Disproportionation conjugated systems

Until now, the overwhelming majority of disproportionations of larger molecules studied have concerned conjugated systems. Since all respective oxidation levels possess the same number of a bonds, consideration can be restricted to rr-electron energies. In solution, however, solvation energy should be taken into account. 

The mechanism is not as straightforward as it may appear in the first step, in fact, after OH radical addition to any aromatic ring, a new colored product is formed, i.e. the cyclohexadienylic type radical may reform the conjugated system in disproportionation reactions [7,1 Oj.The process is exemplified with a substituted benzene (Scheme 1). Using high doses, and especially in the presence of O2 or O3, aldehydes and organic acids are formed from the OH-dye adducts and finally decomposed to CO2 and H2O (see upper part of Scheme 1). 

Formation of 1,1,4-trimethylcycloheptane from car-3-ene depends on two properties of palladium, the strong tendency of palladium to promote isomerization and to hydrogenolyze vinylcyclopropane systems. In the bicyclic unsaturated acid, IV, prior isomerization is not necessary to move the double bond into conjugation, yet here, too, platinum and palladium give quite different results. The saturated bicyclic acid is formed over platinum, whereas over palladium a mixture of cyclohexane-carboxylic acid and benzoic acid results through hydrogenolysis and disproportionation (20). 

The hydroperoxyl radical (HOO-) is the conjugate acid of superoxide ion (02 ) (equation 93) and constitutes about 1% of the 02 that is formed in aqueous systems at pH 7. Although the 0-0 bond of HOO- traditionally is viewed to be the same as the single a bond of HO-OH (A//dbe, 51 kcalmoH ), its bond energy (AHdbe) is about 85kcalmoH, which is more consistent with the 1.5 bond order of 02. However, HOO- is unstable in protic media (such as water and alcohols) and rapidly decomposes via het-erolytic and homolytic disproportionation (equations 94 and 95). ... 

Vitamin C is another enol-based redox system like the hydroquinones and tyrosine, but it has no aromatic character. The enediol component is stabilized by a conjugated lactone group. The oxidation (Ej = +58 mV) occurs in two one-electron steps, but the ascorbate radical is not as stable as the semiquinone radical (Bielski, and Richter, 1977). The anion radical disproportionates via an initial dimerization in a similar way as semiquinone radicals (Bielski et al., 1981 Sawyer, et al., 1982). The lifetime of the radical is in the order of microseconds. The acidity of ascorbic acid (pk = 4.2) stems from the OH group in the P position to the lactone carbonyl group. It corresponds to the OH group of a vinylogous carboxylic acid (Scheme 7.2.11). Its UV maximum occurs at 260 nm (e = 1 x 1(T). 

The stability of phenoxy radicals is determined by the effect of conjugation of the unpaired electron with the system of the remaining bonds and by steric effects. The introduction of such voluminous substituents as tertiary butyl and phenyl, which shield the reaction centers of the radicals, sharply increases their stability. Radicals of imsubstituted or incompletely substituted phenols readily recombine or disproportionate, and do not accumulate in significant concentrations [4]. There is no direct and distinct relationship between the stability of the radical and the effectiveness of the corresponding phenol as an inhibitor, since the effectiveness depends not only on the reactivity of the OH-bond, but also on a number of other factors. However, there is a general correspondence between the stability of the radical and the effectiveness of the corresponding phenol. 

The reaction increases the Hj content of syngas, CO + Hj mixtures, and can be used in conjunction with the Fischer-Tropsch process. There has been renewed interest in this reaction as a source of for fuel cells. Metal carbonyls of Fe, Ru and Rh, under either acidic or basic conditions, are effective homogeneous catalysts. Fachinetti and co-workers have investigated the speciation and reactivity of Rh CCOju " and RujCCOjjj under acidic conditions. These systems are complicated by reactions such as disproportionation and complexation of the catalyst by the conjugate base of the acid. 

The pK of ascorbyl radical is 0.45 and it is present in its anionic form in the pH range of 0-13. The unpaired electron in the ascorbyl radical is delocalized over a highly conjugated tricarbonyl system, which makes ascorbyl radical unreactive. It decays either by disproportionation or by reaction with other radicals. 

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