Mechanistic insights into the Rh(i)-catalyzed transannulation of 1,2,3-thiadiazoles with alkenes, alkynes, and nitriles: Does the intermediacy of α-thiavinyl Rh-carbenoids play an important role?

Abstract

Density functional theory (DFT) calculations were performed to gain an in-depth mechanistic understanding of the Rh(I)-catalyzed transannulation of 1,2,3-thiadiazoles with alkenes, alkynes, and nitriles. Computational results indicate that the denitrogenation of 1,2,3-thiadiazoles promoted by the Rh(I) catalyst may not afford the commonly proposed α-thiavinyl Rh-carbenoid intermediate. Instead, the four-membered cyclometalated Rh(III) complex is suggested to be the key intermediate, which could be formed via the cleavage of the S–N bond of 1,2,3-thiadiazoles to generate a six-membered cyclometalated Rh(III) complex followed by N₂ extrusion. The easy chelation of the S atom with Rh is mainly responsible for the favorable formation of the four-membered cyclometalated Rh(III) intermediate. Next, the substrates alkenes, alkynes, and nitriles could undergo migratory insertion with the four-membered rhodacycle followed by reductive elimination to furnish the corresponding products. The origins of divergent regioselectivities for the Rh(I)-catalyzed transannulation of 1,2,3-thiadiazoles with alkenes, alkynes, and nitriles are discussed, respectively, which are not only determined by the feasible migratory insertion pathway, but also by the feasibility of the subsequent reductive elimination.