 1, entries 5 and 6). During the screening for other ligands, when the L5 ligand was used, no deoxyhydrogenation product 3a was found, and a deoxyhydrosilylation product 4a was obtained instead (Table 1, entry 7). As expected, higher temperatures also promoted the formation of the hydrosilylation product 4a that was obtained in 60% isolated yield (Table 1, entry 8). In addition, we examined a series of NHC ligands, and the results did not improve further (Table 1, entries 9–11). Notably, when NHC ligands were used, there were trace or undesired product 3a (Table 1, entries 7–10), indicating that silyl enol ether was particularly susceptible to further hydrosilylation after hydrogenation in the presence of Ni(cod)₂ and NHC ligands.

Table 1 Optimization of the reaction conditions^a

Entry	Ligand	Temp.	Yield of 3a ^b (%)	Yield of 4a ^b (%)
a Condition: 1a (0.2 mmol), 2a (2 equiv.), Ni(cod)2 (10 mol%), PCy3 (20 mol%), 1,4-dioxane (2 mL), 24 h. b Yields were determined by GC analysis using n-dodecane as the internal standard. c 2a (4 equiv.), ligand (10 mol%), LiO^tBu (20 mol%), 1,4-dioxane (1 mL), 6 h. d Isolated yield.
1	L1	80 °C	60	0
2	L2	80 °C	0	0
3	L3	80 °C	0	0
4	L4	80 °C	0	0
5	L1	100 °C	82(79)	Trace
6	L1	120 °C	63	Trace
7^c	L5	100 °C	Trace	42
8^c	L5	140 °C	0	62(60)
9^c	L6	140 °C	Trace	25
10^c	L7	140 °C	Trace	40
11^c	L8	140 °C	Trace	Trace

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With the optimal reaction conditions determined, the scope of silyl enol ether was investigated. As demonstrated in Scheme 2, the deoxyhydrogenation method turned out to be widely applicable, regardless of the electronic and steric environments on the aromatic rings. Alkoxy (3c–d, 3g), phenyl (3e), ester (3f), and borate ester (3h) were tolerated on the benzene ring, and the corresponding aromatic olefin products were obtained in yields ranging from 57 to 98%. Substrate 3i can be converted to the target product at 60% yield, indicating that C(vinyl)–O[Si] is more reactive than C(aryl)–O[Si]. The presence of electronically unbiased substituents at the ortho position of the reactive site did not impede the deoxyhydrogenation reaction (3b). As expected, the π-extended silyl enol ethers reacted perfectly well (3j–l).^90–98 Importantly, heterocyclic substituents were also compatible under the standard conditions (3m–n). In addition, the internal alkenes could also be opportunely generated from the corresponding internal silyl enol ether by making simple modifications to the standard reaction conditions. Moreover, the olefinic products were dominated by excellent E-configuration. All the silyl enol ethers with different chain lengths reacted smoothly with Et₃SiH to afford the corresponding products 3o–q in good yields. The substituents of alkyl terminal groups had a limited effect on the deoxygenation of silyl enol ethers (3r–t).

Scheme 2 Deoxyhydrogenation of silyl enol ethers. ^a Condition: 1 (0.5 mmol), Et₃SiH (2 equiv.), Ni(cod)₂ (10 mol%), PCy₃ (20 mol%) in 1,4-dioxane (5 mL) for 24 h. Isolated yields. ^b GC yields. ^c Neat, 80 °C. ^d Neat, 100 °C.

Next, we tested the scope of silyl enol ether under the deoxyhydrosilylation conditions (Scheme 3). As shown in Scheme 3, the corresponding benzyl silicones were obtained in good yields and excellent site-selectivity, regardless of whether the substituents on the aromatic ring of the silyl enol ethers were electron-withdrawing (4d) or electron-donating groups (4a–c, 4e, 4g). Naphthalene and fluorene (4h–k) could also be tolerated. Other aromatic rings, such as ferrocene, pyrrole, and furan, were incorporated into products (4l–n). Deoxyhydrosilylation was applicable on internal silyl enol ethers with different carbon chains (4o–q). Notably, different functional groups, such as silyl ethers or amine phenyl ethers, were also compatible with the alkyl chain (4r–s). Steric hindrance seemed to have little effect on deoxyhydrosilylation. For example, polysubstituted silyl enol ethers (4t–u) were converted into the corresponding deoxyhydrosilylation products in moderate yields.

Scheme 3 Deoxyhydrosilylation and deoxy-dehydrosilylation of silyl enol ethers. ^a Condition: 1 (0.5 mmol), Et₃SiH (4 equiv.), Ni(cod)₂ (10 mol%), ICy·HCl (10 mol%), LiO^tBu (20 mol%), 1,4-dioxane (2.5 mL), 140 °C, 6 h. ^b Ni(cod)₂ (15 mol%), ICy·HCl (15 mol%), LiO^tBu (30 mol%) in 1,4-dioxane (0.6 mL). ^c Ni(cod)₂ (10 mol%), PCy₃ (20 mol%), Et₃SiH (5 equiv.), 1-octene (3 equiv.), CPME (0.5 mL), 140 °C, 24 h.

Encouraged by the above findings, we believe that as silyl enol ether can be hydrosilylated via an olefin intermediate, it is reasonable to suppose that Ni–[Si] species¹⁰⁷ can be inserted into the intermediate to obtain dehydrosilylation products as well (see the ESI† for details). As we expected, by using the external hydrogen acceptor^107–113 and simple modification of the conditions, we achieved dehydrogenative silylation of 2-substituted silyl enol ethers, among which CF₃ (5b) and OMe (5c) could be retained. However, polysubstituted silyl enol ethers (5d) did not convert under these conditions. Steric hindrance is a very important parameter affecting dehydrosilylation, and it is necessary to inhibit the olefin polymerization without compromising the insertion ability of the Ni–[Si] species.

To further demonstrate the utility of deoxyhydrogenation and subsequent hydrosilylation methods (Scheme 4A), we subjected several medicinally relevant molecules to these reactions to afford the derivatives of tonalide (3aa, 4aa), citronellol (3ab, 4ab), L-menthol (3ac, 4ac), and L(–)-borneol (3ad, 4ad). Pharmaceutical intermediates, diacetone-D-glucose (3ae, 4ae), can attain altered intrinsic biological activity through our realistic approach. We can also modify complex molecules, such as cholesterol (4af) and diosgenin derivatives (4ag), with functional groups.

Scheme 4 Modifications of complex molecules and gram-scale reaction. ^a Reaction condition: 1 (0.5 mmol), Et₃SiH (2 equiv.), Ni(cod)₂ (10 mol%), PCy₃ (20 mol%), 1,4-dioxane (5 mL), 100 °C, 24 h. ^b 1 (0.5 mmol), Et₃SiH (4 equiv.), Ni(cod)₂ (10 mol%), ICy·HCl (10 mol%), LiO^tBu (20 mol%), 1,4-dioxane (2.5 mL), 140 °C, 6 h.

In addition, without further optimization, these transformations exhibited an excellent performance in gram-scale reactions. Silyl enol ether was selected as an internal substrate of dehydrogenation for a gram-scale reaction, with a yield of 98% (Scheme 4B, eqn (1)). The yield of 1-naphthol silyl enol ether was 58% by hydrosilylation at 5 mmol (Scheme 4B, eqn (2)). These results indicated the potential of the method for synthetic applications in the future.

In principle, two different mechanistic interpretations were conceived for the deoxyhydrogenation of silyl enol ether to afford the corresponding olefin (Scheme 5A). Firstly, Ni–H species (I-a) underwent migratory insertion into the C–C double bond of 1a to form an alkyl nickel species I-b, which would transfer to I-c by β-H elimination/Ni–H reinsertion process. The olefin product 3a was obtained by β-O elimination of alkyl Ni (I-c). Another possible mechanism involved the Ni–SiEt₃ species (II-a) from Ni(0) and Et₃SiH.⁹¹ This Ni–SiEt₃ species (II-a) underwent migratory insertion of II-a into the C–C double bond of 1a to yield II-b.¹¹⁴ Subsequently, intermediate II-d was produced through the Peterson elimination^115–117 from II-b. Finally, the alkenyl nickel species (II-d) went through metathesis with Et₃SiH to produce an olefinic product (3a) and regenerate the Ni–SiEt₃ species (II-a).

Scheme 5 Research mechanisms: (A) proposed pathways of deoxyhydrogenation of silyl enol ether. (B) Deuterium-labeling experiment. (C) Hydrosilylation of styrene. (D) Competition experiment.

Although our experimental data did not allow us to rigorously distinguish between these two mechanisms, we decided to gather indirect evidence for the mechanism by studying the reaction of silyl enol ether with 2a. To exclude the oxidative addition process, 1c was used to react with Ph₃SiD under standard conditions. The deuterated olefin 8 was separated, and the hydrogens on both carbons of the olefins were determined to be deuteriums (Scheme 5B). Deuterated silyl enol ether was used to react with Et₃SiH under standard conditions as well; we observed that deuterium was substituted by hydrogen on the olefin. The control experiments for determining deuterium supported a mechanism involving β-H elimination/Ni–H reinsertion process. To verify the intermediates of hydrosilylation, olefinic 3a was selected as the substrate to react with silane under standard conditions, yielding hydrosilylation products (4a) in 78% isolated yield (Scheme 5C). Subsequently, 1a and olefin 10 were subjected to the standard deoxyhydrogenation conditions, producing 3a in 78% yield, while substrate 10 did not convert. Under the standard deoxyhydrosilylation conditions, 31% of 4a and 84% of 11 were obtained (Scheme 5D). These results indicate that (PCy₃)Ni(I)–H species prefer to insert the electron-rich C–C double bond of silyl enol ether, and (NHC)Ni(I)–H species can insert not only the electron-rich C–C double bond but also continue to insert electron-poor styrenes.

In addition, kinetic experimental data (Fig. 1) showed a positive reaction order of 0.83 in Ni(cod)₂, 0.57 in Et₃SiH, and 0.03 in silyl enol ethers (see the ESI† for details).

Fig. 1 Kinetic experimental data. (A) Plot of initial rates vs. concentration of Ni(cod)₂. (B) Plot of initial rates vs. concentration of Et₃SiH.

According to the previous reports,²⁸ the silane 2a can effectively react with Ni(0) catalysts, followed by a comproportionation reaction to generate Ni(I)–H species INT1A and Ni(I)–Si species INT1B. The INT1A-dimer is the resting state of the catalyst, which can dissociate into monomeric INT1A to initiate the catalytic cycle (Fig. 2A). The intermediate INT1A undergoes reversible migration–insertion reactions with silyl enol ethers 1a in two ways, yielding anti-Markovnikov and Markovnikov intermediates INT2A-1 and INT2A-2, respectively. The transition states, TS2A-1 and TS2A-2, have comparable energy barriers at 22.7 and 22.0 kcal mol⁻¹, respectively, indicating that INT2A-1 and INT2A-2 are in dynamic equilibrium in the catalytic cycle. Due to the higher calculated energy barrier, the migration–insertion process of INT1B with substrate 1a is well excluded (51.9 kcal mol⁻¹ for TS2B-1 and 36.1 kcal mol⁻¹ for TS2B-2, Fig. 2B). The β-carbon of intermediate INT2A-1 undergoes σ-bond rotation, facilitating β-O elimination through transition state TS3A-1 with an energy barrier of 10.2 kcal mol⁻¹, yielding the deoxyhydrogenation product 3a and the Ni(I)–O species intermediate INT3A-1. Subsequent σ-bond metathesis with the silane 2a occurs, generating silyl ethers as by-products and regenerating the Ni(I)–H species INT1A to complete the catalytic cycle. We further investigated the competitive pathways by which product 3a undergoes migration insertion with INT1A and INT1B, respectively. Alkene 3a is more prone to undergoing migration insertion with the intermediate INT1A. These processes are reversible and proceed via the transition states TS5A-1 and TS5A-2, respectively, resulting in deuterium transfer within the substrate, which is consistent with the observed deuterium exchange phenomenon in experiments. In contrast, the migration–insertion process, involving Ni(I)–Si intermediates and 3a, is kinetically unfavourable (Fig. 2C).

Fig. 2 The mechanism calculated for the deoxyhydrogenation of silyl enol ethers. (A) The calculated energy profile for the generation of 3a. (B) Competitive transition states for the migration insertion of 1a. (C) Competitive transition states for the migration insertion of 3a.

Compared with tricyclohexylphosphine, the NHC ligand exhibits a stronger σ-donating ability, which increases the electron density around the nickel metal, thereby enhancing the reactivity of Ni(I)–H and Ni(I)–Si species and facilitating the migration insertion of the substrate 1a. When the ligand NHC is used, both INT1A-NHC and INT1B-NHC can react with silyl enol ethers 1a, leading to the formation of deoxyhydrogenated products through two pathways (Fig. 3A). The intermediate INT1A-NHC selectively inserts into the alkene of 1a to form the intermediate INT2A-NHC-1via a transition state TS2A-NHC-1 that has an energy barrier of 10.9 kcal mol⁻¹. In INT2A-NHC-1, the oxygen atom on the β-carbon coordinates with the nickel center, enhancing the stability of the structure and rendering this process irreversible. The migration–insertion process via the Markovnikov-type transition state TS2A-NHC-2 is reversible. The INT2A-NHC-1 intermediate obtained here undergoes β-O elimination to release 3a. The migratory insertion process of INT1B-NHC with substrate 1a is also calculated. The intermediate INT1B-NHC is kinetically more prone to generating INT2B-NHC-2via the transition state TS2B-NHC-2, which has a smaller steric hindrance, resulting in an energy requirement that is 11.3 kcal mol⁻¹ lower than the energy needed to generate intermediate INT2B-NHC-1 through TS2B-NHC-1 (10.5 vs. 21.8 kcal mol⁻¹). The intermediate INT2B-NHC-2 further undergoes Peterson elimination via the transition state TS3B-NHC-2 with an activation energy of 15.9 kcal mol⁻¹ to generate the vinyl-nickel intermediate INT3B-NHC-2. Subsequent protonation by another molecule, silane 2a, yields 3a, while simultaneously regenerating the active catalyst INT1B-NHC. Product 3a which is generated in situ reacts with the intermediate INT1B-NHC to produce deoxyhydrosilylated product 4a and deoxy-dehydrosilylated product 5a. The alkene 3a inserts into Ni(I)–Si species to generate the intermediate INT5B-NHC-1via the transition state TS5B-NHC-1, which then undergoes protonation to afford product 4a and regenerates the active catalyst INT1B-NHC. The protonation process in this step proceeds via the transition state TS6B-NHC-1 with an energy of 21.4 kcal mol⁻¹, which is the rate-determining step in the catalytic cycle. This pathway is exothermic; hence it is thermodynamically controlled. Alternatively, INT1B-NHC and 3a undergo migration insertion via the transition state TS5B-NHC-2, followed by β-H elimination to generate the deoxy-dehydrosilylated byproduct 5a. This process is reversible and kinetically controlled. When the temperature is elevated to 140 °C, the thermal pathway becomes the predominant route, leading to compound 4a becoming the major product observed.

Fig. 3 The calculated mechanism for the deoxyhydrosilylation and deoxy-dehydrosilylation of silyl enol ethers. (A) The pathways for the generation of 3a. (B) The further transformation of 3a.

Conclusions

We have developed a surprisingly facile mechanism for the ligand-controlled nickel-catalyzed C(sp²)–O bond activation of silyl enol ether. We find that the PCy₃ ligand can make Ni–H insert only the C–C double bond of the more electron-rich silyl enol ether so that the product remains in the olefinic stage. When electron-rich ICy·HCl is used as the ligand, the Ni–H species can be inserted into the silyl enol ether and the styrene derivatives formed in situ, which yield the benzyl silane products. In addition, Ni(0)/PCy₃/hydride acceptor (1-octene) system can form the active Ni–[Si] intermediate in situ, which could react with the styrenes to construct alkenyl silanes. Mechanistic studies suggest that Ni(I)–H is the key species for the cleavage of the alkenyl C(sp²)–O bond in silyl enol ethers.

Data availability

The data supporting this article have been included as part of the ESI.†

Conflicts of interest

The authors declare that they have no conflict of interest.

Acknowledgements

We thank the National Natural Science Foundation of China (No. 22371078, 22001076), the Natural Science Foundation of Guangdong Province (No. 2022A1515010660, 2021A1515220024), and the National Key R&D Program of China (2022YFA1503200) for financial support. We are grateful to the High-Performance Computing Center of Nanjing University for performing the numerical calculations presented in this paper on their blade cluster system.

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Footnotes

† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d4qo01906g

‡ These authors contributed equally to this work.

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