Mechanistic Insights into the Rh-Catalyzed Transannulation of Pyridotriazole with Phenylacetylene and Benzonitrile: A DFT Study.

Computational studies were carried out to explore the mechanisms of Rh(OAc)-catalyzed transannulation of pyridotriazole with phenylacetylene and benzonitrile, respectively. For the Rh(OAc)-catalyzed divergent cyclization with phenylacetylene, the major product (cyclopropene derivative) might be formed via [2 + 2] cycloaddition to yield a metallacyclobutene intermediate followed by a reductive elimination pathway, where carboxylate ligand lability is considered to release a vacant equatorial site for the formation of Rh-carbenoid. The cyclopropenation of phenylacetylene via a commonly proposed [2 + 1] cycloaddition pathway, where the tetrabridged framework of Rh(OAc) is preserved, however, might not play a dominant role. This is because the competing formal [2 + 3] pathway leading to the indolizine as a minor product is more likely to occur. The main factors responsible for the chemoselectivity in cyclization with phenylacetylene are discussed. For the Rh(OAc)-catalyzed transannulation of pyridotriazole with benzonitrile, a stepwise formal [2 + 3] pathway via a ylide intermediate is proposed regardless of whether the tetrabridged framework of Rh(OAc) is preserved or not.