An Unexpected Synthesis of Dihydrophenazines en Route to Benzimidazolium Salts

The oxidation of various N,N′-diarylbenzene-1,2-diamines bearing bulky aromatic substituents with sodium periodate on wet silica gel afforded a series of five new dihydrophenazines instead of the expected cyclohexadiene-1,2-diimines. The reaction most likely proceeds via a 1,6-electrocyclic path and provides a convenient access to an important class of nitrogen heterocycles. Subsequent treatment of the mesityl derivative with chloromethyl pivalate and silver triflate led to the corresponding benzimidazolium salt.