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

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posted on 2018-12-07, 00:00 authored by Jun-Jie Tian, Ning-Ning Zeng, Ning Liu, Xian-Shuang Tu, Xiao-Chen Wang
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.