The Allyl Intermediate in Regioselective and Enantioselective Iridium-Catalyzed Asymmetric Allylic Substitution Reactions

摘要

Indium-catalyzed allylic substitution has become a powerful enantioselective method to form both carbon-heteroatom and carbon-carbon bonds (eq 1). When conducted with the proper combination of iridium precursor and phosphoramidite ligands, these reactions form branched substitution products with high regioselectivity and enanti-oselectivity.rnThese reactions likely occur through an allyliridium intermediate, but the composition and structure of this intermediate has been elusive. Studies from the authors' laboratory led to the isolation of a trapped form of the active catalyst, which possesses a cyclometalated phosphoramidite ligand. However, kinetic studies have implied that an allyliridium complex derived from this metallacycle would be a fleeting intermediate in the catalytic system. Here, we describe the independent synthesis, characterization, and reactivity of allyliridium complexes that are kinetically and chemically competent to be the elusive allyliridium intermediate. Structural data help reveal the origins of regioselectivity and explain how enantioselectivity is controlled by a stereocenter in the catalyst that is distant from that in the emerging product.