Analysis of the effect of changing the heterocycle,
increasing the conjugation, ring expansion and substitution
Effect of factors such as changing the heterocycle, increasing the
conjugation, ring expansion of the N-heterocyclic core and substitution
at the heteroatom has been explored. Oxazol-2-ylidene (IOH) and
Thiazol-2-ylidene (ISH) has been chosen to look at the effect of
changing the heterocycle. IOH is computed to have increased
∆E1 and ∆E2 values in comparison to IH
for acetylene hydro- thiolation and selenation (Table 2). SE studies
revealed that the increase in ∆E1 for both reactions is
caused by predominant TS1 destabilization (Table 3). The more electron
deficient nature of carbene carbon in IOH relative to IH was already
reported. 49 This reduced Bronsted basicity of IOH
supports the increased ∆E for the proton abstraction step
(∆E1).WBI values suggested TS1 in IOH to be a late TS in
comparison to IH in agreement with our earlier conclusion (Table 2, 4).
Similarly, increase of ∆E2 in both reactions has been
found due to the INT stabilization (Table 2, 3). Stronger C2-S/Se5
interaction is observed with INT of IOH relative to IH thus
substantiating the INT stabilization and subsequent increased
∆E2 (Figure 2). ISH is computed to have an increased
∆E1 and decreased ∆E2 in comparison with
IH (Table 2). SE studies proved that TS1 destabilization in the first
step and INT destabilization in the second step accounts for the ∆E
variations (Table 2, 3). The intermediate and TS2 showed much reduced
C2-S/Se5interaction here (Figure 2) (intermediate with much reduced
C2-S/Se5 interaction will hereafter be notated as INT’) and it was found
that the resultant ∆E2 is lowered with respect to IH by
9.95 and 5.36 kcal/mol for acetylene hydro- thiolation and selenation
respectively (Table 2).
With benzimidazol-2-ylidene (BH) as the catalyst, the characterized
pathway proceeded through the INT with marked C2-S/Se5 interaction for
both reactions and thus, with increased ∆E2 in
comparison to IH. WBI studies showed greater C2-S/Se5 interaction in BH
relative to IH and hence increased ∆E2 (Figure 2).
∆E1 also showed an increase. SE studies revealed that it
is the TS1 destabilization/INT stabilization that determines the
∆E1 /∆E2 variation relative to IH (Table
2, 3). The explanation regarding the late TS1 disfavoring the reaction
is applicable here as well (Table 4). Thus, increasing the conjugation
does not positively influence the catalytic activity here.
For tetrahydropyrimidin-2-ylidene (TH) catalyzed acetylene
hydro-thiolation and selenation reactions, ∆E1 is
computed to be lowered in comparison to IH (Table 2). This ring-
expanded NHC exhibit distinct electronic characteristics over their five
membered counter parts. The widening of the NCN angle in six membered
NHC increases the basicity, rather than electron delocalization50 and this makes chalcogenol activation step more
feasible. WBI values also support the formation of an early TS in TH.
The computed ∆E2 for TH catalyzed thiolation and
selenation reactions proceeding through INT are 33.94 and 30.94 kcal/mol
respectively, i.e, IH catalyzed C2-S/Se5 bond formation was slightly
more feasible than TH catalyzed reaction (Table 2). WBI studies
indicated that stronger C2-S/Se5 interaction in TH relative to IH caused
the increased ∆E2 values (Table, Supplementary
information). Permidin-2-ylidene (PH) catalyzed acetylene
hydro-thiolation/selenation was also characterized to have stronger
C2-S/Se5 interaction in INT and increased ∆E2 in
comparison with IH (Table, Supplementary information). Similar to IH
catalyst, TS1 and INT stabilization respectively caused the lowering of
∆E1 and increase of ∆E2 in TH catalyzed
reactions (Table 2, 3).
Catalytic pathway for N,N-dimethyl/N-methyl substituted analogues of IH,
IOH, ISH, BH, TH and PH were explored to evaluate the effect of electron
donating substituents. This substitution rendered the pathway of IMe
different from IH. The reaction proceeded through INT’ with much reduced
C2-S/Se5 interaction (Table, Supplementary information), while all other
systems closely resemble the pathway of their unsubstituted
counterparts. Thus, ∆E2 of IMe is drastically reduced
while only a slight lowering has been observed with all other NHCs
(Table 2) obviously due to the disparity in INT/INT’ destabilization.WBI
analysis revealed reduced C2-S/Se5 interaction in the methyl substituted
intermediates (Table, Supplementary information) and hence accounts for
the observed destabilization. ∆E1 was also reduced with
methyl substitution for all NHC systems. This lowering of
∆E1 in acetylene hydro- thiolation and selenation
reactions is caused by quantitatively predominant TS1 stabilization in
all NHC systems with a few exceptions (IOMe, ISMe, hydroselenation cases
of TMe and PMe, where RC destabilization accounts for the observed
∆E1 lowering) (Table 2, 3). Charge and WBI studies
reveal that TS1 in methyl substituted systems are generally marked with
low positive charges on C2 and H4 and low negative charge on S/Se5and
having geometry close to the reactants (early TS1) (Table 4).