Amine-borane mediated metal-free hydrogen activation and catalytic hydrogenation.

The use of frustrated Lewis pairs (FLPs) as hydrogenation catalysts is attracting increasing attention as one of the most modern and rapidly growing areas of organic chemistry, with many research groups around the world working on this subject. Since the pioneering studies of the groups of Stephan and Piers on the Lewis acid-base pairs, which do not react irreversibly with each other and act as a trap for small molecules, numerous FLPs for hydrogen activation have been reported. Among others, intra- and intermolecular systems based on phosphines, organic carbenes, amines as Lewis bases, and boranes or alanes as Lewis acids were studied. This review presents a progression from the first observation of the facile heterolytical cleavage of hydrogen gas by amines and B(C6F5)3 to highly active non-metal catalysts for both enantioselective and racemic hydrogenation of unsaturated nitrogen-containing compounds and also internal alkynes.