Solvation studies
Solvation studies (Table, Supplementary information) on IH, IMe,
INH2, ICN, ISH, ISMe, IOH and IOMe catalyzed alkyne
hydro- thiolation and selenation reactions has been carried out using
moderately polar solvent THF. Energetics trend calculated using single
point M06-2X/aug-cc-pVTZ values remain almost the same in THF as in gas
phase. In addition, exclusive Z-anti -Markovnikov selectivity
could be observed. In THF ∆E1 decreases for both hydro-
thiolation and selenation reactions, while ∆E2 (rate
determining energy barrier) generally increases for systems with much
reduced C2-S/Se5 interaction (IMe, INH2, ISH and ISMe)
and decreases for the cases with strong C2-S/Se5 interaction (IH, ICN,
IOH and IOMe). For instance, in IH, TS2 stabilization (by 17.78
kcal/mol) in THF accounts for the lowering of ∆E2 in
THF. Inorder to evaluate the effect of solvent polarity, IH and
INH2 catalyzed alkyne hydrothiolation has been studied
in a polar aprotic solvent, DMSO and a polar protic solvent, MeOH.
Similar trend of ∆E1, ∆E2 as well as
selectivity is observed in both these solvents. From these studies it
could be concluded that, generally in solvent media those systems with
C2-S/Se5 interactions in TS2 proceeds with lowered ∆E2in comparison to gas phase. Comparing the ∆E2 values of
IH catalyzed alkyne hydrothiolation in the three considered solvents
(THF, DMSO and MeOH), it is clear that DMSO provides the lowest energy
barrier (by 1.65 and 1.73 kcal/mol with respect to MeOH and THF
respectively). But for INH2 catalyzed alkyne
hydrothiolation, the ∆E2 value differences among the
three solvents remain less than ~0.5 kcal/mol. Thus it
could be concluded that a polar aprotic solvent has the effect of
lowering the reaction energetics by stabilizing the TS2 having C2-S/Se5
interaction.