理论研究“受阻路易斯酸碱对”催化的烯醇硅醚氢化反应机理

doi:10.6023/A21050236

摘要/Abstract

“受阻路易斯酸碱对”(FLPs)催化的烯醇硅醚氢化反应是一类重要的直接合成醇类化合物的方法, 然而目前其反应机理仍不明确. 基于此, 以乙基取代的全氟苯基硼作为路易斯酸(Et-B(C₆F₅)₂), 三叔丁基膦(t-Bu₃P)作为路易斯碱, 烯醇硅醚化的苯乙酮(Me-TMS)作为底物建立了模型反应, 并使用密度泛函理论系统研究了其催化氢化反应机理. 结果显示: FLPs催化的烯醇硅醚氢化反应从Et-B(C₆F₅)₂和t-Bu₃P形成B-P-FLPs开始, 随后会依次经过H₂裂解, H⁺和H^-转移等过程, 其中H⁺转移为决速步, H^-转移无势垒, B-P-FLPs生成及H⁺转移是吸热反应. 升高温度不利于氢化反应发生, 但是增大压力可促进反应进行. 底物取代基效应会影响H^-转移过程, 可能使反应不发生.

关键词: 受阻路易斯酸碱对, 烯醇硅醚, 氢化反应, 密度泛函理论

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

引用此文

王英辉, 魏思敏, 段金伟, 王康. 理论研究“受阻路易斯酸碱对”催化的烯醇硅醚氢化反应机理[J]. 化学学报, 2021, 79(9): 1164-1172.

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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图/表 10

图1. 文献报道的“受阻路易斯酸碱对”催化的烯醇硅醚氢化反应

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

图2. “受阻路易斯酸碱对”催化的烯醇硅醚氢化模型反应

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

图3. 在IEF-PCM M06-2X/6-31+G(d)理论水平上优化得到的H2、Et-B(C6F5)2、t-Bu3P和Me-TMS结构, 键长单位为nm

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

图4. 优化得到的FLPs形成及其催化的H2裂解吉布斯自由能曲线, 其中优化的结构及其相对能量分别在IEF-PCM M06-2X/6-31+G(d)和IEF-PCM M06-2X/6-311++G(d,p)理论水平获得, 键长单位为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

图5. 优化得到的两性离子与Me-TMS的H+和H–转移吉布斯自由能曲线, 其中优化的结构及其相对能量分别在IEF-PCM M06-2X/6-31+G(d)和IEF-PCM M06-2X/6-311++G(d,p)理论水平获得, 键长单位为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

图6. IEF-PCM M06-2X/6-31+G(d)理论水平扫描Me-TMS中H–转移势能面

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

图7. 在IEF-PCM M06-2X/6-31+G(d)理论水平上优化得到的Et-TMS和Me-TBS结构

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

图8. 优化得到的两性离子与Et-TMS的H+和H–转移吉布斯自由能曲线, 其中优化的结构及其相对能量分别在IEF-PCM M06-2X/6-31+G(d)和IEF-PCM M06-2X/6-311++G(d,p)理论水平获得, 键长单位为nm

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

图9. IEF-PCM M06-2X/6-31+G(d)理论水平扫描Et-TMS和Me-TBS中 H–转移势能面

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

图10. 优化得到的两性离子与Me-TBS的H+和H–转移吉布斯自由能曲线, 其中优化的结构及其相对能量分别在IEF-PCM M06-2X/6-31+G(d)和IEF-PCM M06-2X/6-311++G(d,p)理论水平获得, 键长单位为nm

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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