Indenes 4,7-dimethylindene

In some cases radical cations may undergo cycloadditions with an acceptor derived intermediate without prior proton transfer. This is observed especially for radical cations without sufficiently acidic protons, although it is not limited to such species. For example, the photoreaction of chloranil with 3,3-dimethylindene results in two types of cycloadducts [141]. In the early stages of the reaction a primary adduct is identified, in which the carbonyl oxygen is connected to the p-position of the indene (type B) in the later stages this adduct is consumed and replaced by an adduct of type A, in which the carbonyl oxygen is connected to the a-position. CIDNP effects observed during the photoreaction indicate that the type B adduct is formed from free indene radical cations, which have lost their spin correlation with the semiquinone anions. 

Indenes lacking benzylic hydrogens (e.g., 1,1-dimethylindene) give no substitution products upon irradiation with acrylonitrile. 

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