The five-, six-, and seven-membered cyclic 1,4-diaza-l,3-dienes have been irradiated under a variety of conditions. The six-membered ring (dihydropyrazine) opens in a 6ne valence isomerization reaction. The resulting enediimine is stable at low temperatures and has been characterized (i.r.). Irradiation of 2,3,5,5-tetramethyl-5,6-dihydropyrazine (lb) results in formation of the enediimine 26 which reacts further to give the rearranged monomer 12 and/or dimers 13 depending upon the temperature. The fiveand seven-membered systems are stable to irradiation in inert solvents. However, irradiation of 2,2-dimethyl-4,5-diphenylisoimidazole (10h) in isopropyl alcohol gives the dihydroderivative 19 and in tetramethylethylene the one-to-one adduct 25.The U.V. spectrum of the six-membered ring (5,6-dihydropyrazine) is very different from that of the five-and seven-membered rings. The long wavelength maximum (ca. 350 nm) of the six-membered ring is attributed to a transition directly into an excited state common to the enediimine 2 (i.e., the excited state has a node between C-5 and -6 of the dihydropyrazine).
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.