Vanadium complexes dioxygen

Valinomycin metal complexes, 969 Vanadium complexes acetylacetone exchange reactions, 380 1,4-diaza-l,3-butadiene, 209 dioxygen mononuclear, 321 hydrazido(2-), 148 hydroxamic adds, 506 phthalocyanines, 865 polypyrazolylborates, 248 porphyrins, 824 dioxygen adducts, 325... 

There are also several situations where the metal can act as both a homolytic and heterolytic catalyst. For example, vanadium complexes catalyze the epoxidation of allylic alcohols by alkyl hydroperoxides stereoselectively,57 and they involve vanadium(V) alkyl peroxides as reactive intermediates. However, vanadium(V)-alkyl peroxide complexes such as (dipic)VO(OOR)L, having no available coordination site for the complexation of alkenes to occur, react homolyti-cally.46 On the other hand, Group VIII dioxygen complexes generally oxidize alkenes homolytically under forced conditions, while some rhodium-dioxygen complexes oxidize terminal alkenes to methyl ketones at room temperature. 

Peroxo examples formed by the direct reaction with molecular dioxygen in the first row of the transition metals are few. One of the first examples of a peroxo vanadium complex formed by the direct reaction with dioxygen is a Tp Pr-bound seven-coordinate pentagonal-bipyramidal Vv complex Tp PrV0(f72-02)(pz PrH), with a relatively short O—O distance of 1.379(6) A and relatively high O—O stretching frequency of 960 cm-1, relative to typical peroxo species.14 First-row transition metalloproteins which bond molecular dioxygen are discussed, with their models, in Volume 8. 

The scorpionate vanadium complexes [VCl3 HC(pz)3 ] (10) and [VCl3 S03C(pz)3 ] (15), which catalyze cyclohexane oxidation with H2O2 (Section 22.2.1,), also operate with dioxygen under solvent-free conditions. Cyclohexane is oxidized to cyclohexanol (the main product) and cyclohexanone (13% conversion), with a high selectivity, typically at the O2 pressure of 15 atm, at 140 °C, 18 h reaction time [6]. The reaction is further promoted (to 15% conversion) by pyrazinecarboxylic acid. The reactions proceed via radical mechanisms with possible involvement of both C-centered and 0-centered radicals. 

The experimental observations were interpreted by assuming that the redox cycle starts with the formation of a complex between the catalyst and the substrate. This species undergoes intramolecular two-electron transfer and produces vanadium(II) and the quinone form of adrenaline. The organic intermediate rearranges into leucoadrenochrome which is oxidized to the final product also in a two-electron redox step. The +2 oxidation state of vanadium is stabilized by complex formation with the substrate. Subsequent reactions include the autoxidation of the V(II) complex to the product as well as the formation of aVOV4+ intermediate which is reoxidized to V02+ by dioxygen. These reactions also produce H2O2. The model also takes into account the rapidly established equilibria between different vanadium-substrate complexes which react with 02 at different rates. The concentration and pH dependencies of the reaction rate provided evidence for the formation of a V(C-RH)3 complex in which the formal oxidation state of vanadium is +4. 

Snbsequent detailed kinetic stndies revealed that the reaction mechanism for the hydroxy-lation of arenes is mnch more complicated than that indicated above Furthermore, the active intermediate is likely an anion radical species formed upon interaction of two molecules of the vanadium peroxo complex. The sequence of the various steps is indicated in equations 17-24. The steps indicated in equations 17-21 refer to a radical chain which accounts for decomposition of the peroxo complex to form dioxygen, whereas the subsequent steps are those required for the functionalization of the substrate. 

Studies have also focused on vanadium-based asymmetric catalysts in addition to these photocatalytic systems. A catalytic achiral version of an oxidative coupling reaction was published in 1999 by Uang and co-workers (see Section 14.4.2) [70]. They developed an air-stable complex (VO(acac)2) that can be used in catalytic quantities in the presence of dioxygen as a re-oxidant. The promising results obtained led to an investigation of chiral versions of this reagent, and the initial reports document that such a reaction was possible with complexes... 

Complexes formed by the action of hydrogen peroxide on vanadium compounds have been known for many years and have recently been studied by n.m.r. spectros-copy A report of the reversible formation of a dioxygen complex by vanadium(IV) catecholates has been re-investigated recently and refuted. ... 

The electrochemistry of dinuclear vanadium o-A-salicylideneamino-ethylphenyl complexes were investigated.372 Conversion of Viv—Viv dimers to Vv—Vv dimers was reported and serves as a representative model system for oxidation reactions.294 296 A related class of these dinuclear complexes form mixed valence vanadium(IV/V) complexes upon oxidation and exhibit an unusual electron delocalization over the V203 3+ core.310 A V111—Vlv mixed valent, dinuclear bis(salicy-lidene)ethylenediamine (salen) complex underwent multielectron oxidation with dioxygen to yield a Vv—Viv mixed valence complex.302... 

The use of a heterogeneous system under nitrogen consisting of a stirred suspension of silica gel with adsorbed ferric-catechol complex in benzene treated with 35% hydrogen peroxide has been reported to result after 2.5 hours in a 60% yield of phenol (ref. 14). The formation of phenol in 56% yield resulted from a mixture of benzene and vanadium(V) catalyst in acetonitrile under nitrogen when reacted for 2 hours at ambient temperature (ref. 15). More recent studies have involved the conversion of benzene in trifluoromethanesulphonic acid to phenol by the electroreduction of dioxygen (ref. 16) and from generation... 

No dioxygen adducts of vanadium porphyrins have been reported. However, a niobium(IV) porphyrin, NbBr2TPP, has recently been reported to react with dioxygen. Characterization was based upon ESR data. A 10-line signal g — 2.002), due to weak coupling to the Nb nucleus (/ = f), was seen. This implied a superoxo-type complex. IR data are consistent with the superoxo formalism showing Vo o = 1220cm". ... 

Vanadium(III) complexes, 473 adenine, 475 alcohols, 478 amides, 474, 480 amines, 474 amino acids, 484 ammonia, 474 aqua, 477 arsines, 476 azide, 475 bipyridyl, 475 bromides, 483 carboxylates, 479 catecholates, 478 chlorides, 482 complexones, 485 cyanides, 474,476 (Wethyl sulfoxide, 480 dioxygen, 478 dithiocarbamates, 481 dithiolates, 481 dithiophosphinates, 481 ethers, 478... 

Oxidation is assumed to occur via 2-electron transfer steps from coordinated ligand to dioxygen without the intervention of vanadium-dioxygen complexes. 

There are few examples of ketone oxidation. Copper(II) and its amine complexes are useful catalysts and vanadium heteropoly acids have been employed. Cyclohexanone can be converted to the 1,2-dione, which is cleaved and dioxygenated to adipic acid. 

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