Ni-Catalyzed Dearomative Cycloaddition of Alkynes to 10π Aromatic Benzothiophenes: Elucidation of Reaction Mechanism

Abstract

Abstract The Ni(0)-catalyzed cycloaddition of alkynes to stable 10π-electron aromatic benzothiophenes involves the insertion of the former into the C–S bond of the latter to form metastable 12π-electron non-aromatic benzothiepines featuring a seven-membered ring, with the reactivity of benzothiophenes strongly influenced by their substituents (e.g., MeO and F). Herein, the mechanism of the above dearomative cycloaddition is elucidated by computational and experimental methods. The promotional effects of benzothiophene substituents (MeO and F) on the catalytic reaction are rationalized by density functional theory computations. Moreover, computations demonstrate the superiority of Ni catalysts over Pd ones. The theoretical findings are confirmed by spectroscopic (operando X-ray absorption fine structure) analysis, and the combined results are used to design an alternative reaction-promoting substituent, the effect of which is then experimentally validated. A deep mechanistic understanding of this unprecedented dearomative cycloaddition is important for the design of a new range of reactions involving dearomatization.