The very useful singlet-oxygen ene reaction of reluctant alkenes, e.g. l , 5 , 7, 8 , and, in particular, the mono(ally1 hydroperoxides) 10 and 11, is handicapped by the instability of the sensitiser TPP. A comparative study of TPP, TPFPP, and TDCPP in the singlet-oxygen ene reaction of these alkenes shows that 1) the persistence increases in the order TPP < TPFPP < TDCPP, rendenng TDCPP the sensitiser of choice for the generation of singlet-oxygen in non-polar solvents, 2) the persistence of the tetraarylporphyrins decreases in the presence of an alkene, 3 ) this decrease strongly depends on ~ ~ the nature of the alkene, being most pronounced in the case of cyclohexene. These results are interpreted in terms of unknown substrate-denved species which induce oxidative destruction of the tetraarylporphynns Alternatively, abstraction of allylic hydrogen atoms from the alkenes by the excited sensitisers may give rise to the observed substrate-dependent photobleaching.-Because the singlet-oxygen ene reaction is the key step of a 1,2-carbonyl transposition with concomitant dehydrogenation, the scope and usefulness of this sequence are distinctly improvedRecently, we have devised a novel method for the 1,2-carbonyl transposition with concomitant dehydrogenationL21 of symmetrical ketonesL31 involving their conversion into alkenes which react with singlet-oxygen to afford ally1 hydroper~xides[~I. These were dehydrated in the next step to afford unsaturated carbonyl compounds [5]. When this sequence was applied to 1,5-dimethylbicyclo[3.3.0]octane-3,7-dione, problems were encountered in the twofold photooxygenation of the dienes 7a f Sa, because the mono(ally1 hydroperoxides) 10a and l l a react only very slowly with singlet-oxygen. The reluctance of 10a and l l a , apparently caused by electronic and steric effects, necessitated a high excess of singlet-oxygen and prolonged reaction times. Therefore, the only moderate persistence of the sensitiser 5,10,15,20-tetraphenylporphyrin (TPP) under the vigorous reaction conditions became the critical factor, even when small amounts of pyridine were added as stabiliser and the immersion well was cooled with an aqueous solution of potassium dichromate as light filterL31. Thus, for complete conversion of the readily formed hydroperoxides 10a and l l a , often repeated renewal of TPP was necessary, whose accumulating decomposition products were detrimental to the progress of the reaction, however. Furthermore, the unknown decomposition products appeared to enhance the rate of photobleaching of TPP [4c,6]. Results obtained with Rose Bengal bis(triethy1ammonium) in dichloromethane as solvent were even worse[*]. Clearly, the search for i I more stable singlet-oxygen sensitiser, soluble in aprotic solvents, was advisable. To this end, we tested 5,10,15,20-tetra(pentafluoropheny1)porphyrin (TPFPP) and 5,10, 1 5,20-tetra(2,6-dichlorophenyl)porphyrin (TDCPP). We now report that TDCPP is the sensitiser of choice for the generation of singlet-oxygen in aprotic organic solvents.
