Catalytic oxidation with hydrogen peroxide catalyzed by 'sandwich' type transition metal substituted polyoxometalates

The oxidation of organic substrates catalyzed by 'sandwich' type transition metal substituted polyoxometalates of the general formula, Na(x)M₂Zn₃W₁₉O₆₈, (M = Ru, Mn, Zn, Pd, Pt, Co, Fe, Rh) was examined in three different reaction media. The manganese analog was dissolved in a 1,2-dichloroethane phase using a lipophilic quaternary ammonium counter cation. Various organic substrates were oxidized with 30% aqueous H₂O₂. Alkenes reactivity increased as a function of the nucleophilicity of the double bond, but decreased as a function of steric crowding in the cyclohexene series. Alkenols with primary hydroxyl groups reacted chemo- and stereoselectively to form the corresponding epoxy alcohols. On the other hand, alkenols with secondary hydroxyl units did not react chemoselectively; both ketones and epoxy alcohols were formed. Diols were oxidized in most cases to ketols, except for 1,4-butanediol which yielded γ-butyrolactone. Secondary amines yielded hydroxyl amines except for piperidine which reacted with the solvent. A manganese containing catalyst supported on a functionalized silica particle was as active and selective as the organic solvent containing biphasic system for the oxidation of alkenes and alkenols. Reactions were also carried out by dissolving Na(x)M₂Zn₃W₁₉O₆₈ in aqueous solutions of 30% H₂O₂, 70% t-butylhydroperoxide or 0.02 M potassium persulfate in the absence of solvent. Hydrogen peroxide degraded all the TMSP compounds. One degradation product was an effective and chemo- and stereoselective catalyst for the epoxidation of primary alkenols. In alcohol oxidation only the ruthenium precursor was active. For oxidations with 70% t-butylhydroperoxide all compounds were stable but only the Na₁₁Ru₂Zn₃W₁₉O₆₈ compound was active. Alcohols were oxidized selectively, however, alkenols yielded a mixture of products. With persulfate, some catalytic effects were observed in double bond oxidation.