Aerobic oxidation vanadium complexes

L — maltolate the coordination environment of the vanadium in K[V02(malt)2] H2O is approximately octahedral, the two 0x0 ligands being in cis positions. [K(H20)e] units link adjacent vanadium(V) complex anions to give a chain structure 166). The main products of aerobic oxidation of [V O(dmpp)2l in aqueous solution are [V02(dmpp)] and [VOo(dmpp)2]. High pH favors these V products, whereas at low pH V species predominate 171). Vanadium(V) also forms a VO(OR)(malt)2 series, readily prepared from ammonium vanadate, maltol, and the appropriate alcohol in a water-alcohol-dichloromethane medium 172), and 3-hydroxy-4-pyridinonate analogues V0(0R)L2 on oxidation of their oxovanadium(IV) precursors in solution in the appropriate alcohol ROH 168). 

An inner-sphere hydrogen atom abstraction from the alcohol by a peroxo metal complex, thus forming a coordinated ketyl radical [(CH3)2—C —O—V(0)(00H)]" , has been proposed for the aerobic oxidation of alcohols catalyzed by peroxidic molybdenum and vanadium derivatives (Scheme 16). While in the case of Mo-catalyzed reaction the H2O2 produced is quantitatively converted to products (ketone and H2O), in the vanadium mediated process, hydrogen peroxide accumulates . In this latter case, the direct involvement of a vanadium monoperoxo species has been substantiated by ESI-MS data. 

Another system is relevant to this discussion of the oxidation of bromide by hydrogen peroxide. It does not catalyze precisely the same chemistry instead, the vanadium complex of tetraethylene glycol (H2teg) catalyzes the aerobic oxidation of bromide (from HBr) in 1,2-dichloroethane (30). 

Scheme 9.18 Enantioselective and aerobic oxidative coupling of 2-naphthol derivatives using a chiral dinuclear vanadium(v) complex in water, reported by Sasai and Takizawa. - ...

Vanadium. and NMR spectroscopy has been used to study the peroxyvanadium intermediates in vanadium catalysed enantioselective oxidation of sulfides. The aerobic oxidation of Pr OH catalysed by per-oxovanadium complexes has been investigated using NMR spectroscopy. ... 

Related V complexes show activity toward the oxidative decomposition of pinacol with C—C bond cleavage and aerobic oxidation of 4-methoxybenzylalcohol and other lignin model compounds." Other oxidovanadium(V) complexes with c 5-2,6-bis-(methanolate)-piperidine ligands of the type depicted on Scheme 3 were appHed as catalysts to convert prochiral alkenols into 2-(tetrahydrofiiran-2-yl)-2-propanols, 2-(tetrahydropyran-2-yl)-2-propanols, oxepan-3-ols and epoxides, upon oxidative alkenol cyclization with TBHP as oxidant (Scheme 3)." These catalysts are rather stable and possess improved chemoselectivity, e.g., epoxidation of geraniol occurs enantioselectively. It was ruled out the vanadium(V) ieri-butyl peroxy complex formation is a key step to activate peroxides. 

Zhang G, ScottBL, Wu R, Silks LAP, Hanson SK. Aerobic oxidation reactions catalyzed by vanadium complexes of bis(phenolate) ligands. Inorg Chem. 2012 51 7354-7361. 

Complexes of the first two ligands are prepared by reaction of vanadyl sulfate with the ligand, preformed in situ in aqueous ethanol (38). Isolated as the vanadium(IV) complexes, they can be oxidized in aerobic methanol and reisolated as the methoxymethanol vanadium(V) derivatives (39). Alternatively, the reduced complexes can be dissolved in DMF and allowed to oxidize aerobically before use. Their reactivity parallels that of (HPS)VO(OEt)(EtOH). They have reaction rates for catalyzing bromide oxidation comparable to (HPS)VO(OH) (Table I). Interestingly, the rates of these reactions are quite sensitive to the amount of water. The increase from 0.5% to 1.0% added water causes a 10-20% decrease in rate. 

Aerobic alcohol oxidations with vanadium catalysts continue to widen their substrate scope and applications. The recendy studied mechanism of the intramolecular oxidation of benzyl alcoholate Hgands in 8-hydroxyquinoHnato(L) vanadium(V) complexes of the type [LV(0) (OR)] resembles those proposed for certain metaHoenzyme-catalyzed... 

Catalytic systems based on palladium [208], mthenium [209] and vanadium [210] complexes with tert-butyl hydroperoxide as oxidant or a cobalt catalyst under aerobic conditions [211] have also been applied in this reaction. In carbohydrate chemistry in particular [212] other cyclic acetals and cyclic hemi-acetals have been oxidized to the corresponding lactones with a plethora of oxidants. 

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