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 PV2Mo10O405-/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 PV2Mo10O405- with carbon matrices as active supports.
Abstract. Two major reaction modes have been perceived for the catalytic activity of polyoxometalates in oxidation reactions. In one case, the catalytic cycle has been described by the division of the reaction into two stages. First, the substrate is oxidized by consecutive electron and proton transfer by the polyoxometalate in the oxidized form to yield the product and the reduced polyoxometalate catalyst. The reduced polyoxometalate catalyst is then reoxidized, importantly by molecular oxygen to form water, in the second and possibly separate stage completing the catalytic cycle. The polyoxometalates often most effective in this reaction are the phosphovanadomolybdates of the Keggin structure, ( especially ). Now, new investigations of the reactivity of with aldehydes and quinones enables the differentiation between the reactivity of the five inseparable isomers of using NMR and ESR spectroscopy, and UV-vis absorption-time profiles. The 1,11 isomer with vanadium in distal positions is the most abundant, although the isomers with vanadium in vicinal positions appeared to be the most kinetically viable. For example, alkane/aldehyde/ oxidizing systems were found to be quite effective and selective for oxidation of alkanes to ketones. Further studies of -quinone interactions has shown the formation of semiquinone intermediates. The later are active in the dehydrogenation of benzylic and allylic alcohols to aldehydes and can be used as models for the reactivity of on carbon supports . The second reaction type views the oxidation catalyzed by the polyoxometalate as an interaction with a primary oxidant. This interaction yields an activated catalyst intermediate eg a peroxo, hydroperoxo or oxo species which can be used to oxidize the organic substrate. In this mode, one can consider reaction at a transition metal substituted position within the polyoxometalate. Here the polyoxometalate acts as an "inorganic ligand" for transition metals such as cobalt, manganese, ruthenium, etc. In mechanistic scenarios for such reactions, the catalytically active site is a tetragonally (pyrimidal) oxo coordinated transition metal while the polyoxometalate as a whole functions as a ligand with a strong capacity for accepting electrons. In this last group of oxidation reactions the actual reaction mechanism certainly varies as a function of the transition metal and oxidant, but can be conceived to take place via a general intermediate "transition metal -oxidant" species. The ruthenium substituted "sandwich" type polyoxometalate, has been shown to activate molecular oxygen in a dioxygenase type mechanism, and selectively catalyze thereby a) the hydroxylation of alkanes at the tertiary carbon position and b) the stereoselective epoxidation of alkenes. For comparison, catalytic oxidation of a novel ruthenium substituted polyoxometalate, in similar reactions appears to occur by a metal catalyzed autooxidation.
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