Computationally unraveling the mechanism and selectivity of five and six membered N‐heterocyclic carbene‐catalyzed alkyne hydrochalcogenation

The present work is intended to bring to the forefront a relatively less explored area of N‐Heterocyclic Carbene (NHC) catalyzed alkyne hydro‐ thiolation and selenation reactions by exploring the reaction mechanism. Reaction mechanism involves chalcogenol activation followed by alkyne insertion and the second step is the rate determining step. A comparison with the reported uncatalyzed gas phase reaction showed that a simple imidazol‐2‐ylidene catalyst can lower the free energy barrier by 28.74 and 21.73 kcal/mol respectively for acetylene hydro‐ thiolation and selenation reaction. All the employed NHCs are proved to be better catalyst for both hydrothiolation and hydroselenation. Factors such as changing the heterocycle, increasing the conjugation, ring expansion and electronic/steric substitution at the heteroatom were found to affect the energy barriers due to the geometric variations at NHC‐carbon and S/Se in the intermediate and the transition structure corresponding to alkyne insertion. Effect of solvent polarity on the reaction energetics and selectivity has also been analyzed employing THF, DMSO and MeOH as the solvents. NHC catalyzed alkyne hydro‐thiolation and selenation reactions proceed through chalcogenol activation followed by alkyne insertion. A comparison with reported uncatalyzed reaction showed considerable reduction in energy barriers. Effect of solvent polarity on the reaction energetics and selectivity are also analyzed.