title: Substrate-controlled and site-selective [3+2] cycloadditions of N-heterocyclic carbene derived ambident dipoles.
abstract: 2-aryl thiocarbamoyl benzimidazolium and imidazolinium inner salts derived from benzimidazole and imidazoline carbenes are unique ambident C-C-S and C-C-N 1,3-dipolar systems, which undergo highly efficient and site-selective cycloaddition reactions with dimethyl acetylenedicarboxylate or dibenzoylacetylene to furnish spiro(imidazole-2,3'-thiophene) derivatives in excellent yields. When treated with ethyl propiolate, methyl acrylate or acrylonitrile, spiro(imidazole-2,3'-pyrrole) derivatives were formed in good yields. Theoretical studies revealed an asynchronous concerted mechanism for both the C-C-S and C-C-N 1,3-dipolar cycloaddition reactions. The site selectivity in the [3+2] cycloaddition reaction of ambident 1,3-dipoles was predictably regulated by both the electronic and steric effects of dipolarophiles.
authors.affiliation: Chemistry Department, Beijing Normal University, Beijing 100875, China. ycheng2@bnu.edu.cn
authors.forename: Ying
authors.initials: Y
authors.lastname: Cheng
authors.forename: Mei-Fang
authors.initials: MF
authors.lastname: Liu
authors.forename: De-Cai
authors.initials: DC
authors.lastname: Fang
authors.forename: Xue-Mei
authors.initials: XM
authors.lastname: Lei