Bis-benzimidazolium salts as bifunctional organocatalysts for the cycloaddition of CO2 with epoxides

Abstract

A series of 1,2-bis(benzimidazolium)ethane dihydrohalide salts was prepared and characterized, including five structures elucidated by single-crystal X-ray diffraction. These compounds were tested as organocatalysts for the cycloaddition reaction of CO2 with epoxides to produce cyclic carbonates. Effects of counteranions, N-substituents and C-5 substituents, and how they affect solubility in the catalytic system, were evaluated. The salts containing 1,2-bis(5-methyl-benzimidazolium)ethane cation presented the best activities and the one with bromide as counteranion (2Br) was selected to carry out a study of several parameters of the catalytic system. Various epoxides could be selectively converted to the respective cyclic carbonate and, reuse experiments were conducted, showing that catalyst 2Br can be reused at least six times, with no decline in conversion and selectivity. In addition, a catalytic mechanism was carefully proposed, taking into account 1H NMR spectroscopy experiments. Simple, nontoxic and low cost synthetic route to obtain the catalysts, as well as the high conversions using low catalyst loading, make this class of catalysts promising for the use of carbon dioxide in the production of cyclic carbonates.