Mechanistic Basis for the Iridium-Catalyzed Enantioselective Allylation 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 carbonates. The reaction proceeds through an allylation-induced 1,2-metalate shift of the alkenyl boronate to form non-adjacent stereocenters. Mechanistic investigations outline the overall catalytic cycle and reveal trends in reactivity and selectivity. Analysis of relative stereochemistry in products derived from 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 dependence in boronate, the catalyst, and both the slow- and fast-reacting enantiomer of allylic carbonate as well as the turnover-limiting step of the reaction. Determination of nucleophile-specific parameters N and sN for alkenyl boronate complexes enabled comparison to other classes of nucleophiles. DFT calculations indicate the addition of the alkenyl boronate to the cationic Ir(π-allyl) intermediate and the 1,2-metalate shift occur in a concerted mechanism. The stereoselectivity is determined by ligand-substrate steric repulsions and dispersion interactions in the syn addition transition state. Hammett studies supported the computational results with regard to electronic trends observed with both aryl-derived alkenyl boronates and aryl carbonates.