PHOTOREARRANGEMENT AND ELECTRON-TRANSFER PHOTOOXIDATION OF 1-ACETOXY-1,2-DIPHENYLCYCLOPROPANE

Direct irradiation of the title compound 4 in hexane led to 1,3-diphenyl-1,4-pentanedione (5) in 75 % yield. This is explained through carbonyl-oxygen bond cleavage (process iii), to afford a cyclopropyloxy radical (1) which rearranges to a beta-carbonyl radical (II) prior to in cage recombination. In methanol, the solvent addition product 6 was obtained as byproduct. It formation is rationalized by means of the symmetrical cationic intermediate IV, on the basis of deuterium incorporation to C(1) and C(3) in MeOD. Photosensitization by acetone resulted exclusively in trans to cis isomerization, via the diradical III (process i). Finally electron transfer activation by means of cerium(IV) ammonium nitrate (CAN) or excited triphenylpyrylium tetrafluoroborate (TPT) gave rise to chalcone (7) or the beta-functionalized ketones 8 or 9. These products must arise from a common cationic precursor (V), after cleavage of the central carbon-carbon bond (process i) in the radical cation 4+-.. From the preparative point of view, the obtention of diketone 5 in 75 % yield by photolysis of 4 in hexane is exploitable, since it constitutes a synthetic equivalent of the addition of acyl anions or acyl radicals to enones.