Photochemische Reaktionen. 53. Mitteilung [1]. Photochemische Addition von Alkylaromaten an Aryl‐Carbonylverbindungen

In the preceding paper [1] a novel primary photochemical process of triplet excited α,β‐conjugated cycloalkenones in toluene solution has been reported: the abstraction of a benzylic hydrogen from the solvent by the β‐carbon (cf. 1 → 2 + 3 + 4 ). The reaction has been attributed to the π,π* triplet. Aromatic aldehydes and ketones ( 5–11a ), the triplet state reactivity of which is known to be mostly π,π* in nature, have now been examined under the same irradiation conditions. However, a reaction similar to that of cycloalkenones — expected to result in the addition of hydrogen to the ortho and para positions of the aryl moiety and the formation of benzylcyclohexa‐1,3‐and 1,4‐diene derivatives — could not been found. Compounds 5 – 10 remained essentially unchanged. 4‐Methoxyacetophenone ( 11a ) reacted slowly to form the same type of products [ tert ‐carbinol 12a , pinacol 13a and dibenzyl ( 4 )] as the aromatic carbonyl compounds 11b‐d , benzophenone and cyclopropylphenylketone, which exhibit typical n ,π* triplet reactivity (hydrogen abstraction by the carbonyl oxygen).