Photoepoxidation of Olefins with Benzoins and Oxygen. Epoxidation with Acylperoxy Radical

Article abstract of DOI:10.1021/ja00398a029

Benzoin was photooxidized to yield benzaldehyde, peroxybenzoic acid, and hydrogen peroxide.The addition of styrenes to this photooxidation system afforded high yields of epoxides together with some C-C cleaved products.While the C-C cleavage products increased with increasing concentration of olefins, the epoxide yield was constant and nearly quantitative; the stoichiometry at the infinite olefin concentration was such that 1 mol of benzoin produces 1 mol of epoxide and 2 equiv of C-C cleaved product.The effect of olefin concentration is analyzed to show that the C-C cleavage is ascribed to benzoyloxy and α-hydroperoxy radicals, PhCH(OH)OO^..The photoepoxidation proceeds by a way of acylperoxy radical, affording predominantly trans-epoxide (e.g., 100percent trans-epoxide from stilbenes and 77percent trans-epoxide from 2-octenes).The relative reactivities of olefins differ significantly from those with molecular peroxyacid; the additivity of methyl substitution does not hold, and aliphatic olefins are less reactive than the corresponding aromatic ones.The relative reactivities of photoepoxidation are examined on the basis of the stability of a resulting adduct radical between the acylperoxy radical and olefin.The peroxy radical is not reactive toward pyridine or sulfoxides.The photoepoxidation with benzoin ethers proceeds similarly except the formation of esters, which is explained by the β-scission or the 1,2-phenyl migration of intermediary α-alkoxy radical.These results are discussed in relation to practical photopolymerizations and to an origin of photoxic substances.