Alexander M. Khenkin

Aerobic hydrocarbon oxidation catalyzed by the vanadomolybdophosphate polyoxometalate, H5PV2Mo10O40, supported on mesoporous MCM-41

The impregnation of H5PV2Mo10O40 polyoxometalate onto MCM‐41 and amino‐modified MCM‐41 materials provided mesoporous active catalysts with large surface areas for aerobic hydrocarbon oxidation using isobutyraldehyde as a reducing agent. The results of the oxidation of alkenes and alkanes gave product selectivities similar to those observed in the corresponding homogeneous reaction although catalytic activity was somewhat reduced. Under appropriate experimental conditions there was no leaching and the solid catalyst could be recovered and reused without loss in activity.

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 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.

A new dinuclear rhodium(III) ‘sandwich’ polyoxometalate, [(WZnRHIII2)(ZnW9O34)2]10−. Synthesis, characterization and catalytic activity

Abstract The rhodium containing polyoxometalate, [(WZnRhIII2)(ZnW9O34)2]10− was prepared by reaction of [(WZn3)(ZnW9O34)2]12− with RhCl3. The X-ray diffraction showed this compound to be isostructural to other compounds of this series. This was confirmed by the IR spectra. The elementary analysis and cyclic voltammetry measurements indicated that the polyoxometalate was of high purity. Epoxidation of alkenes with 30% aqueous H2O2 in 1,2-dichloroethane/water biphasic systems using [(WZnRhIII2)(ZnW9O34)2]10− as catalyst showed high overall activity as compared to other noble metal substituted polyoxometalates of this series. The reactions were highly selective to epoxidation of cyclohexene and similar to previously reported [(WZnMnII2)(ZnW9O34)2]12−, except that hydrogen peroxide yields were measurably higher in the rhodium case. The [(WZnRhIII2)(ZnW9O34)2]10− catalyst appears to be stable under turnovers conditions. Using IR spectroscopy as a probe to compare active polyoxometalates (Rh and Mn) versus an inactive one (Zn), showed that in the former case peroxo-tungstate intermediates could be identified whereas for the inactive compound no such intermediate was observed. A possible mechanism taking into account reactivity, steric constraints and IR spectra assuming structural integrity under catalytic turnover conditions was put forth.

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

Abstract The oxidation of organic substrates catalyzed by ‘sandwich’ type transition metal substituted polyoxometalates of the general formula, NaxM2Zn3W19O68, (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 NaxM2Zn3W19O68 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.

Vanadium-substituted MCM-41 zeolites as catalysts for oxidation of alkanes with peroxides

Alkanes are oxidized selectively to ketones using V-MCM-41 with isobutyraldehyde/dioxygen as the preferred oxidant in terms of product selectivity and catalyst stability and recycle.

The contribution of tunnelling to high values of kinetic isotope effect in aliphatic hydroxylation by a cytochrome P-450 model

High values of kinetic isotope effect (KIE), up to 21.9 ± 1.9 at 20 °C, are obtained in cyclohexane oxidation by an iron tetramesitylporphyrin (TMPFeCl)–hypochlorite system; their very sharp dependence on temperature suggests tunnelling contribution to the C–H bond cleavage step.

Alkane oxidation with manganese substituted polyoxometalates in aqueous media with ozone and the intermediacy of manganese ozonide species

Manganese substituted polyoxometalates (POMs), such as Li12[MnII2ZnW(ZnW9O34)2] were effective catalysts for the oxidation of alkanes to ketones with ozone in an aqueous reaction medium; a green intermediate compound observable by UV–VIS and ESR at –78 °C was postulated to be a reactive manganese ozonide species.

Aerobic Oxidations Catalyzed by Polyoxometalates

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, H 3+x PV x Mo 12−x O 40 (x = 1 − 6, especially x = 2). Now, new investigations of the reactivity of PV 2 Mo 10 O 40 5− with aldehydes and quinones enables the differentiation between the reactivity of the five inseparable isomers of α-H 5 PV 2 Mo 10 O 40 using 31 P 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/O 2 oxidizing systems were found to be quite effective and selective for oxidation of alkanes to ketones. Further studies of PV 2 Mo 10 O 40 5− - 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 PV 2 Mo 10 O 40 5− 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, [WZnRu III 2(ZnW 9 O 34 )2] 11− , 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, [RuII(H2O)W17O55F6NaH2]9−, in similar reactions appears to occur by a metal catalyzed autooxidation.