Regioselectivity for the Rh(I)-catalyzed Annulation of 1,2,3-Thiadiazoles with Alkynes: Experimental and Computational Analysis Reveal the Surprising Role of the Alkyne Substituent

The Rh(I)-catalyzed denitrogenative annulation reactions of 1,2,3-thiadiazoles are a new direct approach to synthesizing densely functionalized heterocycles. The synthesis of multisubstituted thiophenes is possible using this methodology, however the difficulty to predict which regioisomer will form is a current limitation. In this current work, systematic computational and experimental studies were performed addressing how the terminal alkyne substituent effects regioselectivity. The data revealed that the electronic and steric properties of the alkyne substituent effects regioselectivity, including whether the group is aryl or alkyl. The insight gained from this study has allowed the development of a framework for predicting regioselectivity for the Rh(I)-catalyzed denitrogenative transannulation of 1,2,3-thiadiazoles with terminal alkynes.