Bifunctional enantioselective ligands of chiral BINOL derivatives for asymmetric addition of alkynylzinc to aldehydes

Four analogous binaphthyl compounds (R)-3a-3d containing (R)-3,3'-bis(2-pyridyl) groups were synthesized by the conjugation of (R)-2,2'-dimethoxy-1,1'-binaphthyl-3,3'-diboronic acid [(R)-2] with 2-bromopyridine, 2-bromo-5-methylpyridine, 2-chloro-4-fluoropyridine and 2-chloro-3-(trifluoromethyl)pyridine via Pd-catalyzed Suzuki reactions, respectively. The application of the four chiral ligands in combination with Et2Zn and Ti(Oi-Pr)(4) to the asymmetric addition of phenylacetylene to various aldehydes has been studied. The results show that (R)-3a and (R)-3b are not good catalysts for the alkynylzinc addition to aldehydes, (R)-3d shows good enantioselectivity only for the alkynylzinc addition to aliphatic aldehydes, and (R)-3c exhibits excellent enantioselectivity for phenylethynylzinc addition to both aromatic and aliphatic aldehydes. All the four chiral ligands produced the opposite configuration of the propargylic alcohols to that of the chiral ligands.