Intramolecular Cyclizations of Vinyl-Substituted N,N‑Dialkyl Arylamines Enabled by Borane-Assisted Hydride Transfer

Catalytic amounts of B­(C₆F₅)₃ have been found to be able to promote the intramolecular cyclization of vinyl-substituted N,N-dialkyl arylamines to afford nitrogen-containing heterocycles. Our mechanistic studies indicate the reaction is initiated by abstraction of an α-hydride from an N-alkyl substituent by B­(C₆F₅)₃, which is followed by cyclization, and is concluded by delivery of the hydride to the cyclic cationic intermediate. The dual roles of B­(C₆F₅)₃, first as an oxidant and then as a hydride-carrying reductant, have enabled a rare redox-neutral cyclization process between a sp³ carbon and an electron-rich olefin without using a transition metal or an external oxidant.