Silyl peroxides as effective oxidants in the Baeyer-Villiger reaction with chloroaluminate(III) ionic liquids as catalysts

A new application of silyl peroxides as oxidants in the Baeyer-Villiger oxidation of cyclic ketones in chloroaluminate(III) ionic liquids is described. Among the silyl peroxides, the reactivity of two groups of peroxides was studied: bis(silyl) and t-butyl silyl peroxides possessing different structured substituents attached to the Si atom. It was shown that the acidic 1-hexyl-3-methylimidazolium chloroaluminate(III) ionic liquid (molar ratio of AlCl₃ in ionic liquid: 0.67) present in the oxidation of cyclic ketones with bis(silyl) peroxides acts as the catalyst. In this variant of the reaction, the reactivities of bis(silyl) peroxides decrease in the following order: bis(trimethylsilyl) peroxide > bis(vinyldimethylsilyl) peroxide > bis(phenyldimethylsilyl) peroxide > bis(diphenylmethylsilyl) peroxide. A variety of cyclic ketones such as cyclobutanone, 3-substituted cyclobutanones, cyclopentanone, cyclohexanone, 2-methylcyclohexanone, 4-methylcyclohexanone, 2-adamantanone and norcamphor were oxidised to their corresponding lactones with high yields (49-100%). When t-butyl silyl peroxides and neutral chloroaluminate(III) ionic liquids (molar ratio of AlCl₃ in ionic liquid: 0.5) were utilised in the Baeyer-Villiger oxidation, the studied ionic liquid acted as the reagent. Here, phenyldimethyl(t-butylperoxy)silane was the most efficient oxidant in the oxidation of cyclobutanone to γ- butyrolactone (70% yield). Other peroxides, including trimethyl(t-butylperoxy) silane, vinyldimethyl(t-butylperoxy)silane and diphenylmethyl-(t-butylperoxy) silane, were less reactive oxidants. Two variants of the Baeyer-Villiger reaction mechanism are postulated.