Oxidation Potential Schemes

The potential schemes (1) for Iodine species at unit activity and 10 unit activity In acidic and basic solutions are shown below. 

The only expected thermodynamic differences between the behavior of Iodine at low concentrations and Its behavior at normal concentrations Is due to the decreased stability of I2 at low concentrations I2 being both a better oxidizing and a better reducing agent when the concentration of I2 Is low. For example. consider the hydrolysis of l2  

In an aqueous solution at pH 7. I, Is 8 percent hydrolyzed when the total Iodine -3  

Latimer, W. M. The Oxidation States of the Elements and their Potentials In Aqueous Solutions , 2nd edition, Prentice-Hall, Inc. (1952), p. 67. 

Iodine Species at Unit Activity Iodine Species at 10 Unit Activity 

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Pyrroles, furans and thiophenes undergo photoinduced alkylation with diarylalkenes provided that the alkene and the heteroaromatic compound have similar oxidation potentials, indicating that alkylation can occur by a non-ionic mechanism (Scheme 20) (81JA5570). 

One of the most prominent characteristics of Fe(+2) is its ability to undergo oxidation leading to Fe(+3). This was used by Uchiyama et al. when they reported on Fe(+2)-ate complexes as potent electron transfer catalysts [7, 8]. These ferrates are accessible from FeCl2 and 3 equiv. of MeLi. The Fe(+2/+3) oxidation potential of [Me3Fe(+2)]Li 19 in THF is —2.50 V, thus being in between those of Sml2 (—2.33 V) and Mg (—3.05 V). With these alkyliron-ate complexes it was possible to realize a reductive desulfonylation of various A -sulfonylated amines 20 with different basicity. By using Mg metal to restore the active Fe(+2) species 19 a catalytic reductive desulfonylation process was achieved (Scheme 4). 

The free-radical scheme, however, fails to account for the following (i) It cannot be easily generalised to cover the identical kinetics of the Mn(lII) sulphate oxidation if -CH(C02H) has an oxidation potential comparable with Mn(Ill)/ Mn(II) pyrophosphate then it cannot appreciably reoxidise Mn(ll) sulphate, (a) If -CH(C02H) reoxidises Mn(II) sulphate then it should be capable of re-oxidising both V(1V) sulphate (of the V(V)/V(IV) pair, potential 1.0 V) and Mn(II) sulphate in the V(V) oxidation of malonic acid that it does neither can be seen from the rate laws of these oxidations which show no Mn(II)-retardation vide infra). Hi) The not dissimilar kinetics of the Mn(III) sulphate oxidation of formic acid vide supra) and mercurous ion °. 

The ideal electropolymerization scheme (Eq. (5.5.39)) is further complicated by the fact that lower oligomers can react with nucleophilic substances (impurities, electrolyte anions, and solvent) and are thus deactivated for subsequent polymerization. The rate of these undesired side reactions apparently increases with increasing oxidation potential of the monomer, for example, in the series ... 

Scheme 1 Structures of the Sa and Sd hairpin linkers, electron donor nucleobases and their oxidation potentials, and the hairpins 3G and 3GAGG shown in Fig. 1...

FIGURE 9.2 Dependence of scavenging ability of carotenoids in Scheme 9.3 (I-VI) with oxidation potential. 

It has been found (Polyakov et al. 2001c) that when carotenoids are involved in a reaction cycle with the participation of iron as Fe2+, an increase of the total radical yield or a prooxidant effect will occur and will increase with decreasing carotenoid oxidation potential and its scavenging activity. The mechanism of the participating carotenoid is shown in Scheme 9.4 (Polyakov et al. 2001c). 

Determination of electrochemical oxidation potentials and electrochemical reduction of 13 p-phosphorylated acyclic nitrones shows that phosphorylated compounds have a clear anodic shift of potentials of both, oxidation (Ep 1.40 to 2.00 V versus SCE in CH3CN) and reduction (Ep—0.94 to —2.06 V). This is caused by a strong electron-acceptor influence of the diethoxyphosphoryl group (430). In contrast, a reversible one-electron oxidation of azulene nitrones (233) (Scheme 2.80) occurs 0.6 V below the Ep potential of PBN, that is at the value one observes the oxidation of AH -imidazole-1,3-dioxides (219) (428, 429). In other words, the corresponding RC (234) is 14 kcal more stable than the RC of PBN. Although the EPR spectrum of RC (234) was not recorded, RC (236) from dinitrone (235) turned out to be rather stable and gave an EPR spectrum (170). 

Moreover, one should mention that in spite of similar electronic structures, PBN and the isoquinoline nitrone (278) react in a different way. Under no circumstances does PBN give an oxidative methoxylation product, whereas nitrone (278) reacts readily to form a,a-dialkoxy-substituted nitroxyl radical (280) (517). Perhaps this difference might be due to the ability to form a complex with methanol in aldo-nitrones with -configuration. This seems favorable for a fast nucleophilic addition of methanol to the radical cation (RC), formed in the oxidation step. The a-methoxy nitrone (279), obtained in the initial methoxylation, has a lower oxidation potential than the initial aldo-nitrone (see Section 2.4). Its oxidation to the radical cation and subsequent reaction with methanol results in the formation of the a,a-dimethoxy-substituted nitroxyl radical (280) (Scheme 2.105). 

Eq. (8) requires determination of the two-electron oxidation potential of L M by electrochemical methods. When combined with the two-electron reduction of protons in Eq. (9), the sum provides Eq. (10), the AGh- values of which can be compared for a series of metal hydrides. Another way to determine the AGh-entails the thermochemical cycle is shown in Scheme 7.3. This method requires measurement of the K of Eq. (11) for a metal complex capable of heterolytic cleavage of H2, using a base (B), where the pK., of BH+ must be known in the solvent in which the other measurements are conducted. In several cases, Du-Bois et al. were able to demonstrate that the two methods gave the same results. The thermodynamic hydricity data (AGh- in CH3CN) for a series of metal hydrides are listed in Table 7.4. Transition metal hydrides exhibit a remarkably large range of thermodynamic hydricity, spanning some 30 kcal mol-1. 

Schafer reported that the electrochemical oxidation of silyl enol ethers results in the homo-coupling products. 1,4-diketones (Scheme 25) [59], A mechanism involving the dimerization of initially formed cation radical species seems to be reasonable. Another possible mechanism involves the decomposition of the cation radical by Si-O bond cleavage to give the radical species which dimerizes to form the 1,4-diketone. In the case of the anodic oxidation of allylsilanes and benzylsilanes, the radical intermediate is immediately oxidized to give the cationic species, because oxidation potentials of allyl radicals and benzyl radicals are relatively low. But in the case of a-oxoalkyl radicals, the oxidation to the cationic species seems to be retarded. Presumably, the oxidation potential of such radicals becomes more positive because of the electron-withdrawing effect of the carbonyl group. Therefore, the dimerization seems to take place preferentially. 

Anodic conversion of aromatics proceeds in most cases by le-transfer to the anode to form a radical cation (34) (Scheme 9). Oxidation is facilitated by extension of the 7T-system ( 1/2 vs. Ag/Ag+ benzene 2.08 V, pyrene 0.86 V) and by electron donating substituents ( 1/2 vs. Ag/Ag+p-phenylenediamine —0.15 V). Oxidation potentials of polycyclic aromatics and substituted benzenes are collected in Ref [140-142]. 

Scheme 28 Lowering the oxidation potential of amide by an electroauxiliary.

Both anodic oxidation reactions proceeded well. As illustrated in Scheme 44, an anodic methoxylation of menthyl pyroglutamate followed by the trapping of an incipient A-acyliminium ion with allyl-silane in the presence of Lewis acid led to (138) [86]. While the stereoselectivity of this reaction was not high, the major product from the reaction could be fractionally crystallized from hexane and the route used to conveniently prepare (138) on a scale of 10 g. In this case, a platinum wire anode was used in order to keep the current density high. This was required because of the high oxidation potential of the secondary amide relative to the methanol solvent used in the reaction. 

Both inter- and intramolecular [5 + 2] cycloaddition modes have been utilized in the synthesis of natural products. Successful intermolecular cycloaddition depends on making an appropriate selection of solvent, supporting electrolyte, oxidation potential, and current density. This is nicely illustrated in Schemes 23 to 25. For example, in methanol the controlled potential oxidation of phenol (101) affords a high yield (87%) of (102), the adduct wherein methanol has intercepted the reactive intermediate [51]. In contrast, a constant current electrolysis conducted in acetonitrile rather than methanol, led to an 83% yield of quinone (103). 

The anodic oxidation of 1,4-diphenyl-1,2,3,4-tetrahydrocinnohne, leads to the corresponding 1,4-dihydrocinnoline and cinnohnium salt, at a more anodic potential (Scheme 91) [8]. 

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