Synthesis of enantiomerically pure binaphthyl derivatives. Mechanism of the enantioselective, oxidative coupling of naphthols and designing a catalytic cycle

Three different mechanisms have been identified for the CuCl 2 -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 of 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, enantiodifferentiation in the formation of 5 and 7 results from the diastereoselective crystallization of the corresponding Cu(II)-amine-binaphtyl complex, wheras an enantioselective coupling operates in the preparation of 6. A catalytic cycle has been designed, employing 10 mol% of CuCl 2 (with AgCl to regenerate Cu II ) and sparteine(20 mol%); when applied to the asymmetric cross-coupling 1+3→6, the product (-)6 was obtained in 41%yield