THE COMPETITION BETWEEN CROSS-COUPLING AND THE EXCHANGE-REACTION IN THE PHOTOREDUCTION OF AROMATIC KETONES

Four aromatic ketones K with different reduction potentials and a seri es of hydrogen atom donors AH(2) with different oxidation potentials o f AH were irradiated in pairs and the product composition analyzed. Ba sed on the product analysis the rate k(ex) of the exchange reaction K + AH . --> KH .+ A, was characterized as slow, medium fast or fast, re spectively, relative to the radical-radical coupling reactions of K . and AH .. The results clearly point to the value of a knowledge of the oxidation potential of the AH . radicals and the reduction potential of the ketones for the prediction of the outcome of the photoreduction of a ketone by an alcohol or another hydrogen atom donor.