Polyazaacenes - On the way to stable, fluorescent and redox-active derivatives

A new synthetic method for the preparation of polyazaacenes is described starting from two different nucleophilic building blocks. Disubstituted oxalic amidines I can be cyclized under mild conditions with 2,3-dichloro-5,6-dicyanopyrazine (3) to yield 5,6-dihydropyrazino[2,3-b]pyrazines 4a-c. By employing higher temperatures and 2 equiv. of 3, octaazanaphthacene 6 can be isolated. Similarly, pyrazino[2,3-b]pyrazines 2 and bielectrophile 3 yielded novel dodecaazahexacenes 8 in addition to semicyclized derivative 9. When tetraazafulvalene 10d was heated in the presence of an amine in xylene in the presence of oxygen, octaazahexacene 13 was isolated as the main product. Instead of pyrazino[2,3-b]pyrazines 2, this highly fluorescent polyazaacene was formed from a cascade reaction which involves a dyotropic rearrangement, an intramolecular Diels-Alder reaction and a multistep redox reaction. ((C) Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2007).