Quinones as co-catalysts and models for the surface of active carbon in the phosphovanadomolybdate-catalyzed aerobic oxidation of benzylic and allylic alcohols: Synthetic, kinetic, and mechanistic aspects

Quinones have been considered as reactive compounds present on the surface of active carbon. Thus, the co-catalytic use of quinones combined with the phosphovanadomolybdate polyoxometalate, PV₂Mo₁₀O₄₀^5-, has been studied as an analogue of the known PV₂Mo₁₀O₄₀^5-/C catalyst in oxidative dehydrogenation reactions. From the synthetic point of view both biphasic the quinone (org)-Na₅PV₂Mo₁₀O₄₀-(aq) and monophasic quinone (org)-⁴Q₅PV₂Mo₁₀O₄₀-(org) [⁴Q = (nC₄H₉)₄-N⁺] systems are effective for the selective oxidation of benzylic and allylic alcohols to their corresponding aldehydes. Kinetic measurements carried out on the model oxidative dehydrogenation of 4-methylbenzyl alcohol in the presence of p-chloranil, ⁴Q₅PV₂Mo₁₀O₄₀, and molecular oxygen showed that the reaction was non-elementary, although the 4-methylbenzyl alcohol oxydehydrogenation was the rate-determining step. ESR measurements showed the presence of the semiquinone of p-chloranil, probably as a complex with the polyoxometalate. This proposed complex was shown to be a more potent oxidant than p-chloranil. Thus, for the oxidation of 4-methoxytoluene the semiquinone complex was active, whereas p-chloranil alone was inactive. Beyond the importance of understanding quinone-phosphovanadomolybdate polyoxometalate-catalyzed reactions, insight gained from the formation of semiquinone active species can be applied for heterogeneous and aerobic oxidative transformations catalyzed by PV₂Mo₁₀O₄₀^5- with carbon matrices as active supports.