Rh(I)-Catalyzed Denitrogenative Transformations of 1,2,3-Thiadiazoles: Ligand-Controlled Product Selectivity and the Structure of the Key Organorhodium Intermediate Revealed

Ligand-controlled rhodium­(I)-catalyzed denitrogenative transformations of a range of 4-vinyl-1,2,3-thiadiazoles possessing electron-donating substituents at the C5-position of the heterocycle have been demonstrated. With [Rh­(COD)2]­BF4, vinylic 1,2,3-thiadiazoles undergo an intramolecular transannulation reaction to afford substituted furans. In contrast, the [Rh­(COD)­DPPF]­BF4 catalytic system inhibits the intramolecular reaction but promotes intermolecular transannulation with both electron-deficient and electron-rich terminal alkynes, providing access to densely functionalized thiophenes with unexpected regioselectivity. Experimental and computational mechanistic studies were performed to gain insights into the Rh­(I)-catalyzed intramolecular transannulation of vinylic 1,2,3-thiadiazoles, with a focus on understanding the influence of the C5-substituent on reactivity and the role of the DPPF ligand. Importantly, our crystallographic data uncovered that the true structure of the organorhodium intermediate involved in Rh­(I)-catalyzed denitrogenative reactions of 1,2,3-thiadiazoles is likely to be a four-membered cyclometalated Rh­(III) complex.