Catalytic Oxidative Coupling of 2-Naphthols by New Oxidovanadium Complex

The oxidative coupling of 2-naphthols is a straightforward pathway synthetic transformation as 1,1 -bi-2-naphthols (BINOLs) are widely used chiral inducers in synthetic organic chemistry [1]. Catalytic coupling of 2-naphthols by using transition metal complexes such as Cu, Fe, Ru, V, etc. is one of the most important methods for preparing BINOLs from the various starting naphthol derivatives [2]. Among the transition metal complexes that catalytically couple 2-naphthol derivatives to BINOLs, vanadium compounds are particularly useful for this process. Vanadium-mediated couplings occur via a favorable one-electron phenolic oxidation in mild reaction conditions with the further advantage that only water is formed as side product. Although several catalytic systems have been reported for preparing BINOLs, development of new catalysts and methods are gaining much attention [3]. By taking the advantage of vanadium complexes toward oxidation reactions, we report here a simple and convenient method for the aerobic oxidative coupling of 2-naphthol to BINOL in excellent yield by using a new vanadium complex as a catalyst. The vanadium complex was synthesized by the reaction of H3L (H3L= 2-(5- Bromo-2-hydroxyphenyl)-1,3-bis(2-(5-bromo-2-hydroxybenzylideneamino)ethyl)imidazolidine) and VO(acac)2 in methanol. The vanadium was characterized by elemental analysis and spectroscopic methods (FT-IR and UV-Vis) and was employed as catalyst for oxidative coupling of 2-naphthol. The effects of various parameters, including the temperature, the solvent and the reaction atmosphere were studied. The results of this study indicated that the oxidovanadium complex efficiently coupled 2-naphthol in CCl4 solvent and the conversion was high when the reaction was carried out at 40 C and in oxygen rich atmosphere.