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

Abstract

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⋅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⋅HX and evaluated by DFT calculations, is correlated with the rate of deprotonation of C2 in IPr⋅HX, which could be monitored by the capture of the free carbene formed in situ with elemental sulfur. The deprotonation of C2 in IPr⋅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.

Proton Transfer Reactions of Triazol-3-ylidenes: Kinetic Acidities and Carbon Acid pKa Values for Twenty Triazolium Salts in Aqueous Solution

“…By contrast, azolium cations and counter anions are known to form hydrogen bonds both in the solid state and in concentrated solutions in non-hydroxylic solvents, which often results in the observation of large anion effects. [51][52][53][54][55][56] Under the present fully aqueous conditions, hydrogen bonding with solvent clearly outcompetes any interactions between the triazolium cation and counteranion.…”

Heavier chalcogenone complexes of bismuth(iii)trihalides: potential catalysts for acylative cleavage of cyclic ethers

“…24 Although, 5 was isolated from reaction between Cy 3 P(E) and IPr by chalcogen transfer route, 25 herein, we report the straight forward route to isolate 5 in good yield, together with a crystal structure. 24 Although, 5 was isolated from reaction between Cy 3 P(E) and IPr by chalcogen transfer route, 25 herein, we report the straight forward route to isolate 5 in good yield, together with a crystal structure.…”

A Stable Anionic Dithiolene Radical

“…37 However, di- and tri- sulfurization of the imidazole ring may be achieved by sulfurization of C4-metalated N-heterocyclic carbenes. The first anionic N-heterocyclic dicarbene (NHDC, 2 ) was synthesized by this laboratory in 2010 via C4-lithiation of a NHC ligand [L: = :C{N(2,6-Pr i 2 C 6 H 3 )CH} 2 ].…”