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).