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Studies Dealing with the Cycloreversion Reactions of Phenyl-Substituted 2,3-Diazabicyclohex-2-enes
Padwa, Albert,Kumagai, Tsutomu,Tohidi, Mahrokh
, p. 1834 - 1840 (2007/10/02)
A study of the reactivity of 2-diazopropane with a series of diphenyl-substituted cyclopropenes has been carried out.The cycloadditions afford 1,5-diphenyl-2,3-diazabicyclohex-2-enes in high yield.Thermolysis of the diazabicyclohexene system for extended periods of time afforded a mixture of 1,3-dienes as well as a substituted bicyclobutane.By studying the distribution of products as a function of time it was possible to show that the 2,3-diazabicyclohexene system first rearranges to an isomeric 1,2-diazabicyclohex-2-ene.This reaction was attributed to an initial cycloreversion to produce a diazoalkene.Attack of the terminal nitrogen atom of the initially generated diazo group onto the neighboring double bond affords the bicyclic aziridine system.Reasonable mechanistic options for the 1,1-cycloaddition include a concerted pathway or a stepwise process.On further heating the 1,2-diazabicyclohexene ring regenerates the allyl-substituted diazoalkane.Loss of nitrogen produces a carbene which undergoes a hydrogen or vinyl shift to give the observed dienes.The formation of the bicyclobutane ring can be viewed in terms of a process involving initial C-N bond cleavage of the 2,3-diazabicyclohexene to yield a diazenyl radical followed by nitrogen loss and radical coupling.In contrast to the thermal results, direct irradiation of the 2,3-diazabicyclohexene results in a cycloreversion to give a 3,4-diaza-substituted 1,3,5-hexatriene derivative.The results obtained indicate that the total mechanistic picture for the thermal and photochemical decomposition of bicyclic pyrazolines can be changed dramatically by the presence of phenyl substituents.
