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

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