2-Naphthol oxidative homo-coupling

As early as in 1993, Ohkubo and co-workers reported a photocatalytic system consisting of a chiral Ru complex A-[Ru(menbpy)3] [menbpy = 4,4 -di-(lR,2S,5R)-(-)-memthoxycarbonyl-2,2 -bipyridine] possessing a C3 molecular helicity as the photosensitizer and Co(acac)3 (acac = acetylacetonato) as the oxidant which could catalyze the oxidative homo-coupling of 2-naphthols (Scheme 3.13). Under the radiation of visible light, A-[Ru(menbpy)3] " can... 

It should be noted that in most cases it is Cu-eatalyzed asymmetric oxidative biaryl homo-coupling reactions that are deseribed. On the contrary, successful examples of asymmetric oxidative biaryl eross-coupling reactions are rare. Habaue and co-workers discussed a highly eross-selective oxidative biaryl coupling between 3-hydroxy-2-naphthoates and 2-naphthols without ester functionality by a chiral Cu-BOX catalyst.The reactions proceed in a highly cross-eoupling selective manner with moderate enantioselectivity (up to 70% ee) (Scheme 3.7). Later, it was found that Lewis acids such as... 

Uang and co-workers, on the other hand, employed similar chiral oxo-vanadium(iv) complexes to realize asymmetric oxidative biaryl coupling reactions. It was found that the addition of acidic additive was crucial for obtaining superior reactivity, probably due to the disproportionation of V" species to V " and V species under strong acidic conditions. Systematic evaluation of various acid additives revealed that TMSCl was the best choice. The 2-naphthol derivatives eould be converted to the corresponding homo-coupling products at room temperature within 24 hours. The loading of complex 23 and TMSCl could be lowered to only 2 mol% (Scheme 3.9). 

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