PHOTOINDUCED REACTIONS .28. COMPETITION BETWEEN INTRAMOLECULAR HYDROGEN ABSTRACTION AND FRAGMENTATION IN PHOTOREACTION OF HINDERED AROMATIC KETONES AND A CORRELATION WITH MASS SPECTRA

The photoreactions of various alkyl 2,4,6-trimethylphenyl ketones bearing a sterically hindered CO group have been investigated. Depending upon the bulkiness of the alkyl side chain, two types of reactions have been observed. Intramolecular hydrogen abstraction leading to formation of a novel benzocyclobutenol is predominant with a less bulky alkyl side chain and fragmentation leading to a carbonyl-alkyl bond fission with a more bulky one. A correlation is found between the photoreactions and the mass spectrometric decomposition of the ketones. These photoreactions have been interpreted in terms of stereoelectronic requirements for the hydrogen abstraction from the n,π* triplet state. Unusual photoreduction of 2,4,6-trimethylbenzophenone is also reported. © 1969.