Metal promoted asymmetry in the 1,2-diboroethylarene synthesis: diboration versus dihydroboration

Abstract

Metal catalysed addition of diboranes to vinylarenes produces the desired 1,2-bis(boronate)ester and mono(boronate)esters as by-products. Their relative rate is a sensitive function between the nature of the catalytic system and the electronic effects of the substrate, that influences the mechanistic steps of the catalytic cycle. However, asymmetry is only induced as moderate enantiomeric excess values, providing an enantioface differentiation, between the bis- and mono(boronate)esters. Alternatively, the method based on the catalytic asymmetric dihydroboration/oxidation of alkynes as diphenylacetylene can provide 1,2-diphenyl-1,2-ethanediol (hydrobenzoin) with a selectivity of 68% mainly as the erythro isomer.