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Sulfamic acid, an eco-friendly and zwitterionic solid, proved to be a very efficient catalyst for the reaction of ortho-phenylenediamine with aryl aldehydes in ethanol at room temperature to furnish both 1-arylmethyl-2-aryl-and 2-arylbenzimidazoles in very good to excellent overall yields.
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Seeberger et al. (2006) studied this question.
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