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?

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 alpha-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 N-2 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.

Automated summary

In-depth mechanistic understanding of the Rh(i)-catalyzed transannulation of 1,2,3-thiadiazoles with alkenes, alkynes, and nitriles was gained through Density Functional Theory (DFT) calculations. The key intermediate in this reaction is suggested to be a four-membered cyclometalated Rh(iii) complex due to the easy chelation of the S atom with Rh. The regioselectivities in the product formation are not only determined by the feasible migratory insertion pathway, but also by the feasibility of the subsequent reductive elimination.