Exceptional activity of gallium(iii) chloride and chlorogallate(iii) ionic liquids for Baeyer–Villiger oxidation

Abstract

Baeyer–Villiger oxidation of cyclic ketones, using H₂O₂ as the oxidising agent, was systematically studied using a range of metal chlorides in different solvents, and in neat chlorogallate(III) ionic liquids. The extremely high activity of GaCl₃ in promoting oxidation with H₂O₂, irrespective of solvent, was reported for the first time. The activity of all other metal chlorides was strongly solvent-dependent. In particular, AlCl₃ was very active in a protic solvent (ethanol), and tin chlorides, SnCl₄ and SnCl₂, were active in aprotic solvents (toluene and dioxane). In order to eliminate the need for volatile organic solvent, a Lewis acidic chlorogallate(III) ionic liquid was used in the place of GaCl₃, which afforded typically 89–94% yields of lactones in 1–120 min, at ambient conditions. Raman and ⁷¹Ga NMR spectroscopic studies suggest that the active species, in both GaCl₃ and chlorogallate(III) ionic liquid systems, are chlorohydroxygallate(III) anions, [GaCl₃OH]⁻, which are the products of partial hydrolysis of GaCl₃ and chlorogallate(III) anions; therefore, the presence of water is crucial.