Relay Catalysis: Manganese(III) Phosphate Catalyzed Asymmetric Addition of β-Dicarbonyls to ortho-Quinone Methides Generated by Catalytic Aerobic Oxidation.

The combination of an in situ formed MnL3 complex (HL = Hacac or R(C═O)CH2CO2R) and a chiral phosphoric acid HX* allows for a fully catalytic, asymmetric synthesis of 4H-chromenes starting from 2-alkyl-substituted phenols. The aerobic oxidation toward a transient ortho-quinone methide was efficiently catalyzed by a manganese(III) species MnL3 while the ensuing Michael addition of β-dicarbonyl compounds proved to be catalyzed by a chiral manganese phosphate MnL2X*.