Benzonitriles photoadditions

Reports of the photoaddition of nitriles to alkenes have also been published. An intermediate azetine 282 has been identified in the photoaddition of benzonitrile to 1,2-dimethylcyclohexene (283) to give adducts 284 and 285,235 and stable azetines have been prepared by direct photoaddition of... 

Bryce-Smith and Gilbert have shown that toluene, t-butylbenzene, chlorobenzene, o- andp-xylene, and biphenyl all undergo photoaddition to maleic anhydride, yielding 1 2 adducts [33], All the substituents decrease the rate of formation of adducts. Benzonitrile, nitrobenzene, phenol, methyl benzoate, durene, hexamethylbenzene, naphthalene, and biphenylene fail to undergo the addition of these, benzonitrile, nitrobenzene, methyl benzoate, and biphenylene do not form charge-transfer complexes. Bradshaw [34] found that various alkylben-zenes give 1 2 adducts with maleic anhydride upon photosensitization with acetophenone. 

Table 3. Free energy change, AG°ct, and rate constants, of photoinduced electron transfer from group 14 organometallic electron donors to C, observed rate constants, obs, triplet quenching rate constants, kq, and limiting quantum yields, Oco, in the photoaddition of the donors to in benzonitrile at 298 K [212].

Photochemical meta cycloaddition reactions of benzonitrile and the methylbenzonitriles with alkenes and dienes have been reported previously, and the results are summarized in a recent review. Examples include benzonitrile itself with seven alkenes and dienes (Table 2 in Reference 9) and each of the three isomeric methylbenzonitriles with -l,2-dichloroethene, fiiran, and cyclopentene (Table 3 in Reference 9). In all cases, cycloadditions were observed to give, along with other products, the dihydrosemibullvalenes expected from the trapping of a bicyclic intermediate analogous to that proposed for the phototranspositions and photoadditions to TFE. However, none of these products resulted from an intermediate that had the cyano group at C6. 

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