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

Organic & Biomolecular Chemistry Pub Date: 2017-10-27 DOI: 10.1039/C7OB02212C

Abstract

Stereochemical models and mechanistic insights are proposed for [2t + 2i + 2i] annulations of thioketenes and imines on the basis of experimental and computational investigations. In the [2t + 2i + 2i] 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-[2t + 2i + 2i] 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.

Graphical abstract: Stereochemistry and mechanistic insights in the [2t + 2i + 2i] annulations of thioketenes and imines
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