Cooperation between an alcoholic proton and boryl species in the catalytic gem-hydrodiborylation of carboxylic esters to access 1,1-diborylalkanes

Abstract

Cooperation between an alcoholic proton and boryl species in the selective gem-hydrodiborylation of carboxylic esters is described. The simple iron salt FeBr₂ was used as an effective catalyst. Experimental evidence suggests that the ester is initially hydrogenated with the alcoholic proton as the hydride source to provide an intermediate aldehyde, followed by a gem-diborylation with B₂pin₂ (bis(pinacolato)diboron) to selectively achieve gem-hydrodiborylation. The structure of the alkoxide used is important for chemoselective transformation of the ester functional group. When tert-butoxide is used, protonation of the iron boryl species to form an iron hydride species is rate-limiting, making hydrodiborylation of the ester selectively occur. When ethoxide or methoxide is used, the protonation of the iron boryl species to form an iron hydride species is relatively facile, making hydrogenation of the ester to an alcohol selectively occur with the alcoholic proton acting as the hydride source. With NaO^tBu as the base and EtOH as the proton source, various aliphatic esters like ethyl acetate were selectively converted to 1,1-diborylalkanes in the presence of B₂pin₂. Diverse transformations of the obtained 1,1-diborylalkanes exhibit the advantages of this methodology for hetero multi-functionalization of ester functional groups for the synthesis of various value-added molecules.