Peroxo complexes decomposition

Density functional calculations reveal that epoxidation of olefins by peroxo complexes with TM d° electronic configuration preferentially proceeds as direct attack of the nucleophilic olefin on an electrophilic peroxo oxygen center via a TS of spiro structure (Sharpless mechanism). For the insertion mechanism much higher activation barriers have been calculated. Moreover, decomposition of the five-membered metallacycle intermediate occurring in the insertion mechanism leads rather to an aldehyde than to an epoxide [63]. 

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. 

Early work on peroxo compounds of molybdenum has been reviewed.286 The stoichiometri-cally simplest Mo peroxo complex is the red-brown Mo(02)4 ion formed by the reaction of MoO2- with H202. Although the complex is not stable in solution and decomposes slowly with the evolution of 02, the anion can be crystallized as the Zn(NH3)4+ salt whose structure has been determined. In a sense, Mo(02)4 is an intermediate in the thermodynamically favored decomposition of hydrogen peroxide to water and oxygen. 

Results on epoxidation of cyclohexene with H2O2 with freshly prepared catalysts are given in Table 2. With Mo blue, exchanged on Mg,Al-LDH, the olefin conversion is low, even if all peroxide is consumed within 4 h. Upon addition of the H2O2 to the reaction mixture, the suspended catalyst has the yellow hue of the Movl form of the isopolyacid. However, the suspension soon turns brick red. This color is characteristic for tetraperoxomolybdate Mo(02)42 [17], This indicates that the isopolyacid structure degrades rapidly, with formation of Mo monomers. Peroxo complexes such as Mo(0 )42 or particularly MoO(C>2)32 are known to decompose with formation of 02 the overall process is a decomposition of two molecules of H2O2 into water and C>2 [18] ... 

Although stable in protic solvents (H20, MeOH), they decompose homolytically in nonprotic solvents (CH2C12, MeCN) to give 02 and the corresponding Vv-dioxo complexes. The reactivity of Vv-peroxo complexes toward hydrocarbons parallels their decomposition rate in nonprotic solvents, with the most reactive compounds being V0(02)(pic)(H20)2 (12) and [V0(02)(pic)2rH+-HMPA. 

The decomposition of the peroxometallacycle (72a) or (72b) occurs in a way different from that previously shown to occur in the epoxidation of alkenes by molybdenum-peroxo complexes (equation 26). The three possibilities are shown in equations (58)-(60) and involve (a) a [C-/6, C-a] hydride shift which directly produces the methyl ketone and the rhodium-oxo complex, or the hydroxo species from (72b equation 58) (b) a [C-/3,0-/3] hydride shift which gives enol (equation... 

The decomposition mode is somewhat similar to that previously shown in equation (28) for the Baeyer-Villager oxidation of ketones by Mo-peroxo complexes (c) a /3-hydride abstraction by rhodium followed by a nucleophilic attack at the coordinated alkene by the hydride (equation... 

The complex is green (e = 306 M-1 cm.-1 at 700 nm.) and paramagnetic.4 7 From an x-ray crystallographic study,8 the 02 bridge is known to be cobalt atoms (Co—O—O angles 120°), and the 0—O distance is 1.32 A. The five atoms in the ring are very nearly coplanar. The complex is stable in acidic solutions, but in neutral and alkaline solutions it is reduced to the corresponding peroxo complex, with subsequent decomposition of the latter. Ferrous ion also reduces the complex, in the first place to the peroxo complex. 

The subsequent reaction of the peroxo complexes with bromide gives oxidized bromine species (see Scheme 2). HOBr, Br2, and Br3 rapidly equilibrate, and Br3 is the predominant spectrophotometrically observed intermediate (Xmax 267 nm e = 36,100 M-1 cm-1) in the absence of an organic substrate. Tribromide is stabilized with respect to HOBr, Br2 and decomposition products by high bromide and acid concentrations (34). HOBr is reduced by excess hydrogen peroxide to yield bromide, water, and dioxygen, of which dioxygen can be measured. In the presence of TMB, the oxidized species is rapidly consumed in the bro-mination of TMB to BrTMB. Quantitation of BrTMB demonstrates that bromination is stoichiometric with respect to the concentration of H202 added. Thus TMB is a rapid, quantitative trap for the oxidized bromine species (33). 

Apart fi om those cases where the addition of acid hydrolyses a second bridging group, the kinetics of decomposition of the /<-peroxo complex are generally independent... 

The last three reactions of Fig. 16 (D, E, F) involve the decomposition of a. -peroxo dicobalt complex. The interpretation of the results is complicated by the presence of equilibria 12 and 13, and it is not always clear whether the reaction studied involves then-peroxo complex or one of the complexes formed by equilibria 12 and 13. 

Formation of Hydrogen Peroxide. There have been several reports of the decomposition of /4-peroxo complexes in aqueous solution to give Co(III) complexes and H2O2, although the hydrogen peroxide is frequently found in less than stoichiometric quantities. 

Eu has been studied and found to be inversely dependent on the concentration of H" " suggesting that the protonation of the peroxo unit stabilises it towards reduction as well as towards dissociation It is not always easy to distinguish these two rections and the apparent reduction of a series of / -peroxo complexes by Fe has been shown to proceed at the same rate as the decomposition in acid solution in the absence of Fe ... 

The oxidation of organic compounds by /<-peroxo complexes has not been studied in any detail. There have been several reports of the decomposition of /<-peroxo complexes containing dipeptide ligands in which the peptide is oxidised. A recent investigation has shown that this involves oxidation of the N-terminal position of the peptide, leading to a coordinated imine . The complex [(bipy)2Co(M-OH)( -02)Co(bipy)2] has recently been reported to catalyse the oxidation of 2,6-di-tert-butylphenol in methanol solution ), but it is not clear if the oxidation is catalysed by thecomplex... 

The schemes of radical-chain oxidation including reaction of Cat with R02 -radicals with intermediate formation peroxo-complexes [LM-OOR] [26-29] and further homolytic decomposition of peroxo-complexes ([LM-OOR]—>R C=0 (ROH) + R ) (cage latent radical mechanism) may explain parallel formation of alcohol and ketone under ethylbenzene oxidation in the presence of M(L )n (L1 = acac") and their complexes with R4NBr (wpr° (Cat + R02 ->)). 

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