Origin of Asymmetric Induction in Bicyclic Guanidine-Catalyzed Thio-Michael Reaction: A Bifunctional Mode of Lewis Acid-Brønsted Acid Activation

In addition to a bifunctional Brønsted acid activation mode, an unconventional bifunctional mode of Lewis and Brønsted acid activations was revealed in a DFT study of bicyclic guanidine-catalyzed thio-Michael reaction. This activation mode provides an alternate reaction pathway for the C–S bond forming step and influences the final stereochemical outcome. The calculated turnover frequencies of the R- and S-products, based on the energetic span model, are in good accord with the observed high stereoselectivity toward the S-product.