Metal-free synthesis of benzimidazoles via pyridinium-catalyzed silane activation under atmospheric CO₂

Abstract

The conversion of CO₂ into valuable nitrogen-containing heterocycles represents an attractive and sustainable strategy in green chemistry. In this work, an efficient metal-free catalytic system was developed for synthesizing benzimidazoles under mild conditions. This system utilizes the ionic liquid 1-butyl-4-(dimethylamino)pyridinium bromide ([C₄DMAP]Br) in combination with triethoxysilane as a hydride donor. The ionic liquid provides a synergistic Brønsted acidic and nucleophilic environment that effectively activates both o-phenylenediamine and the CO₂-derived intermediates, facilitating the hydride-transfer pathway. Under optimal conditions (atmospheric CO₂ pressure, 120 °C, 0.12 mmol of IL and 8 h) the reaction produced the target benzimidazoles in an excellent yield of 93%. Substrate scope studies confirmed the applicability of this system to a variety of substituted o-phenylenediamines. Notably, the catalyst could be readily recovered and reused for several consecutive cycles without significant loss of activity, demonstrating its good stability and operational practicality. Overall, this work provides a promising and sustainable approach for the fixation of CO₂ into valuable benzimidazole derivatives, contributing to the development of green, recyclable catalytic systems.