Radical ions in photochemistry. 15. The photosubstitution reaction between dicyanobenzenes and alkyl olefins

The photosubstitution (electron transfer) reaction between 1,4-dicyanobenzene (1) and 2,3-dimethyl-2-butene (2), which gives 1-(4-cyanophenyl)-2,3-dimethyl-2-butene (3) and 3-(4-cyanophenyl)-2,3-dimethyl-1-butene (4), has been extended to other dicyanobenzene–olefin mixtures. Substitution of a cyano group occurs when both 1 or 1,2-dicyanobenzene (5) are irradiated in acetonitrile solution, in the presence of 2 or cyclohexene (16). Under comparable conditions, 1,3-dicyanobenzene (6) failed to react. Little or no substitution was observed in any case when the olefin was methylpropene (19). The results for 1 and 5 are in agreement with empirical free energy calculations (Weller equation) for the electron transfer process which, however, fail to explain the general lack of reactivity of 1,3-dicyanobenzene. Phenanthrene (11) has been shown to photosensitize the photosubstitution reaction between dicyanobenzenes and 2. Under these conditions the olefin reacts with 6 predominantly at the 4-position, resulting in overall substitution of a hydrogen atom. This reaction occurs regiospecifically, resulting in the formation of only one of the two possible isomeric side chains. The mechanistic details of these reactions have been substantiated by means of deuterium labelling studies. The aromatic nitriles also undergo photosubstitution by 2, in acetonitrile–methanol solution, resulting in methanol-incorporated products. Whereas reaction with 1 or 5 results in substitution of a cyano group, 6 was observed to give isomeric dicyanocyclohexenes, resulting from initial reaction at the 4-position, followed by reduction. A detailed mechanism for this secondary photoreduction has been substantiated by deuterium labelling studies. The anomalous behaviour of 1,3-dicyanobenzene has been attributed to a difference in the reactivity of the radical anion.