Oxidative addition outer-sphere electron-transfer

The oxidation of Pt(II) complexes is thought to proceed in most cases via addition of an electrophile to the Pt(II) center (141,178-182). This process does not involve outer-sphere electron transfer. The initial product of the electrophile addition is possibly a five-coordinate Pt(IV) species, however, the observable Pt(IV) product is six-coordinate. Coordination of a sixth ligand, e.g. solvent, occurs (183). This may proceed concertedly with the addition of the electrophile, in particular if the sixth ligand is solvent, or alternatively after the addition of the electrophile, as shown in Scheme 35. 

There are two possible routes by which electron transfer could result in the oxidation of the methyl substituent on heme O. The first is via outer-sphere electron transfer, as depicted in Figure 5. In this mechanism, the cofactor heme B binds and activates O2 to form compound I, and then heme O is oxidized via a peroxidase-type mechanism. In the second, related mechanism, HAS oxidizes heme O via autoxidation. In this case, heme O binds and activates O2 to form compound I, while heme B is presumably involved in shuttling electrons from a putative ferredoxin to the active site. Heme O would then be oxidized by internal electron transfer, similar to the mechanism of heme cross-linking elucidated by Ortiz de Montellano and coworkers (22). While the labeling experiments of HAS strongly suggest that heme O is oxidized via electron transfer, they do not allow us to distinguish between these two possible scenarios, and additional experiments are required. 

Ruthenium porphyrins represent an additional possibility, viz. outer-sphere electron transfer to dioxygen, followed by phosphine oxidation by the produced ... 

Oxidative addition of alkyl halides can also occur in certain cases by an outer-sphere electron transfer mechanism involving a coordinatively saturated metal center and an alkyl halide. This pathway is shown in Scheme 7.7. Oxidative addition by this initial outer-sphere electron transfer pathway tends to occur instead of an 5, 2 pathway when the electrophile is particularly susceptible to electron transfer, when the electrophile possesses some steric hindrance, when the electrophile possesses a weak C-X bond, and when the metal lacks an available coordination site. Because of the lack of a coordination site at the metal, the initial electron transfer occurs without prior coordination of the electrophile to the metal. This initial step parallels the electron transfer and subsequent radical chemistry that occurs when some carbanions are treated with alkyl halides. ... 

Scheme 3.19 Oxidative addition via an outer-sphere electron transfer.

Electron self-exchange reaction between O2 and 02 was then discussed, and developments before and after an experimentally determined rate constant for this reaction was published, were also summarized. Related to this, the problem of size differences between O2 or 02 and their typical metal-complex electron donors or acceptors was recently solved quantitatively by addition of a single experimentally accessible parameter, A, which corrected the outer-sphere reorganization energy used in the Marcus cross relation. When this was done, it was found that rate constants for one electron oxidations of the superoxide radical anion, 02 , by typical outer-sphere oxidants are successfiiUy described by the Marcus model for adiabatic outer-sphere electron transfer. 

Oxidation—Reduction. Redox or oxidation—reduction reactions are often governed by the hard—soft base rule. For example, a metal in a low oxidation state (relatively soft) can be oxidized more easily if surrounded by hard ligands or a hard solvent. Metals tend toward hard-acid behavior on oxidation. Redox rates are often limited by substitution rates of the reactant so that direct electron transfer can occur (16). If substitution is very slow, an outer sphere or tunneling reaction may occur. One-electron transfers are normally favored over multielectron processes, especially when three or more species must aggregate prior to reaction. However, oxidative addition... 

Most of the kinetic models predict that the sulfite ion radical is easily oxidized by 02 and/or the oxidized form of the catalyst, but this species was rarely considered as a potential oxidant. In a recent pulse radiolysis study, the oxidation of Ni(II and I) and Cu(II and I) macrocyclic complexes by SO was studied under anaerobic conditions (117). In the reactions with Ni(I) and Cu(I) complexes intermediates could not be detected, and the electron transfer was interpreted in terms of a simple outer-sphere mechanism. In contrast, time resolved spectra confirmed the formation of intermediates with a ligand-radical nature in the reactions of the M(II) ions. The formation of a product with a sulfonated macrocycle and another with an additional double bond in the macrocycle were isolated in the reaction with [NiCR]2+. These results may require the refinement of the kinetic model proposed by Lepentsiotis for the [NiCR]2+ SO/ 02 system (116). 

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