Alkene Oxidation Catalyzed by a Ruthenium-Substituted Heteropolyanion, SiRu(L)W₁₁O₃₉: The Mechanism of the Periodate Mediated Oxidative Cleavage

A ruthenium-substituted heteropolyanion SiRu(H₂O)W₁₁O₃₉^5- was synthesized and characterized. The hydrophobic quaternary ammonium salt of the heteropolyanion ((C₆H₁₃)₄N)₅SiRu^III(H₂O)W₁₁O₃₉ was used as a catalyst for the oxidation of alkenes with tert-butyl hydroperoxide, potassium persulfate, iodosobenzene, and sodium periodate as primary oxidants. Reactivity and selectivity were found to be dependent on the oxidant used; several different types of oxidation processes could be identified including allylic oxidation, epoxidation, and oxidative cleavage. Use of sodium periodate as oxidant enabled selective bond cleavage with aldehydes as the exclusive product. Different product selectivity and UV-vis and IR spectra of the ruthenium heteropoly compound in the presence of the various oxidants shows that unique mechanisms are operating in each case. A series of further experiments into the oxidation of styrene derivatives to benzaldehydes by sodium periodate including investigation of the reaction kinetics, substituent effects, and isotope incorporation enabled the formulation of a reaction mechanism. The reaction proceeds by interaction of the styrene with the catalyst forming a metallocyclooxetane which rearranges in the rate-determining step to a cyclic diester through two different transition states depending on the ring substituent. In the final step the cyclic diester decomposes yielding the cleavage products.