Inversion effect with electron transfer

Correcting /3 for the contribution from the 180 KIE upon 02" oxidation gives the 180 KIE = 0.982 on the reduction of 02 by Cu fSOD). The inverse isotope effect is inconsistent with electron transfer, which would result in weakening the 0—0 bond. The formation of a copper(II) peroxide, H02, or 022 is therefore characterized by an isotope effect of unity or greater. Since the estimated lsO KIE is inverse, it is instead more consistent with rate-determining protonation to form H02" (lsO EIE 0.978). Due to its positive redox potential, H02 would be expected to easily reoxidize the Cu SOD. 

Fig. 5.76 Enrichment, depletion and inversion effects with regard to the partial conductivities and the total conductivity. In the concentration presentation carrier (1) is enriched and the counterdefect (2) is depleted. Note too that, in the case of a mixed conductor (electronic and ionic defects) the space charge profile causes changes in the transference number (Ueon/u, < ion/o )-...

The neutral carboxyl group is not very effective in increasing the reduction rate of the complex. However, when the proton is removed from the carboxyl, the effect can increase and is greatest when the carboxyl ion is in a configuration favorable to chelation. Thus, the inverse (H+) path is not even observable for acid succinate in the same acidity range as that for which this path is important in the acid malonato reaction. The acid dissociation constants are known well enough so that the behavior difference between acid malonato and acid succinato can not be entirely ascribed to different acidities of the complexes. The results obtained with the acid malonate complexes, as reported in Table II, incidentally provide no support for the hypothesis (22) that electron transfer takes place by remote attack across hydrogen bonds. 

Hence, the first clearcut evidence for the involvement of enol radical cations in ketone oxidation reactions was provided by Henry [109] and Littler [110,112]. From kinetic results and product studies it was concluded that in the oxidation of cyclohexanone using the outer-sphere one-electron oxidants, tris-substituted 2,2 -bipyridyl or 1,10-phenanthroline complexes of iron(III) and ruthenium(III) or sodium hexachloroiridate(IV) (IrCI), the cyclohexenol radical cation (65" ) is formed, which rapidly deprotonates to the a-carbonyl radical 66. An upper limit for the deuterium isotope effect in the oxidation step (k /kjy < 2) suggests that electron transfer from the enol to the metal complex occurs prior to the loss of the proton [109]. In the reaction with the ruthenium(III) salt, four main products were formed 2-hydroxycyclohexanone (67), cyclohexenone, cyclopen tanecarboxylic acid and 1,2-cyclohexanedione, whereas oxidation with IrCl afforded 2-chlorocyclohexanone in almost quantitative yield. Similarly, enol radical cations can be invoked in the oxidation reactions of aliphatic ketones with the substitution inert dodecatungstocobaltate(III), CoW,20 o complex [169]. Unfortunately, these results have never been linked to the general concept of inversion of stability order of enol/ketone systems (Sect. 2) and thus have never received wide attention. 

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