A Direct Intramolecular C−H Amination Reaction Cocatalyzed by Copper(II) and Iron(III) as Part of an Efficient Route for the Synthesis of Pyrido[1,2-<i>a</i>]benzimidazoles from<i>N</i>-Aryl-2-aminopyridines

A novel and efficient synthesis of pyrido[1,2-a]benzimidazoles through direct intramolecular aromatic C-H amination of N-aryl-2-aminopyridines has been developed. The reaction, cocatalyzed by Cu(OAc)(2) and Fe(NO(3))(3)·9H(2)O, is carried out in DMF under a dioxygen atmosphere. Diversified pyrido[1,2-a]benzimidazoles containing various substitution patterns are obtained in moderate to excellent yields by using this procedure. The results of mechanistic studies suggest that a Cu(III)-catalyzed electrophilic aromatic substitution (S(E)Ar) pathway is operating in this process. The unique role of iron(III) is believed to lie in its ability to facilitate formation of the more electrophilic Cu(III) species. In the absence of iron(III), a much less efficient and reversible Cu(II)-mediated S(E)Ar process takes place.