PHOTOCHEMISTRY OF VICINAL TRIKETONES

Vicinal triketones are converted to the corresponding diketones by irradiation in the presence of electron donors such as 1,4-diazabicyclo[2.2.2]octane (DABCO) or hydroquinone dimethyl ether. The primary step is suggested to be electron transfer to the excited state of trione. Irradiation of triketones alone in toluene, p-xylene, cyclohexane or benzene solutions proceeds with very low quantum yields in the absence of oxygen to give complex mixtures of products. Gas chromatography-mass spectrometry (GC-MS) studies of reactions of diphenyl triketone (1) in p-xylene show that benzaldehyde, p-methylbenzyl phenyl ketone, p,p'-dimethylbibenzyl and products derived from reaction with the traces of oxygen present are formed. Analogous results were observed with di-tert-butyl triketone. The suggested primary photochemical processes are initial alpha-cleavage of excited trione or hydrogen atom abstraction from the solvent. The equilibrium constant for the hydration of 1 in dioxan at 25 degrees C is 27.