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Impact of counteranions on N-heterocyclic carbene gold(i)-catalyzed cyclization of propargylic amide†
Hafiz Saqib Ali,Aqeel A. Hussein,Mohammed Obies
RSC Advances Pub Date : 01/19/2023 00:00:00 , DOI:10.1039/D2RA06210K
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.

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