Alkenyl- and Aryl-Borane Nucleophiles in Enantioselective Iridium-Catalyzed Allylic Substitution of Vinyl Epoxides.

Abstract

The catalytic asymmetric Petasis reaction represents a practical approach for synthesizing highly valuable chiral amine building blocks. However, despite the potential that this reactivity provides, the extension of the mechanistic framework to alternative electrophilic fragments is noticeably underdeveloped. We report herein the first Ir-catalyzed allylation of alkenyl, aryl, and alkynyl boranes with racemic vinyl epoxides or vinyl aziridines via an enantioselective 1,4-boronate rearrangement. Mechanistic studies reveal that the high levels of stereoselectivity arise due to tandem dynamic kinetic resolution and kinetic resolution processes, with computational analysis suggesting that a stabilizing interaction between the alkenyl boronate π-system and the electrophile facilitates the key transition state. The utility of this methodology is demonstrated in a concise, enantioselective two-step synthesis of the phytotoxin (R)-pyricuol.