Syntheses of Formamidines and Benzimidazoles

Abstract

In view of the developing interest in and impor- of formamidines¹ and benzimidazoles,^2–3 we wish to review recent developments in syntheses applicable to both of these two classes of compounds starting from the appropriate amine or diamine. Although structurally similar in that both formamidines and benzimidazoles contain the group, the formation of benzimidazoles involves ring closure to give an aromatic product, while formation of formamidines does not. One would thus expect formation of benzimidazoles to be thermodynamically more favorable. In fact, syntheses of formamidines outlined below work well for benzimidazoles, although the inverse is not necessarily true. For example, Phillip's⁴ widely used synthesis of benzimidazoles from aromatic diamines with refluxing formic acid/HC1 is not applicable to formamidines. In addition to a general review of recent synthetic methods, results obtained in this laboratory will be presented where appropriate.