Development of dinuclear vanadium catalysts for enantioselective coupling of 2-naphthols via a dual activation mechanism.

This review describes our recent efforts in the development of chiral dinuclear vanadium complexes which work as dual activation catalysts for oxidative coupling of 2-naphthols. A chiral dinuclear vanadium(IV) complex (R(a),S,S)-1a possessing (S)-tert-leucine moieties at the 3,3'-positions of the (R)-binaphthyl skeleton was developed, and found to promote oxidative coupling of 2-naphthol to afford (S)-1,1'-bi-2-naphthol (BINOL) with 91% ee. To verify the dual activation mechanism, mononuclear vanadium(IV) complex (S)-10 was also prepared. Kinetic analysis revealed that the reaction rate of oxidative coupling of 2-naphthol promoted by (R(a),S,S)-1a is 48.3 times faster than that of (S)-10. In the coupling reaction, the two vanadium metals in the chiral complex activate two molecules of 2-naphthol simultaneously achieving a high reaction rate with high enantiocontrol. Since the dinuclear vanadium(IV) complex was found to be readily oxidized to afford a corresponding vanadium(V) species during preparation in air, a new synthetic procedure using VOCl3 and a convenient one-pot procedure using VOSO4 under O2 have been applied towards dinuclear vanadium(V) complexes (R(a),S,S)-2. The coupling of 2-naphthol was catalyzed by (R(a),S,S)-2 with reaction rates enhanced 2.3 times faster than that of (R(a),S,S)-1a, which is likely a precatalyst. To the best of our knowledge, (R(a),S,S)-1a, 2, and 3 show considerably higher catalytic activity than previously reported vanadium complexes for oxidative coupling of 2-naphthols.