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

Abstract

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.