Ketones, reductive cleavage triplet

Photooxetane formation is quite inefficient, a fact which usually points to the presence of an intermediate which can partially revert to ground state reactants. Cleavage of the diradical must be responsible for some of the inefficiency in oxetane formation 129>. However, in the past few years convincing evidence has appeared that a CT complex precedes the diradical iso.isi). The two most telling pieces of evidence are the relative reactivities of different alkenes 130> and the absence of any measurable secondary deuterium isotope effect on quenching rate constants 131>. Relative quenching rates of sterically un crowded olefins are proportional both to the ionization potentials of the donor olefins 130> and to the reduction potentials of the acceptor ketones 131>, as would be expected for a CT process. Inasmuch as n,n triplets resemble electron-deficient alkoxy radicals, such substituent effects would also be expected on direct radical addition of triplet ketone to olefin. However, radical addition would yield an inverse isotope effect (in, say, 2-butene-2,3-d2) and would be faster to 1,1-dialkylethylenes than to 1,2-dialkylethylenes, in contrast to the actual observations. 

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