Towards phase-transfer catalysts with a chiral anion: inducing asymmetry in the reactions of cations

The ability of chiral anions, for example bis[1,1′-bi-2-naphtholato]borate, to induce asymmetry in the reactions of prochiral cations was investigated. Ion-pairing of a borate anion with an aziridinium ion was demonstrated by NMR spectroscopy. The addition of N-methyl indole to an iminium ion (benzylidenedimethylammonium) and the ring-opening of an aziridinium ion (1,2-diphenyl-3-azonia-spiro[2.4]heptane) with benzylamine were studied. Low, but significant, (<15%) enantioselectivities were induced in the formation of the diamine benzyl-(1′,2′-diphenyl-2-pyrrolidin-1-yl-ethyl)-amine. The counterion of the additive was found to have a remarkable effect on the yield and the sense of enantioselectivity of these reactions.