The Surface of Silica as a Medium for the Radical and Ionic Decomposition of Diacyl Peroxides

Article abstract of DOI:10.1021/ja00416a013

The rates of decomposition of β-phenylpropionyl peroxide 1, β-phenylisovaleryl peroxide 2, and β-phenylisovaleryl p-nitrobenzoyl peroxide 3 are much higher on silica surfaces than they are in solution.The products formed on silica surfaces are derived from both radical and ionic precursors except in the case of 3, for which the reaction may be entirely ionic.The relationships between the medium effects on the rates and on the products suggest that the ionic and radical parts of the reaction branch from a common polar intermediate.Both the ionic and radical products differ significantly from those formed in solution.In the neophyl radical, adsorption inhibits the migration of phenyl to form phenyl-tert-butyl radical.In the ionic reaction, there is extensive migration of methyl in competition with phenyl, in contrast to the behavior of neophyl derivatives in solvolysis reaction.Dihydrocoumarin, from 1, and dimethyldihydrocoumarin, from 2 and 3, are not formed at all in solution.Esters, which are often formed via carboxy inversion and related reactions when polar diacyl peroxides decompose in solution, appear to arise from electron transfer in radial-pair precursors.There is no evidence of carboxy inversion compounds or carbonic acid esters in the physically adsorbed products, although some RO-C(=O)⁺ groups (1-2percent) appear to be trapped by the silica.Rearranged and unrearranged neophyl cations are trapped more extensively as silyl ethers.Other ion-derived products are one of the phenylisobutylenes, several phenylbutenes, and the carboxylic acids.Coadsorbed oxygen or acetonitrile alters the product distribution partly by trapping radicals and partly by site preemption effects.