PHOTOGENERATION OF THE XANTHYL CATION - BETA-CLEAVAGE FROM EXCITED-STATE KETONES

Irradiation of 9-xanthylacetone or 9-xanthylacetophenone in dilute acidic aqueous solution results in facile formation of the xanthyl cation. This cation photogeneration is not subject to acid catalysis over the acid range of 5-30% H2SO4. In contrast, the thermal reaction requires considerably stronger acidic media, and its rate shows a strong dependence on acid concentration. Laser flash photolysis studies demonstrate that the excited-state mechanism proceeds through an initial homolytic carbon-carbon beta-bond cleavage to generate the xanthyl radical. The xanthyl cation is produced from the xanthyl radical in a subsequent step, with oxygen acting as the oxidizing agent. The intermediacy of the xanthyl radical in the mechanism is also supported by the isolation of the radical coupling product bixanthyl from preparative irradiations. Triplet sensitization and quenching experiments demonstrate that the reaction proceeds through an excited triplet state of the ketone substrates.