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

Abstract

We present a novel strategy for organocatalytic transfer hydrogenations 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 combination of a chiral cation with a structurally flexible anion resulted in a novel chiral framework for asymmetric transfer hydrogenations with enhanced selectivity through synergistic effects. The optimized catalytic system, in combination with a Hantzsch ester as hydrogen source for biomimetic transfer hydrogenation, enabled high enantioselectivity and excellent yields for a series of α,β‐unsaturated cyclohexenones under mild conditions. 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.