SYNTHESIS OF ENANTIOMERICALLY PURE BINAPHTHYL DERIVATIVES - MECHANISMOF THE ENANTIOSELECTIVE, OXIDATIVE COUPLING OF NAPHTHOLS AND DESIGNING A CATALYTIC CYCLE

Three different mechanisms have been identified for the CuCl2-mediated oxidative coupling of naphthols to binaphthyls in the presence of chi ral amines (sparteine or alpha-methylbenzylamine); fair to excellent e nantioselection has been observed (up to 100% ee). The choice of the d ominant mechanism appears to be critically dependent on the structure of the partners to be coupled. Thus, for the self-coupling of 2-naphth ol (1) the enantioselection is mainly controlled via a second-order as ymmetric transformation of the product 4 (up to 100% ee). The same mec hanism is responsible for the formation of enantiomerically enriched b iphenanthrol 9 (76% ee). By contrast, enantiodifferentiation in the fo rmation 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 p reparation of 6 (71% ee). A catalytic cycle has been designed, employi ng 10 mol % Of CuCl2 (with AgCl to regenerate Cu(II)) and sparteine (2 0 mol %); when applied to the asymmetric cross-coupling 1 + 3 --> 6, t he product (-)-6 (32% ee) was obtained in 41% yield.