Outer-sphere oxidation

The following outer-sphere oxidation mechanism is favoured over the radical-cation formation which is a feature of similar oxidations by Mn(III) acetate (p. 375). With toluene... 

The main oxidation product from dibenzyl ether is benzaldehyde (up to 80% yield) with smaller amounts of benzyl alcohol and benzoic acid. The rates of oxidation are only slightly affected by major stereochemical changes, and it is considered that an outer-sphere oxidation of the ether is followed by radical breakdown, viz. 

Hydroxyl radical is a strong indiscriminate outer-sphere oxidant (generating OH ) and H-atom abstractor (generating H2O) [Huie and Neta, 1999]. Simple Fe porphyrins are known to promote 0-0 bond homolysis in reaction with H2O2 [Watanabe, 2000]. Because of its high reactivity, once generated, "OH probably reacts with the... 

Homogeneous Processes with Tris-phenanthroline Metal(III) Oxidants. The rates of electron transfer for the oxidation of these organometal and alkyl radical donors (hereafter designated generically as RM and R, respectively, for convenience) by a series of tris-phenanthroline complexes ML33+ of iron(III), ruthe-nium(III), and osmium(III) will be considered initially, since they have been previously established by Sutin and others as outer-sphere oxidants (5). 

This gap in our knowledge is now closed, as the first paper on the uncatalyzed outer-sphere oxidation of an aliphatic thiol was recently published (12).This work selected thioglycolic acid (TGA, mercaptoacetic acid, HSCH2CO2H) as a representative thiol because of its high water solubility, low vapor pressure, and simple structure. The oxidant was [IrCle]2-, a well-characterized one-electron oxidant that frequently reacts through an outer-sphere mechanism. As is typical of such... 

The reactivity of RhL32+ or RhL33+ toward outer-sphere oxidation or reduction is a function of the intrinsic reorganization barrier for the couple as well as the driving... 

The pattern for outer-sphere oxidation by Co(NH3)5 compared with Co(en)j+ (usually it is —10 times slower) towards inorganic reductants can be used to support an estimate of the proportion of electron transfer (Marcus-dependent) and charge transfer which Ru(bpy) + displays towards these oxidants (45 and 11%, respectively). Sec. 2.2.1(b). Finally, Eqn. 5.35 can be used to determine K 2 for a reaction in which the other kinetic parameters are known. The value of A, 2 can be used, in turn, to estimate the oxidation potential of one couple, which is normally inaccessible. Thus the potentials of the o-, m- and /7-benzene diol radicals 1T2A4 were determined from kinetic data for the oxidation of the diols (H2A) by Fe(phen) + (5.45) ... 

The isotope effect suggests abstraction of H(D) from the NH(ND) group of Co(sep) + is necessary for the OJ reaction to form the (thermodynamically stable) H02 product. Direct outer sphere oxidation of Co(sep) + by O2 will not produce an isotope effect. 

Outer-sphere oxidations in bimolecular steps. The substrate is generally not coordinated... 

The most important classes of bimolecular reactions of transition metal complexes are ligand substitutions, reactions of the coordinated ligands and inner and outer sphere oxidation-reduction reactions28. ... 

Cobalt. Outer sphere oxidations of Fe(CN)6- by a MC excited state of Co(en)3+ 30S and alcohol by a LMCT excited state of Co(EDTA)- 306 have been reported. 

Platinum. Photolysis of some Pt(IV) complexes in aqueous solutions or in a frozen matrix results in the outer-sphere oxidation of anions and reduction of platinum to Pt(II)311-314, probably via a Pt(III) intermediate. 

For non-electrophilic strong oxidants, the reaction with an alkane typically follows an outer-sphere ET mechanism. Photoexcited aromatic compounds are among the most powerful outer-sphere oxidants (e.g., the oxidation potential of the excited singlet state of 1,2,4,5-tetracyanobenzene (TCB) is 3.44 V relative to the SCE) [14, 15]. Photoexcited TCB (TCB ) can generate radical cations even from straight-chain alkanes through an SET oxidation. The reaction involves formation of ion-radical pairs between the alkane radical cation and the reduced oxidant (Eq. 5). Proton loss from the radical cation to the solvent (Eq. 6) is followed by aromatic substitution (Eq. 7) to form alkylaromatic compounds. 

Amatore C, Thiebault A, Verpeaux J-N (1989) Unexpected single electron-transfer catalyzed cyclization of prenyl sufore dimer - evidence for radical-anion coupling in the outer-sphere oxidation of prenyl sulfone carbanion. J Chem Soc Chem Commun 20 1543-1545... 

The nature of the coordinated dioxygen within the Ru porphyrins remains uncertain. Some data (lack of an ESR signal, visible spectrum) support a Ru(III)—02 formulation, while the other data (comparison of visible spectral data with the corresponding carbonyl, binding constant for 02, and binding of ethylene) imply a Ru(IV)—022- formulation (8, 135). We have reconstituted apomyoglobin with Ru porphyrins, and characterized in solution the Ru(II) species RuMb and the oxidized met-form RuMb+ (142) as well as the carbonyl RuMb(CO), which has been reported previously (143). Unlike myoglobin itself, RuMb is hexacoordinate and low-spin treatment with 02 leads to the met-form via an outer-sphere oxidation (cf. Reaction 35). Our hopes of... 

An outer sphere oxidation of free CN1 by TPPFeIII(CN)21", which is mechanistically described by Reaction 13. The electrochemical oxidation of cyanide ion in acetonitrile occurs at potentials more positive than +0.5 volt vs. SCE (22). The electrochemical reduction of TPP-FeIII(CN)21" in acetonitrile occurs at a potential of —0.5 volt vs. SCE (25). Thus an outer sphere mechanism is considered unlikely. 

Detailed kinetics of some of these and other processes have been studied, e.g. oxidation by [Os(bipy)3]3+ of J , SCN",167 OH" 122 reaction of [Os(LL)3]3+ with OH- to give 02 (LL = bipy, phen, 5-Mephen, 4,4-Me2bipy),168 oxidation of ascorbic acid by [Os(LL)3]3+ (LL = bipy, 4,4 -Me2bipy) 169 and the photoinduced electron transfer reactions of acidopentaamminecobalt(III) complexes with [Os(bipy)3]2+.170 The outer-sphere oxidation of ascorbic acid by [Os(bipy)3]2+ incorporated in a coating on a graphite electrode has recently been reported.170... 

The first estimate of E° for the SCN/SCN- couple was performed by Shizuka and co-workers (289) their result was based on an equation (of unclear provenance) that relates the energy of the ctts band of the anion and its E°. Such an approach cannot be expected to yield exact results. The remaining estimates were determined in two ways. One involved measuring the rate of oxidation of SCN- by an outer-sphere oxidant such as [Fe(bpy)3]3 + and then assuming a rate constant for back-electron transfer that was diffusion controlled. The other method was to use an established E° for the I/I- couple and then to derive the SCN/SCN- potential by using the equilibrium constants reported by Schoneshofer (272) for the following reactions ... 

A third possibility, in which a highly favorable outer-sphere oxidation of disulfide radical anions to neutral disulfides, Eq. (38), becomes ratedetermining, will yield... 

Marcus theory showed a good correlation between experimental and calculated rate constants using Eq. (5). The 22 value was set at 10 M sec for this purpose and is considered as an upper limit for selfexchange of the diethyldithiocarbamate radical/anion pair. From the oxidation rates it was also estimated that E (edtc /edtc ) = 0.43(3)V vs SCE. A free-energy analysis for the oxidation of diethyldithiocarbamate (edtc ) by [FeiCNlgT also showed that the initial outer-sphere oxidation of the thiolate anion to its thio radical (Eq. 36) is the main energy barrier to be crossed along the reaction coordinate. 

The strongly oxidizing radical S04 is often considered to be a natural one-electron (outer-sphere) oxidant, Eq. 25 ... 

Most chemical applications of metal-polypyridine complexes are based on the high reactivity of their unusual oxidation states. Their use as stoichiometric reagents [258] seems to be largely limited to outer-sphere oxidations with tris-polypyiidine complexes of Fe Ru , and of the less oxidizing Co or Cr . Redox potentials of Fe and Ru complexes can be finely tuned. Section 5.3.3. By contrast, reduced polypyridine complexes are usually not employed as stoichiometric reductants, probably because of an abundance of other reducing agents which are more easily handled. 

Outer-sphere oxidations in bimolecular steps. The substrate The reaction occurs in the coordination sphere of the metal, is generally not coordinated The substrate is generally activated by coordination. 

Studies by cyclic voltammetry of anodic oxidation of organotin compounds at a platinum electrode in acetonitrile show that the primary irreversible process is the outer sphere oxidation of R4Sn to the radical cation, followed by rapid fragmentation into R3Sn+ and R which is then rapidly oxidised further to R+. The rate constants of the reactions correlate with those for the oxidation by Fe(III) complexes. Values for the oxidation potentials and the ionisation energies are given in Table 5-3. 

Complexes of ligands with O-donors and two or three N-donors are known. The kinetics of the outer-sphere oxidation of cis-aq uaoxovanad i u m (IV) complexes of [2-(pyridylmethyl)imino]diacet-ate and its derivatives were determined.675 Complexes with Schiff bases have been used to mimic the structure and chemistry of vanadium bromoperoxidase.275 The ligation of an imidazole functionality in the ligand has been found to readily dissociate, and is important to the functional aspects of this complex.275 A variety of five-coordinate complexes with tridentate Schiff base complexes have been prepared, several of which have been found to form supramolecular polymeric structures through association between the V=0 groups in a V=0 V=0 V=0 pattern (140).627... 

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