O-Atom Transfer

A number of mechanistic pathways have been identified for the oxidation, such as O-atom transfer to sulfides, electrophilic attack on phenols, hydride transfer from alcohols, and proton-coupled electron transfer from hydroquinone. Some kinetic studies indicate that the rate-determining step involves preassociation of the substrate with the catalyst.507,508 The electrocatalytic properties of polypyridyl oxo-ruthenium complexes have been also applied with success to DNA cleavage509,5 and sugar oxidation.511... 

The oxidation of dimethyl sulfide to dimethyl sulfoxide and dimethyl sulfoxide to dimethyl sulfone by [Ru(0)(bpy)2(py)] " also occurs by O-atom transfer with respective rate constants of 17.1 s and 0.13 M s in MeCN at 298 K. The rate-determining step in the oxidation of... 

For the one-electron reduction of nitrite to NO by nitrate reductase, an O atom transfer mechanism is unlikely, inasmuch as O atom transfer is inherently a two-electron mechanism. 

Oxygen atom transfer fnechanism of nitrite dehydrogenase from Nitrobacter agilis. The filled-in oxygen atom tepresents O. The lower pathway provides for O exchange with water, a process that competes with O atom transfer from nitrate to nitrite. M represents a molybdenum atom. From Friedman et... 

The SO 3 is readily converted to sulfuric acid in the presence of water vapor [6,7], An experiment employing 1803 provided evidence for acyclic adduct formation as originally proposed by Martinez et al. [127]. The ratio /c55//c54 was calculated to be (4.9 2.0) x 10 15 cm3 molecule. At any rate, the reaction of CH200 with S02 does not appear to be a simple bimolecular O-atom transfer reaction. 

Oxo-metal species participate in a wide range of biological and chemical oxidation reactions. Representative oxidizing enzyme, cytochrome P-450, which carries iron(III)-porphyrin complex as its active site, catalyzes various O-atom transfer reactions such as epoxidation, hydroxy-lation of C-H bond, and oxidation of sulfides. These reactions have been proven to proceed through cationic oxoiron(IV)-porphyrin species, which are generated by the oxidation of Fe(III) complex with molecular oxygen. This conversion from Fe(III) to 0=Fe(IV) species is a... 

Less attention has been paid, however, to C02 organometallic chemistry during the past decade. Whilst many reduction or coupling reactions are known to proceed in the presence of stoichiometric or catalytic amounts of transition metal complexes, very few examples remain where the formation of a metal-C02 complex has led to an effective, catalytic reduction reaction of C02. Carbon dioxide complex photoactivation also represents an attractive route to CO bond cleavage, coupled with O-atom transfer. However, progress in the area of C02 utilization requires a better understanding of the reaction mechanisms, of the thermodynamics of reaction intermediates, and of structure-reactivity relationships. 

Another group of reagents that can distinguish between Fe O2 + and HO" radicals are the sulfoxides, R2SO, which react with Fe 02+ by O-atom transfer to yield the sulfones, and with HO" radicals by addition and elimination of CH3" radicals ... 

Complexes of Tc(III) where O-atom transfer occurs by reacting [TcOC14] with several phosphine ligands in 4-picoline as solvent yield the mcr-[Cl3(pic)3Tc] and mer-[Cl3(pic)(PMe2Ph)2Tc] complexes. The products were characterized by x-ray diffraction and spectrophotometric methods [501]. 

Nitrous Oxide (N20). In contrast to nitric oxide, N20 is not readily reduced by hydrogen atoms. However, it is a good O-atom transfer reagent and spontaneously reacts with metal surfaces at elevated temperatures. Under alkaline conditions with a freshly formed platinum surface (Pt ), N20 induces a vol-tammetric reduction peak (Figure 11.8) 10... 

The proposed catalytic cycle and the structure of 9.36 are shown in Fig. 9.7. Nucleophilic S v2-typc attack by the olefin to the distal oxygen atom produces the epoxy alkoxide. The chiral environment around the Ti atom ensures that the allyl alcohol is oriented in such a way that O atom transfer takes place only on one particular enantioface. The discrimination between the two possible faces is stereoelectronic rather than steric in nature. The epoxy alkoxide is then replaced by allyl alcohol to give the epoxy alcohol and 9.37. The latter can react with more f-butyl hydroperoxide to regenerate 9.36. 

In some cases, singlet oxygen is formed when ozone reacts by O-atom transfer, for example with sulfides, disulfides, methanesulfinic acid, nitrite, etc. A detailed description of these results is beyond the scope of this paper for more information see [38]. 

In the case of complex ions wsuch as the oxy-halogen negative ions there is good evidence from tracer work that charge transfer from these ions takes place via O atom transfer. Anbar and Taube found that in the reaction CIO- + NOiT — Cl- + NOa"", the extra O in NOl comes exclu-... 

Tracer work with O has demonstrated that, in the reaction of SOs with the species XO, XOf, XO , CIO2, or CI2O, there is O atom transfer from oxidizer to SO3. The final products in acid solution are S04 and X. The same authors also studied the disproportionation of CIO2 dissolved in alkali ... 

As part of the work on model heme FeNO complexes, mechanistic studies on the reversible binding of nitric oxide to metmyoglobin and water soluble Fe, Co and Fe porphyrin complexes in aqueous solution, ligand-promoted rapid NO or NO2 dissociation from Fe porphyrins, reductive nitrosylation of water-soluble iron porphyrins, activation of nitrite ions to carry out O-atom transfer by Fe porphyrins, demonstration of the role of scission of the proximal histidine-iron bond in the activation of soluble guanylyl cyclase through metalloporphyrin substitution studies, reactions of peroxynitrite with iron porphyrins, and the first observation of photoinduced nitrosyl linkage isomers of FeNO heme complexes have been reported. 

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