Insight into the Role of the Counteranion of an Imidazolium Salt in Organocatalysis: A Combined Experimental and Computational Study

N-Heterocyclic carbenes (NHCs) can serve as very reactive nucleophilic catalysts and exhibit strong basicity. Herein, we initiate a combined experimental and computational investigation of the NHC-catalyzed ring-closing reactions of 4-(2-formylphenoxy) but-2-enoate derivatives 1 to uncover the relationship between the counteranion of an azolium salt, the nucleophilicity and basicity of the carbene species, and the catalytic performance of the carbene species by taking imidazolium salts IPr center dot HX (X = counteranion, IPr = 1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene) as the representative precatalysts. The plausible mechanisms of IPr-mediated ring-closing reactions have been investigated by using DFT calculations. The hydrogen-accepting ability, assigned as the basicity of the counteranion of IPr center dot HX and evaluated by DFT calculations, is correlated with the rate of deprotonation of C2 in IPr center dot HX, which could be monitored by the capture of the free carbene formed in situ with elemental sulfur. The deprotonation of C2 in IPr center dot HX with a more basic anion gives rise to a higher concentration of the free carbene and vice versa. At a relatively low concentration, IPr prefers to show a nucleophilic character to induce the intramolecular Stetter reaction. At a relatively high concentration, IPr primarily acts as a base to afford benzofuran derivatives. These data comprehensively disclose, for the first time, that the counteranions of azolium salts significantly influence not only the catalytic activity, but also possibly the reaction mechanism.