Aryloxide-Facilitated Catalyst Turnover in Enantioselective ,-Unsaturated Acyl Ammonium Catalysis

A new general concept for ,-unsaturated acyl ammonium catalysis is reported that uses p-nitrophenoxide release from an ,-unsaturated p-nitrophenyl ester substrate to facilitate catalyst turnover. This method was used for the enantioselective isothiourea-catalyzed Michael addition of nitroalkanes to ,-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 ,-unsaturated p-nitrophenyl ester, and a recently reported variable-time normalization kinetic analysis method was used to delineate the complex reaction kinetics.