Dual activation in oxidative coupling of 2-naphthols catalyzed by chiral dinuclear vanadium complexes

Abstract

An efficient enantioselective oxidative coupling of 2-naphthol derivatives based on a concept of dual activation catalysis is realized. A chiral dinuclear vanadium(IV) complex ( R a, S, S)- 1e possessing ( S)- tert-leucine moieties at the 3,3′-positions of the ( R)-binaphthyl skeleton was developed, which was found to promote the oxidative coupling of 2-naphthol to afford ( S)-BINOL with 91% ee. To verify the dual activation mechanism, mononuclear vanadium(IV) complex ( S)- 8 was also prepared. Kinetic analysis revealed that the reaction rate of oxidative coupling of 2-naphthol promoted by ( R a, S, S)- 1e is 48.3 times faster than that of ( S)- 8. The two vanadium metals in the chiral complex activate two molecules of 2-naphthol simultaneously in an intramolecular manner coupling reaction, achieving a high reaction rate with high enantiocontrol. Reaction mechanisms of the oxidative coupling reaction promoted by either vanadium(IV) or vanadium(V) complexes are also described.