Novel achiral biphenol-derived diastereomeric oxovanadium(IV) complexes for highly enantioselective oxidative coupling of 2-naphthols.

Abstract

Optically pure 1,1’-binaphthol and its derivatives have been evaluated as versatile chiral auxiliaries and ligands in asymmetric transformations. Research in this area has provided many efficient and useful methods for the preparation of key chiral building blocks, some of which have been used for the construction of complex natural products.[1] They have also been extensively applied to the preparation of chiral organic materials.[2] The wide-ranging and important applications of such compounds in organic synthesis have stimulated great interest in developing efficient methods for their preparation.[3] Compared to the well-established resolution of racemic binaphthol for the preparation of optically pure BINOL,[3a±f] catalytic asymmetric preparation of chiral binaphthols has developed much more slowly. The discovery of efficient catalysts for the highly enantioselective formation of optically active binaphthol and its derivatives is an attractive target. The oxidative coupling of 2-naphthols in the presence of a catalytic amount of a copper complex of a chiral amine has provided several promising results, but high enantioselectivity has been achieved only for the coupling of 3carboalkoxyl-2-naphthols (93% ee).[4] A photo-activated chiral [RuII(salen)(NO)] complex catalyzes the aerobic oxidative coupling of 2-naphthols with 33±71% ee.[5] Chen et al. and Uang et al. independently designed similar oxovanadium(iv) complexes of chiral Schiff bases for the asymmetric oxidative coupling of 2-naphthols with moderate enantioselectivities of up to 62% ee.[6] We developed the catalyst (R,S)-1c for the oxidative coupling of 2-naphthol with high enantioselectivity, and found that the chiral centers on the amino acid part and the axially chiral binaphthyl unit are both crucial to stereocontrol by the catalyst.[7] However, a drawback is that the chiral oxovanadium complex must be prepared from an optically pure 3,3’diformyl-2,2’-dihydroxy-1,1’-binaphthol and (S)-amino acid. The catalyst is only highly enantioselective when the two COMMUNICATIONS