Computational insights into the mechanisms of Ru-catalyzed cycloisomerization of 2-ethynylaniline and 2-(2-propynyl)tosylanilide: The role of pyridine in assisting the metal-vinylidene formation

Abstract

Mechanistic studies of ruthenium-catalyzed cycloisomerization of 2-ethynylaniline and 2-(2-propynyl)tosylanilide were carried out using DFT calculations. A pyridine assisted hydrogen transfer pathway was unveiled for the formation of Ru-vinylidene complex. The proposed pathway was applied to rationalize the origin of the regioselectivity, 5-exo vs 6-endo cyclization, in the ruthenium catalyzed cycloisomerization of 2-(2-propynyl)tosylanilide. The cycloisomerization of 2-(2-propynyl)tosylanilide in the presence of pyridine produced a 6-endo cyclized product via the Ru-vinylidene intermediate assisted by pyridine while its absence afforded a 5-exo cyclized product via a direct nucleophilic addition. This study sheds more light on alternative pathways for reactions involving alkyne-vinylidene species.