Reactivities of Acylperoxy Radicals in the Photoreaction of α-Diketones and Oxygen

Article abstract of DOI:10.1021/jo00192a018

The photoepoxidation of olefins with α-diketones and oxygen has been studied mechanistically focusing on the reactivities of intermediate radicals.One mole of α-diketone resulted in the formation of 2 mol of epoxide together with 2 equiv of C-C cleavage of olefins.The photoepoxidation proceeds via acylperoxy radicals RCO3* and the C-C cleavage of olefins is caused by acyloxy radical RCO2*.The addition of RCO3* to olefins was found to be ca.10⁵-fold faster than that of acylperoxy radical ROO*.The relative reactivities of olefins suggest that acylperoxy radicals behave as a strongly electrophilic radical.That is, ρ values of -1 (vs.?⁺) obtained in the photoepoxidation of substituted styrenes are of the same magnitude as those in the epoxidation with molecular peracids.Although the relative reactivities of olefins toward the photoepoxidation roughly parallel those for the peracid epoxidations, the additivity of methyl substituent is not always operative.This is explained by a steric retardation by too many substituents on the carbon attacked by RCO3* in addition to the relative stabilities of resulting adduct radicals between olefins and RCO3*.Since acylperoxy radicals are not reactive towards sulfides, sulfoxides, or pyridine, a selective epoxidation of double bonds is possible.The relative reactivities of olefins toward benzoyloxy and methylperoxy redicals revealed a much less electrophilic nature of these oxy radicals, the ρ values for styrenes being -0.1 to -0.2.