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.