Aryloxide‐Facilitated Catalyst Turnover in Enantioselective α,β‐Unsaturated Acyl Ammonium Catalysis

A new general concept for α,β‐unsaturated acyl ammonium catalysis is reported that uses p‐nitrophenoxide release from an α,β‐unsaturated p‐nitrophenyl ester substrate to facilitate catalyst turnover. This method was used for the enantioselective isothiourea‐catalyzed Michael addition of nitroalkanes to α,β‐unsaturated p‐nitrophenyl esters in generally good yield and with excellent enantioselectivity (27 examples, up to 79 % yield, 99:1 er). Mechanistic studies identified rapid and reversible catalyst acylation by the α,β‐unsaturated p‐nitrophenyl ester, and a recently reported variable‐time normalization kinetic analysis method was used to delineate the complex reaction kinetics. Turnover a new leaf: A new concept in α,β‐unsaturated acyl ammonium catalysis uses aryloxide release from an α,β‐unsaturated aryl ester substrate to facilitate catalyst turnover. Enantioselective isothiourea‐catalyzed Michael addition of nitroalkanes to α,β‐unsaturated p‐nitrophenyl esters was achieved in good yield and with excellent enantioselectivity. Kinetic analysis was used to probe the reaction mechanism.