FACILE SYNTHESIS OF CHIRAL BENZIMIDAZOLIUM SALTS AND THE APPLICATION IN ASYMMETRIC CATALYTIC BORYLATION

摘要

A synthetic method towards chiral benzimidazolium salts is developed. The stereocenter is introduced by direct aromatic substitution of 2-fluoronitrobenzene with optically pure amines. After nitro group reduction, selective arylation of the primary amine is achieved via copper catalyzed Chan-Lam coupling reaction. Finally, cyclization of the diamine with HC(OMe)(3) afforded the desired chiral benzimidazolium salts. In situ generated benzimidazole carbenes show potential application for asymmetric catalytic borylation of alpha,beta-unsaturated esters, providing up to 85% ee value with a catalyst loading of only 0.5 mol%.