Stereochemistry and mechanistic insights in the [2 + 2 + 2] annulations of thioketenes and imines

Stereochemical models and mechanistic insights are proposed for [2 t + 2 i + 2 i ] annulations of thioketenes and imines on the basis of experimental and computational investigations. In the [2 t + 2 i + 2 i ] annulations involving cyclic imines, the zwitterionic intermediates generated from monosubstituted thioketenes and the cyclic imines undergo a stepwise nucleophilic endo -addition/ Si -face attack pathway with a second imines molecule, giving initially (2,4)- cis -(4,5)- cis -[2 t + 2 i + 2 i ] annuladducts, which completely epimerize into the corresponding (2,4)- cis -(4,5)- trans -annuladducts under basic reaction conditions. The annuloselectivity of thio-Staudinger cycloadditions is dependent on the substituents of both thioketenes and imines. The reactivities and annuloselectivities of Staudinger, thio-Staudinger, and sulfa-Staudinger intermediates are compared. The mechanism and stereochemistry of [2 t + 2 i + 2 i ] annulations between one thioketene molecule and two imine molecules have been studied experimentally and theoretically.