Conclusions
The present study unraveled the reaction mechanismof NHC catalyzed alkyne- hydrothiolation and selenation reactions, identifying the second step as the rate determining step. Catalytic activity has been affirmed by comparing with reported uncatalyzed gas phase reactions. NHC catalyzed alkyne hydroselenation reaction was found to be more feasible than hydrothiolation. Stabilization energy studies identified TS1 stabilization in the first step and INT destabilization in the second step as the predominant factors that make hydroselenation more facile. The study also elaborates the energetic variations caused by changing the heterocycle, increasing conjugation, ring expansion and electronic/steric substitution at the heteroatom on NHC. Among these changes only sulfur heteroatom incorporation in the ring and substitution at the heteroatom with electron donating and sterically bulky group reduced the energy barriers. The computed natural charges and WBI values suggests that transition structure for step1 having geometry close to the reactants (early TS) and intermediate in the second step with much reduced C2-S/Se5 interaction favor the reaction. An overall Z-anti-Markovnikov selectivity could be observed for majority of the catalysts studied both in gas phase as well as in solvents, THF, DMSO and MeOH.