Research Progress on Ammonia-Borane Based Direct Transfer Hydrogenation Reactions

Abstract

Direct transfer hydrogenation reaction is a H transfer process from a hydrogen donor to a hydrogen acceptor without the presence of a catalyst——the so-called metal-free reaction. It's an idea elementary process for mechanistic study, but was rarely reported due to very high energy barriers. However, it is revealed both theoretically and experimentally that increase in polarity of the substrates can efficiently lower the energy barrier, meaning that polar hydrogen donor and hydrogen acceptors might make such reactions happen. Ammonia borane is well-known as a chemical hydrogen storage material with very high H content. In the meanwhile, it's an ideal polar hydrogen donor containing both hydridic and protic H atoms. Applying ammonia borane as the polar hydrogen donor and some polar hydrogen acceptor molecules such as imines, polarized olefins, aldehydes and ketones, direct transfer hydrogenation reaction occurred under mild conditions. The reaction scope, similarity and differences in mechanism are reviewed herein. Keywords ammonia borane; direct transfer hydrogenation; reaction mechanism; green chemistry 不饱和键的氢化是有机合成中最重要的基本反应