Electron paramagnetic resonance spectroscopic study of the radicals formed during the photodecomposition of some bis(alkenoyl) peroxides, a bis(alkenoyl) peroxide and some peroxydicarbonates

Abstract

Photolysis, at low temperatures in cyclopropane and/or CFCl₃, of bis[(E)-4,4-dimethylpent-2-enoyl]-peroxide 1, bis(4,4-dimethylpent-2-ynoyl) peroxide 2 and dipropyl peroxydicarbonate 3 yielded the corresponding carbonyloxyl radicals,viz. Me₃CCHCHC(O)O˙, Me₃CCCC(O)O˙ and PrOC(O)O˙, which were identified by EPR spectroscopy. Carbonyloxyl radicals were not detected on photolysis of bis(3-methylbut-2-enoyl) peroxide 4, di[(E)-cinnamoyl] peroxide 7, and bis(1-methylpropyl) peroxydicarbonate 9. All the peroxides also give other radicals many of which were identified from their EPR spectral parameters. Thus, the two bis(alkenoyl) peroxides give the ‘corresponding’ alkenylacyl radicals, viz. s-cis- and s-trans-Me₃CCHCHĊO from 1 and s-trans-Me₂CCHĊO from compound 2. Evidence is presented which indicates that these acyl radicals may be formed directly during the photolysis of substrates 1 and 4via a Norrish type-1 process. This would represent a new route for the photodecomposition of acyl peroxides with C–O bond scission occurring in competition with the usual O–O bond scission.