Mono-oxygen species, atom transfer

The data of Figs. 2-1 and 2-2 and of Tables 2-3 and 2-4 provide persuasive evidence that the electron-transfer and atom-transfer chemistry for dioxygen species and mono-oxygen species is strongly dependent upon proton activity, anion solvation, and relative X-OO and Y-O covalent bond energies. Hence, the solvation energy of HO is 22.5 kcal less in acetonitrile than in water, which causes the HO-/HO- redox potential to be 4-0.92 V versus NHE in MeCN instead of 4-1.89 V versus NHE in H2O. 

Atom Transfer Reactions. The apparent catalysis of the auto-oxidation of triphenylphos-phine by (Ph3P)2Pt02 aroused considerable interest as a possible model for enzymatic mono-oxygenase reactions. This reaction was recently reinvestigated by Halpem and Sen who showed that intramolecular oxygen transfer does not occur. The reaction takes place via coordination of free phosphine to the platinum followed by loss of H2O2 or HO2 which can react with free phosphine. The Pt(II) species can be reduced to Pt(0) and may react with molecular oxygen. The reaction of triphenylphosphine with dioxo-... 

In summary, it seems obvious that a generally applicable, simultaneous solution of the spin and stoichiometry problems is rather difficult to achieve. Indeed, as far as single 0-atom transfer is concerned, the two problems are inherently linked. Thus, a metal catalyst enable of both activating 02 and mono-oxygenating a substrate has to release one 0-atom equivalent. For example, in the oxygenation of an alkane (R-H) by dioxygen, the residual O-atom may be bound as a metal-oxo species or as a peroxide (Reaction 7). 

Re has recently come to the forefront in liquid phase oxidation catalysis, mainly as a result of the discovery of the catalytic properties of the alkyl compound CH3Re03 [methyltrioxorhenium (MTO)]. MTO forms mono-and diperoxo adducts with H2O2 these species are capable of transferring an oxygen atom to almost any nucleophile, including olefins, allylic alcohols, sulfur compounds, amides, and halide ions (9). Moreover, MTO catalysis can be accelerated by coordination of N ligands such as pyridine (379-381). An additional effect of such bases is that they buffer the strong Lewis acidity of MTO in aqueous solutions and therefore protect epoxides, for example. 

Methyltrioxorhenium (MTO) is an extensively used catalyst for olefin epoxidation1241 and a number of reviews are available on its application.125"281 The complex reacts with hydrogen peroxide to form the mono r 2-peroxo species mpMTO and the diperoxide dpMTO. Both complexes are capable of transferring an oxygen atom to olefins or other substrates, as shown in Scheme 5.1, but the diperoxide species is about five times more reactive than the monoperoxo complex.1171 Care has to be taken that halide impurities are absent from the solvent as these are oxidised much faster than olefins.1171... 

This mechanism represents a shortcut in biological mono-oxygenase enzyme systems that use O2 as oxygen atom sources and whose mechanism is much more complex because of the requirement to cleave O2. Non-porphyrinic binuclear methane mono-oxygenase model complexes are also able to activate methane in the same way (see Chap. 19). When S = RH, Groves originally proposed the well-known rebound mechanism in which the Fe =0 species removes an H atom from RH, then transfers OH to produce the alcohol ROH ... 

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