Formal intramolecular photoredox reactions of phenylbenzophenone, xanthone and fluorenone methanols in aqueous solution

Abstract

To further explore the structural and chromophoric effects on the formal intramolecular photoredox reaction discovered in our laboratory for aromatic ketones such as benzophenone and anthraquinones in aqueous solution, we have investigated whether this photochemistry can be applied to xanthones, fluorenones, and benzophenones that are “extended” by a conjugated phenyl ring where the oxidizable group is distal to the benzophenone. It was found that 2-(hydroxymethyl)xanthone ( 6) exhibited sufficient photoactivity that the intramolecular photoredox reaction was observable under neutral conditions whereas 2-(hydroxymethyl)fluorenone ( 7) displayed only very low but clean photoreactivity in acid. The last feature of this study focuses on the extension of the electronic transmission from the carbonyl functional group to the benzylic alcohol, by insertion of an additional phenyl group, to give a biphenyl (phenylene) structural feature. The addition of the phenyl group gives rise to bichromophoric molecules ( 8– 10) rather than monochromophoric substrates studied in the past. We find that the location of the hydroxymethyl substituent on the attached phenyl ring plays a pivotal role in the overall photobehaviour: only the ortho-substituted compound gave evidence of an intramolecular photoredox reaction, while the meta- and para-substituted compounds primarily exhibited photobehaviour that can be interpreted as an “aborted” intramolecular redox process. Overall, the results show that photoredox or related reaction can occur for all of these systems but added complexities make them less useful compared to what was observed for the “parent” benzophenone system.