A Direct Comparison Study of Asymmetric Borane Reduction of C = N Double Bond Mediated by Chiral Oxazaborolidines

摘要

A comparison study on asymmetric borane reduction of ketoxime ethers and N-substituted ketimines possesing C = N double bond mediated by the selected chiral oxazaborolidines (1-6) was investigated. Thus, an aromatic ketoxime O-alkyl ether acetophenone oxime O-methyl ether was reduced to the corresponding amine (1-phenylethylamine 8a) with optical yields, such as 58% ee with 1, 86% ee with 2, 3% ee with 3, 99% ee with 4, 60% ee with 5, and 73% ee with 6. However, the reduction of an aliphatic ketoxime derivative 2-heptanone oxime O-methyl ether provided low optical inductions (7-13% ee). For ketoxime O-trimethylsilyl ethers, the reduction of acetophenone O-trimethylsilyl ether afforded 8a with optical yields which were 90% ee with 1, 40% ee with 2, 2% ee with 3, 62% ee with 4, 5% ee with 5, and 60% ee with 6. The reduction of 2-heptanone O-trimethylsilyl ether also gave the product amine with low optical yields (10-40% ee). In the case of N-substituted ketimines, the reduction of acetophenone N-phenylimine afforded the corresponding amine with 79% ee, 78% ee, 9% ee, 73% ee, 78% ee and 67% ee using 1, 2, 3, 4, 5, and 6, respectively, whereas low optical inductions (5-18% ee) for 2-heptanone N-phenylimine were achieved.