Cycloaddition reactions of nitrosoalkenes and azoalkenes with cyclopentadiene and other dienes

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Abstract
α-Nitrosostyrene, when generated from α-chloroacetophenone oxime in the presence of cyclopentadiene, forms a 1 : 1 adduct. This adduct has been formulated as the oxazine derivative (1a) which is formally derived from its two components by 4 + 2 cycloaddition, the nitrosoalkene acting as the 4-electron component. Several similar reactions have been observed between this and related nitrosoalkenes and dienes. Indene and α-morpholinostyrene add in a similar way to α-nitrosostyrene. 2,2-Dichloronitrosoethylene is exceptional in forming a different type of adduct (9) with cyclopentadiene; the reasons for this are discussed. Azoalkenes H2CC(Ph)NNX undergo cycloaddition with cyclopentadiene in an analogous manner to that of α-nitrosostyrene, giving pyridazine derivatives (11). Several similar additions of azoalkenes to electron-rich dienes and olefins have been observed, the ease of the reactions reflecting the strength of the frontier orbital interaction between the two components.

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