Counterion Enhanced Organocatalysis: A Novel Approach for the Asymmetric Transfer Hydrogenation of Enones

Abstract

We present a novel strategy for organocatalytic transfer hydro- genations relying on an ion-paired catalyst of natural l-amino acids as main source of chirality in combination with racemic, atropisomeric phosphoric acids as counteranion. The combina- tion of a chiral cation with a structurally flexible anion resulted in a novel chiral framework for asymmetric transfer hydro- genations with enhanced selectivity through synergistic effects. The optimized catalytic system, in combination with a Hantzsch ester as hydrogen source for biomimetic transfer hydrogena- tion, enabled high enantioselectivity and excellent yields for a series of ,-unsaturated cyclohexenones under mild condi- tions. Moreover, owing to the use of readily available and chiral pool-derived building blocks, it could be prepared in a straightforward and significantly cheaper way compared to the current state of the art.