Mechanism of Silyl Enol Ethers Hydrogenation Catalysed by Frustrated Lewis Pairs: A Theoretical Study

doi:10.6023/A21050236

Abstract

Silyl enol ethers have attracted enormous attention as they could serve as a test bed for the development of novel frustrated Lewis pairs (FLPs) catalytic systems. However, the reaction mechanism of hydrogenation catalysed by metal-free FLPs for these compounds to the corresponding secondary alcohols remains elusive to a large extent in previous studies. We thus performed a thorough investigation on the reaction mechanism by density functional theory (DFT). To illustrate the reaction mechanism of FLPs-catalysed hydrogenation for silyl enol ethers, trimethyl((1-phenylvinyl)oxy)silane (Me-TMS) was chosen as the prototype substrate and toluene as the solvent, where the FLPs were generated by ethylbis(perfluorophenyl)- borane (Et-B(C₆F₅)₂) and tri-tert-butylphosphine (t-Bu₃P). The M06-2X functional in connection with 6-31+G(d) basis set was used to optimize the structures of related species including in the Gibbs free energy profiles, and the energies were obtained at M06-2X/6-311++G(d,p) level of theory, where the solvent effect was simulated with the integral equation formalism, polarized continuum mode (IEF-PCM) in both calculations. Our results suggest that the FLPs-catalysed hydrogenation of silyl enol ethers in toluene begins with the formation of B-P-FLPs followed by hydrogen activation, proton transfer and hydride transfer to complete the process. It is obvious from the Gibbs free energy profile that the proton transfer is rate-determining step, the formation of B-P-FLPs and proton transfer are endothermal and the hydride transfer is no barrier. This indicates that the amount of H₂ and prototype substrate have significant influence on the FLPs-catalysed hydrogenation of silyl enol ethers. A higher temperature (328.15 K) is disadvantageous to hydrogenation reaction catalysed by FLPs but the reaction could be accelerated under higher pressure (4040 kPa). The Gibbs free energy profile calculations for trimethyl((1-phenylprop-1-en-1-yl)oxy)silane (Et-TMS) and tert-butyldimethyl((1-phenylvinyl)oxy)silane (Me-TBS) reveal that substituent group may inhibit the hydride transfer as the absence of a suitable construction for R-H^–-transfer, where the hydride does not direct to the C⁺ of silyl enol ethers and the distance between C⁺ and hydride is longer. These results would be helpful to design another novel FLPs-catalysed hydrogenation reaction for silyl enol ethers.

Key words: frustrated Lewis pairs, silyl enol ethers, hydrogenation, density functional theory

Cite this article

Yinghui Wang, Simin Wei, Jinwei Duan, Kang Wang. Mechanism of Silyl Enol Ethers Hydrogenation Catalysed by Frustrated Lewis Pairs: A Theoretical Study[J]. Acta Chimica Sinica, 2021, 79(9): 1164-1172.

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Fig. & Tab. 10

Figure 1. FLPs-catalyzed hydrogenation of silyl enol ethers developed by previous study

Figure 2. Model reaction for the silyl enol ethers hydrogenation catalysed by frustrated Lewis pairs

Figure 3. Optimized structures at IEF-PCM M06-2X/6-31+G(d) level of theory for H2, Et-B(C6F5)2, t-Bu3P and Me-TMS. Selected bond distances are given in nm

Figure 4. Optimized structures as well as Gibbs free energy profile of related species involved in the generation of B-P-FLPs and H2 split. The structures and energies reported in this figure were calculated at the IEF-PCM M06-2X/6-31+G(d) and IEF-PCM M06-2X/6-311++G(d,p) level of theory, respectively. Selected bond distances are given in nm

Figure 5. Optimized structures as well as Gibbs free energy profile of H+ and H– transfer between intermediate of ion pair and Me-TMS. The structures and energies reported in this figure were calculated at the IEF-PCM M06-2X/6-31+G(d) and IEF-PCM M06-2X/6-311++G(d,p) level of theory, respectively. Selected bond distances are given in nm

Figure 6. Total energy scan of hydride transfer for Me-TMS at IEF-PCM M06-2X/6-31+G(d) level of theory

Figure 7. Optimized structures at IEF-PCM M06-2X/6-31+G(d) level of theory for Et-TMS and Me-TBS

Figure 8. Optimized structures as well as Gibbs free energy profile of H+ and H– transfer between intermediate of ion pair and Et-TMS. The structures and energies reported in this figure were calculated at the IEF-PCM M06-2X/6-31+G(d) and IEF-PCM M06-2X/6-311++G(d,p) level of theory, respectively. Selected bond distances are given in nm

Figure 9. Total energy scan of hydride transfer for Et-TMS and Me-TBS at IEF-PCM M06-2X/6-31+G(d) level of theory

Figure 10. Optimized structures as well as Gibbs free energy profile of H+ and H– transfer between intermediate of ion pair and Me-TBS. The structures and energies reported in this figure were calculated at the IEF-PCM M06-2X/6-31+G(d) and IEF-PCM M06-2X/6-311++G(d,p) level of theory, respectively. Selected bond distances are given in nm

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