Competitive Diene and Dienophilic Character of 2,3,4,5,5-Pentachloro-1-azacyclopentadiene in Diels-Alder Reaction with Conjugated Dienes

Article abstract of DOI:10.1021/jo00328a009

The reaction of the title azadiene 1 with cyclopentadiene via the ene moiety of 1 as the dienophile yielded 5 quantitatively.Chlorination of 5 gave rise to a single dichloro derivative 6.The cycloadduct 7, obtained from reacting cyclopentene with 1, was found to be different from 9, the hydrogenation product of 5, thus eliminating any product which requires 1 behaving as diene.Also, the use of 15N label in the sequence rules out 5a, an imine isomer, as a candidate.However, the existence of the diene character of 1 in reaction with a conjugated diene was found upon heating 1 with cyclohexadiene.Two adducts were isolated in a ratio of 4:1.The minor product was assigned structure 12 which is similar to 5 derived from 1.The cyclohexene adduct of 1 was found to be identical with 14, the dihydro derivative of the major adduct from cyclohexadiene.The latter is assigned structure 10, analogous to the regiospecific formation of endo-5-substituted-2-azanorbornenes from 1 and olefins such as styrene.In the case of an acyclic diene addend such as trans-piperylene, 1 behaves only as a diene, yielding the adduct 16 only.It appears that the steric factor is an important one in deciding the diene or dienophilic reactivity of 1.