Synthesis of Enantiomerically Pure Binaphthyl Derivatives. Mechanism of the Enantioselective, Oxidative Coupling of Naphthols and Designing a Catalytic Cycle

Abstract

Three different mechanisms have been identified for the CuCl2-mediated oxidative coupling of naphthols to binaphthyls in the presence of chiral amines (sparteine or α-methylbenzylamine); fair to excellent enantioselection has been observed (up to 100% ee). The choice of the dominant mechanism appears to be critically dependent on the structure of the partners to be coupled. Thus, for the self-coupling of 2-naphthol (1) the enantioselection is mainly controlled via a second-order asymmetric transformation of the product 4 (up to 100% ee). The same mechanism is responsible for the formation of enantiomerically enriched biphenanthrol 9 (76% ee). By contrast, enantio-differentiation in the formation of 5 and 7 results from the diastereoselective crystallization of the corresponding Cu(II)-amine-binaphthyl complex (46% and 44% ee, respectively), whereas an enantioselective coupling operates in the preparation of 6 (71% ee). A catalytic cycle has been designed, employing 10 mol % of CuCl2 (with AgCl to regenerate CuII) and sparteine (20 mol %); when applied to the asymmetric cross-coupling 1 + 3 → 6, the product (−)-6 (32% ee) was obtained in 41% yield. © 1993, American Chemical Society. All rights reserved.