Oxo-Transfer Reactions

N2O is a potentially useful and environmentally friendly oxidant because it is inexpensive and also the only byproduct from an 0x0 transfer reaction would be dinitrogen. However, N2O is extremely (kinetically) inert [187] and is one of the least reactive species among nitrogen oxides. The very large activation energy (59 kcal moh ) [8] for the decomposition of N2O to N2-hO makes it very difficult to use N2O as a practical 0x0 transfer agent at ambient temperature. 

However, there has been progress in metal-mediated N2O activation. Oxo-transfer from N2O can lead to formation of metal-oxo complexes or oxidation of ligands contained in the metal complex. In pioneering research of N2O reactivity, Bottomley and coworkers [188-192] utihzed N2O in the preparation of a series of unusual oxo-bridged clusters with titanium, vanadium and chromium metal ions M-(N20)-M-bridged intermediates were suggested to form. 

Groves and coworkers demonstrated that [Ru (TMP)(THF)2] (TMP=tetrames-itylporphyrinate ) is efficiently oxidized by N2O to form a dioxo-Ru(VI) species, [Ru (TMP)(0)2] [193]. A dinitrogen complex, [(TMP)Ru N2(THF)] and N2O-bridged dinuclear complex, [Ru(TMP)-N=N-0-Ru(TMP)], were observed as key intermediates. 

Bleeke and coworkers [197] found a ligand-based oxidation to occur in the reaction of iridabenzenes with N2O. The transformations appear to proceed through a metallaepoxide intermediate, although the detailed mechanism of the reaction is not known. 

Reduction of O3 by [IrClg] , shown in Eq. (4), is first-order in both reactants and has a second-order rate constant of 1.7x 10 s at 25.0 °C. ° The initial electron transfer is outer-sphere in nature and allows computation of a self-exchange rate of 4M s for the 03/Of couple. Comparisons of this rate constant with the results of other electron transfer reactions of O3 reveal that inner-sphere mechanisms are common. 

The rate law for decomposition of H2O2 by [Mn(edda)] is given by Eq. (5), where k = 5.4M s at pH 7.0. The proposed mechanism involves formation of a mixed valence Mn(III)Mn(IV) dimer which facilitates the oxidation of manganese(II) to manganese(IV) by H2O2 without formation of OH. 

Superoxide dismutase activity of copper(II)-peptide complexes has been investigated. The rate is found to depend inversely on the reduction potentials for the copper(III)/(II) couple suggesting that copper(III) is involved in 

The complex [(bpy)20Ru—O—RuO(bpy)2] containing ruthenium(V) oxidizes water. Oxygen labeling studies etablish that the mechanism does not involve oxidation of noncoordinated water as a dominant pathway, but that some coupling of the coordinated aqua ligands is involved. 

Mechanistic studies on the Sharpless asymmetric epoxidation, Eq. (8), where DIPT is diisopropyl tartrate, have been published.The rate law in CH2CI2 is first-order in substrate, catalyst, and oxidant, and shows an inverse second-order dependence on the inhibiting alcohol, in this case Pr OH. This is consistent with a mechanism in which both substrate and the peroxide displace Pr O to form a key intermediate in the reaction. [Differences in the selectivities of allylic and homoallylic alcohols in this reaction have been exploited to invert the expected enantioselectivity.  

The controlled oxidation of alkanes into alcohols also attracts attention from an industrial point of view. Copper-based catalysts containing Tp ligands have been employed as catalysts for this reaction that led to a very interesting as well as unprecedented transformation with copper. Thus, when cyclohexane was reacted with in the presence of these catalysts, cyclohexane was partially converted into cyclohexanol and cyclohexanone, as expected. However, a certain amount of cyclohexane underwent dehydrogenation affording cyclohexene, in the first example of a copper-mediated alkane dehydrogenation process. Part of the cyclohexene was epoxidized in the reaction 

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In a study published concurrently with the Evans bis(oxazoline) results, Jacobsen and co-workers (82) demonstrated that diimine complexes of Cu(I) are effective catalysts for the asymmetric aziridination of cis alkenes, Eq. 66. These authors found that salen-Cu [salen = bis(salicylidene)ethylenediamine] complexes such as 88b Cu are ineffective in the aziridination reaction, in spite of the success of these ligands in oxo-transfer reactions. Alkylation of the aryloxides provided catalysts that exhibit good selectivities but no turnover. The optimal catalyst was found to involve ligands that were capable only of bidentate coordination to copper. 

The cis alkenes are more reactive and more selective than their trans counterparts. As with the Evans system, this reaction is not stereospecific. Acyclic cis alkenes provide mixtures of cis and trans aziridines. cis-p-Methylstyrene affords a 3 1 ratio of aziridines favoring the cis isomer, Eq. 67, although selectivity is higher in the trans isomer. A fascinating discussion of this phenomenon, observed in this system as well as the Mn-catalyzed asymmetric oxo-transfer reaction, has been advanced by Jacobsen and co-workers (83). Styrene provides the aziridine in moderate selectivity, Eq. 68, not altogether surprising since bond rotation in this case would lead to enantiomeric products. 

The oxo-transfer chemistry of molybdenum in sulfite oxidase is probably the best characterized, in terms of synthetic models, structural and mechanistic data, of all the elements we have described up till now. The reaction cycle (Figure 17.5) involves binding of sulfite to the oxidized MoVI, two-electron reduction of the Mo centre and release of sulfate. The Movl centre is restored by successive one-electron transfers from a cytochrome (bs in mammals). The primary oxo-transfer reaction ... 

Two five-coordinate Mn =0 complexes with N2O2 tetradentate ligands Ph4P[MnOL] (L= 1,2-bis(2-methyl-2-oxypropanamido)benzene and 1,2-bis(2,2-diphenyl-2-hydroxyethanamido)benzene) have been prepared and structurally characterized (12) and (13), respectively.These complexes and the 0x0 complexes discussed in the next section may have utility in oxo-transfer reactions to organic substrates such as olefins and ethers. 

The synthesis of the W02(R2Dtc)2 complexes is accomplished by an oxo transfer reaction from Mo203(L)4 (L = diethyldithiophosphate) to the W(II) complex W(CO)2(PPh3)(R2Dtc)2. This latter complex was obtained (119) according to the reaction 22... 

Following their investigations on nitrene, carbene, and oxo transfer reactions catalyzed by fluorinated silver tris(pyrazoyl)borate (see Chapter 6 on nitrene chemistry), Lovely et al. looked for a combined carbene transfer and [2,3]-sigmatropic rearrangement. On the basis of these mechanistic considerations, these authors showed that diazoacetates, indeed, reacted with allyl halides in the presence of this silver catalyst to give a-halo-y-unsaturated esters (Scheme 3.51).77... 

Mo(V) complex disproportionates as it dissociates to produce mononuclear Mo (IV) and Mo (VI). As Mo (IV) and Mo (VI) are directly interconvertible by an oxo transfer reaction, they are viable participants in catalytic cycles. A dinuclear Mo(V) species of this nature can thus supply either the oxidizing or reducing member of this couple and presents a mechanism by which molybdenum enzymes can channel reducing or oxidizing power. Several inorganic reactions have recently been explained using this scheme (80, 81). To date, however, Reaction 12 only applies when the ligand is a dithiocarbamate or dithiophosphate. Nevertheless, were there known dinuclear active sites in enzymes, this would be an important mechanism to consider. 

For the molybdenum oxidases, the reverse oxo transfer reaction can be postulated wherein an oxomolybdenum( VI) species donates oxo to substrate. For example, the oxidation of aldehydes (Reaction 18) can... 

Pietsch, M. A., and Hall, M. B., 1996, Theoretical studies on models for the oxo-transfer reaction of dioxomolybdenum enzymes, Inorg. Chem. 35 127391278. 

A detailed kinetic study on the disproportionation of (49b) in neutral aqueous solutions has been performed recently in relation to the mechanisms of oxidative DNA cleavage promoted by this complex.208 Such Crv oxo complexes are among the few known types of metal complexes that cause oxidative DNA cleavage in the absence of reactive oxygen species.209 The interactions of (49b) or [CrO(salen)]+ with DNA have been studied in detail and several possible mechanisms of these reactions have been proposed (reviewed in 2000-2003).11,13,210 Several mechanistic studies on the reactions of [CrO(salen)]+ and related complexes with organic reductants have been performed in relation to the roles of these complexes in Crm-salen-catalyzed oxo-transfer reactions (Section 4.6.5.8.4).211-215... 

The oxo-transfer reactions (illustrated by equations (i) (iii) in Scheme 13) form the basis for the syntheses of stable Crlv porphyrinato complexes (equations (i) and (ii) in Scheme 13 see also Section 4.6.4.2.2), as well as for the use ofJCr(tpp)Cl] and related complexes in redox catalysis (Equations (ii) and (iii) in Scheme 13).1 496 497 The related nitrido-transfer reactions (Equation (iv) in Scheme 13) have also been observed.139,498,499... 

The synthesis of the Sn(ll) bis-alkoxide complex [Sn(OSiMe3)2]2 can be brought about through an oxo-transfer reaction that involves a metathetical exchange between carbon dioxide and the divalent tin bis-amides. °... 

Oxo transfer reactions of perchlorate and other substrates catalyzed by rhenium oxazoline and thiazoline complexes 03CC2102. 

Absorbance vs time curves for guanosine-5 -monophosphate (GMP) are not monoexponential, as observed with the other substrates. The decay curves for GMP oxidation are sigmoidal in shape, as shown in Fig. 15. This behavior is consistent with formation of a bound, colored intermediate and eventual overoxidation of the substrate where the first step is much slower than the subsequent steps. Thus, initial oxidation of GMP produces a species that is oxidized more rapidly than GMP itself Oxidation of trans-stilbene by Ru(IV)0 occurs with an initial rate-determining step corresponding to the formation of a bound epoxide complex Ru(III)(epoxide), and other oxo-transfer reactions of Ru(IV)0 proceed via similar initial steps 130, 190). These bound... 

A wide variety of stoichiometric oxidants have been identified to be effective oxygen atom donors in oxo-transfer reactions with (salen)-metal and (porphyrin)-metal catalysts. These include NaOCl [26,27,28], alkyl hydroperoxides [29,30,... 

A number of theoretical calculations related to Compound I and II states and stereoselectivity of oxo-transfer reactions have been reported,and the role of the heme active site and protein environment in structure, spectra, and function of the cytochromes P450 from a theoretical point of view has been reviewed more recently. Theoretical calculations of the electron configuration of the compound I state have suggested that instead of being porphyrin-based, the radical is thiolate-based, and possible roles for a higher-spin state S = and either... 

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