Among the various NHC catalysts considered, IH has been chosen for
detailed substitution studies. N,N-disubstitution with an electron
donating (-NH2) and withdrawing group (-CN) has been
done to see how these affect the energetics of the reaction. On
comparing the ∆E values of N,N-disubstituted systems with parent system,
electron donating –NH2 has been found to lower the
energy barriers (as already observed for IMe) while electron withdrawing
-CN increases both ∆E1 and ∆E2 ( Table
2). The variation of ∆E1 in acetylene hydro- thiolation
and selenation reactions for INH2/ICN with respect to IH
is caused by quantitatively predominant TS1
stabilization/destabilization (Table 2, 3). WBI and AIM studies were
performed on these TSs and the results supports our conclusion of an
early TS1 favoring the reaction (Table 4 and supplementary information).
Similarly the variation of ∆E2 in these systems is due
to INT’ destabilization/INT stabilization respectively (Table 2, 3).
Computed WBI values also support these results (Figure 2).
Effect of a widely studied sterically bulky substituent (Mes) on the
mechanism and energetics of the reaction is also analyzed and it was
clear that the geometry of INT’ and TS2 are the same as in the case of
electron donating groups, with lowered energy barriers as compared to
the parent system but higher with respect to IMe and
INH2. Earlier conclusion of early TS nature and much
reduced C2-S/Se5 interaction in INT’ (destabilized INT’) substantiate
the observed lowering of energy barriers for either steps in both
reactions. SE studies are in good agreement with the other electron
donating N,N-disubstitution cases.