title: Experimental and theoretical characterization of the aromatization, epimerization, and fragmentation reactions of bi-2H-azirin-2-yls prepared from 1,4-diazidobuta-1,3-dienes.
abstract: 1,4-Diazidobuta-1,3-dienes (Z,Z)-10, 17, and 21 were photolyzed and thermolyzed to yield the pyridazines 13, 20, and 23, respectively. To explain these aromatic final products, the generation of highly strained bi-2H-azirin-2-yls 12, 19, and 22 and their valence isomerization were postulated. In the case of meso- and rac-22, nearly quantitative formation from diazide 21, isolation as stable solids, and complete characterization were possible. On the thermolysis of 22, aromatization to 23 was only a side reaction, whereas equilibration of meso- and rac-22 and fragmentation, which led to alkyne 24 and acetonitrile, dominated. Prolonged irradiation of 22 gave mainly the pyrimidine 25. The change of the configuration at C-2 of the 2H-azirine unit was observed not only in the case of bi-2H-azirin-2-yls 22 but also for simple spirocyclic 2H-azirines 29 at a relatively low temperature (75 °C). The fragmentation of rac-22 to give alkyne 24 and two molecules of acetonitrile was also studied by high-level quantum chemical calculations. For a related model system 30 (methyl instead of phenyl groups), two transition states TS-30-31 of comparable energy with multiconfigurational electronic states could be localized on the energy hypersurface for this one-step conversion. The symmetrical transition state complies with the definition of a coarctate mechanism.
authors.affiliation: Organic Chemistry, Chemnitz University of Technology, Strasse der Nationen 62, 09111 Chemnitz, Germany. klaus.banert@chemie.tu-chemnitz.de
authors.forename: Klaus
authors.initials: K
authors.lastname: Banert
authors.forename: Frank
authors.initials: F
authors.lastname: Köhler
authors.forename: Antje
authors.initials: A
authors.lastname: Melzer
authors.forename: Ingolf
authors.initials: I
authors.lastname: Scharf
authors.forename: Gerd
authors.initials: G
authors.lastname: Rheinwald
authors.forename: Tobias
authors.initials: T
authors.lastname: Rüffer
authors.forename: Heinrich
authors.initials: H
authors.lastname: Lang
authors.forename: Rainer
authors.initials: R
authors.lastname: Herges
authors.forename: Kirsten
authors.initials: K
authors.lastname: Hess
authors.forename: Nugzar
authors.initials: N
authors.lastname: Ghavtadze
authors.forename: Ernst-Ulrich
authors.initials: EU
authors.lastname: Würthwein