Comparative Study of Dialkylboron Chlorides and Triflates for the Enolization of Ketones. The Controlled Stereospecific Synthesis of Either (E)- or (Z)-Enol Borinates

摘要

The ready synthesis and ease of handling of dialkylboron chlorides, R2BC1, as compared to the corresponding triflates, R1BOTf, give the chlorides a significant advantage as reagents to achieve the conversion of ketones into enol borinates quantitatively. A systematic study, in the case of two representative ketones, propiophenone and diethyl ketone, of the effect of the steric requirement of the R group (R2 = 9-BBN vs. Chx2), the amine, Et3N vs. i-Pr2EtN, and the leaving group, Cl vs. OTf, has revealed a controlled shift from preferential (Z)-enol borinate to preferential (E)-enol borinate. (mjm)