Metal-Free Hydrogenation of Ketone by Frustrated Lewis Pairs (FLPs)

Stephan et al. have reported the metal-free hydrogenation of alkyl and aryl ketones using H₂, B(C₆F₅)₃ and diethyl ether. In this reaction, sterically encumbered Lewis acid (B(C₆F₅)₃) and Lewis base (diethyl ether) are combined to form the unquenched “frustrated Lewis pairs (FLPs)”, which affords the specific reaction field. In this field, the borane mediated uptake of H₂ and sequential FLP assisted activation of H₂ enables the hydrogenation to proceed under metal free conditions. It has been considered that hydrogenation of ketones using FLPs is difficult to proceed due to the high oxophilicity of the borane atom. However, the reaction is successfully achieved by using diethyl ether as a Lewis base.