Divergent Radical, Dimerization, and Carbene Formation from One-Electron Reduction of 2-Cyanoimidazolium Salts

Abstract

Nitriles are indispensable synthetic building blocks for the construction of nitrogen-containing molecules with broad applications in pharmaceuticals, agrochemicals, and materials science. While their conventional two-electron reductions enable transformations into amines, carboxylic acids, and aldehydes, their one-electron reduction pathways, which lead to radical formation, remain largely unexplored. In this study, we report the rational synthesis of 2-cyanoimidazolium salts and investigate their one-electron reduction pathways. Upon reduction, these compounds yield both organic radicals and free carbenes. Notably, the free carbene formation occurs via electrophilic cyanide transfer proceeding through a mesoionic carbene (MIC) intermediate. We further demonstrate that the steric bulkiness of imidazolium N-substituents plays a critical role in determining radical stability. The imidazolium with a sterically bulky substituent generates the persistent radical, which is characterized by electron paramagnetic resonance (EPR) spectroscopy. In contrast, heating the radical with a less bulky N-substituent leads to dimerization, forming new C(sp)─C(sp²) and C(sp²)─N bonds. Insights into their mechanism are supported by density functional theory (DFT) calculations.