Impact of counteranions on N-heterocyclic carbene gold(i)-catalyzed cyclization of propargylic amide

Abstract

N-Heterocyclic carbene (NHC) Au(I)-catalyzed organic synthesis has recently been receiving increasing attention, especially with the activation of alkynes. In contrast, counteranions, being widely problematic in Au(I)-catalyzed transformations, are commonly considered as innocent partners and are not respectably included in a computational model. Herein, we report density functional theory (DFT) investigations of the Au(I)-catalyzed cyclization of propargylic amides to exploit the mechanistic effect of several counteranions to shed some light for further future developments. Among the counteranions used in this study, NTf₂⁻, ClO₄⁻, TsO⁻, TFA⁻, TfO⁻, MsO⁻, and SbF₆⁻, both the cyclization and protodeauration step favor the 5-exo-dig product over the 6-endo-dig product when the alkyne moiety is terminated with hydrogen. These anions reveal a crucial influence on the energy profile through lowering the barriers of the reaction. Mechanistically, the results obtained from all counteranions show that the protodeauration is slower than the cyclization. By using an energetic span model, the results clearly indicate that the rate-determining state is the protodeauration step for all counteranions, and thus protodeauration is the turnover-limiting step. The turnover frequency (TOF) results for the formation of the 5-exo-dig product show cyclization reactivity in the order of MsO⁻ > TFA⁻ > ClO₄⁻ > NTf₂⁻ > TfO⁻ > TsO⁻ ≫ SbF₆⁻, whereas an order of TFA⁻ > MsO⁻ > NTf₂⁻ > TfO⁻ ≈ ClO₄⁻ > SbF₆⁻ ⋙ TsO⁻ is calculated for the protodeauration, suggesting that SbF₆⁻ and TsO⁻ are disfavored due to their slow protodeauration. In this regard, and for the 6-endo-dig pathway, our conclusions demonstrate an order of TfO⁻ > TFA⁻ > MsO⁻ > NTf₂⁻ > ClO₄⁻ > TsO⁻ ⋙ SbF₆⁻ for the cyclization and TFA⁻ > TsO⁻ > MsO⁻ > TfO⁻ > NTf₂⁻ > ClO₄⁻ ⋙ SbF₆⁻ for the protodeauration, advocating that the anions SbF₆⁻, NTf₂⁻ and ClO₄⁻ are unlikely partners for the 6-endo-dig pathway because of their slow protodeauration. Finally, the findings here advise that any engineering of the counteranion to increase the efficiency of catalytic system would be more effective on the protodeauration step rather than the cyclization step.