School of Chemistry and University Research Centre in Catalysis and Intensified Processing, Bedson Building, University of Newcastle, Newcastle upon Tyne, UK, NE1 7RU.
Beilstein J Org Chem. 2010 Nov 3;6:1043-55. doi: 10.3762/bjoc.6.119.
Propylene carbonate can be used as a green solvent for the asymmetric synthesis of cyanohydrin trimethylsilyl ethers from aldehydes and trimethylsilyl cyanide catalysed by VO(salen)NCS, though reactions are slower in this solvent than the corresponding reactions carried out in dichloromethane. A mechanistic study has been undertaken, comparing the catalytic activity of VO(salen)NCS in propylene carbonate and dichloromethane. Reactions in both solvents obey overall second-order kinetics, the rate of reaction being dependent on the concentration of both the aldehyde and trimethylsilyl cyanide. The order with respect to VO(salen)NCS was determined and found to decrease from 1.2 in dichloromethane to 1.0 in propylene carbonate, indicating that in propylene carbonate, VO(salen)NCS is present only as a mononuclear species, whereas in dichloromethane dinuclear species are present which have previously been shown to be responsible for most of the catalytic activity. Evidence from ⁵¹V NMR spectroscopy suggested that propylene carbonate coordinates to VO(salen)NCS, blocking the free coordination site, thus inhibiting its Lewis acidity and accounting for the reduction in catalytic activity. This explanation was further supported by a Hammett analysis study, which indicated that Lewis base catalysis made a much greater contribution to the overall catalytic activity of VO(salen)NCS in propylene carbonate than in dichloromethane.
碳酸丙烯酯可用作 VO(salen)NCS 催化醛和三甲基氰硅烷合成氰醇三甲基硅醚的绿色溶剂，尽管在该溶剂中的反应速度比在二氯甲烷中的相应反应慢。进行了一项机理研究，比较了 VO(salen)NCS 在碳酸丙烯酯和二氯甲烷中的催化活性。两种溶剂中的反应均遵循总体二级动力学，反应速率取决于醛和三甲基氰硅烷的浓度。确定了对 VO(salen)NCS 的反应顺序，并发现其从二氯甲烷中的 1.2 降低到碳酸丙烯酯中的 1.0，表明在碳酸丙烯酯中，VO(salen)NCS 仅以单核物种存在，而在二氯甲烷中存在双核物种，先前已证明双核物种是大多数催化活性的原因。来自 ⁵¹V NMR 光谱的证据表明，碳酸丙烯酯与 VO(salen)NCS 配位，阻止了游离配位位，从而抑制了其路易斯酸度，并解释了催化活性的降低。这一解释进一步得到了哈米特分析研究的支持，该研究表明，在碳酸丙烯酯中，路易斯碱催化对 VO(salen)NCS 的总催化活性的贡献远大于在二氯甲烷中。
