Mechanistic basis for the Iridium-Catalyzed Enantioselective Al-lylation of Alkenyl Boronates

Abstract

Iridium(phosphoramidite) complexes catalyze an enantio- and diastereoselective three-component coupling reaction of alkenyl boronic esters, organolithium reagents and secondary allylic aryl carbonates. The reaction proceeds through an allylation-induced 1,2-metalate shift of the alkenyl boronate forming non-adjacent stereocenters. Mechanistic investiga-tions outline the overall catalytic cycle and reveal trends in reactivity and selectivity. Analysis of relative stereochemistry in a variety of 1,1-disubtituted alkenyl boronates provides insight into the transition state of the addition and indicates a concerted pathway. Kinetic analysis of the reaction revealed the kinetic order dependance in boronate, catalyst, and both the slow- and fast-reacting enantiomer of allylic carbonate as well as the turnover-limiting step of the reaction. Hammett studies explored substituent effects in both aryl-derived alkenyl boronates and aryl carbonates. Initial investigations into the migratory selectivity of the 1,2-metalate shift were also examined using (bis)alkenyl boronates.