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.