Novel Photoreactions of 2-Aza-1,4-dienes in the Triplet Excited State and via Radical-Cation Intermediates. 2-Aza-di-π-methane Rearrangements Yielding Cyclopropylimines and N-Vinylaziridines

Triplet-sensitized irradiation of 2-aza-1,4-dienes affords N-cyclopropylimines via 2-aza-di-π-methane (2-ADPM) rearrangement pathways. In the case of the pentaphenyl-substituted azadiene 1, irradiation leads to formation of cyclopropylimine 2 as well as N-vinylaziridine 3. The transformations represent the first examples of di-π-methane rearrangement reactions that yield three-membered heterocyclic products. SET-sensitized irradiation of 2-aza-1,4-dienes, by using 9,10-dicyanoanthracene (DCA) as an electron-acceptor sensitizer and biphenyl as cosensitizer, brings about regioselective formation of N-vinylaziridines. Under these conditions, azadiene 1 also affords cyclopropylimine 37, resulting from an aryl-di-π-methane rearrangement. This result demonstrates that di-π-methane reactions can also take place via radical−cation intermediates. In some instances, imine and olefin centered cation−radical intermediates, generated by SET-sensitized irradiation, undergo alternative reactions to produce isoquinoline and benzoazepine products.