Protonation radicals

Electron transfer reduction of pyridines in both acid and alkaline solution generates the protonated radical-anion. This rapidly accepts a further electron and a proton to give a mixture of dihydropyridines. Enamine structures in these dihydro-pyridines can tautomerise to the imine, which is more readily reduced than the original pyridine molecule. Further reaction of the 1,4-dihydropyridine leads to piperidine while reduction of the t, 2-dihydropyridine leads to a tetrahydropyridine in which the alkene group cannot tautomerise to the imine and which is not therefore reduced to the piperidine stage. The reaction sequence is illustrated for 2,6-dimethyl-pyridine 18 which yields the thermodynamically favoured cis-2,6-dimethylpiperidine in which the two alkyl substituents occupy equatorial conformations. 

The rapid rates of reduction of the oxalato (10) (k = 450 + 1,000 (H+)) and of the pyruvate (2) complexes (2A x 103at 25°C. and (H+) = 0.1) can hardly be understood as caused by chelation. Binoxalate does not chelate unless the proton is lost, and the rate law for the reduction of the complex shows that it brings a proton into the activated complex. Pyruvate almost certainly is not chelated in the product. Both groups are rapidly reduced by Craq.+2 when they are feee from the cobalt center. (The reduction of H2C2O4 by Craq+2 was explored by R. Milburn and the present author (29). The observations on pyruvate were made by R. Butler (2)). The complexes of pyridine-2-carboxylate and pyridine-4-carboxylate are rapidly reduced by Cr+2 at least in the forms which present the nitrogen without associated protons. Radical ion intermediates for these structures are not unreasonable. In fact, a stable free radical derived from AT-ethyl-4-carbethoxypyridinyl has been... 

Pyrazine 192 is reduced in acidic solution in two one-electron steps through the protonated radical to 1,4-dihydropyrazine (193) 192 and 193 can react with formation of two PH- radicals.306,307 The 1,4-dihydro- (or... 

The protonation of anion radicals and dianions derived from aromatic hydrocarbons has been studied in some detail by Hoijtink and co-workers 113-11 s). it was shown that apart from the reactions given above (Eqs. (42)—(46)) other disproportionation equilibria also play an important role. These are different for different anion radicals, making the whole picture very complex. Kinetic studies on the disproportionation of the nitrobenzene anion radical and some of its derivati-ves 116,u 7) jlave s 10wn that in aqueous solution at a pH > 11.5, reaction (41) is of great importance, whereas the protonated radical ion and the radical ion are the kinetically active species in the pH interval between 3.2 and 11.5. 

Free radicals also have distinctive Raman stretching frequencies. For example, ribonucleotide reductase (Fig. 9) has a stretch at 1498 cm-1 that is not present in the hydroxyurea-treated radical-free protein [208], This is close to that observed [209] for deprotonated phenoxy radicals (1505 cm-1), and different [210] from that of the protonated radicals (1426 cm-1). Thus it was concluded that the tyrosine radical was the neutral deprotonated radical, a conclusion that is difficult to reach from the EPR spectra alone. 

Fremy s salt, 0N(S03)22 is the last of the stable free radicals in aqueous solution to be described in this review. Quantitative studies of its chemistry are rare, and the reader is referred to a recent paper by Balasubramanian and Gould for details (26). According to these workers the protonated radical has a pK less than 5.6. They cite a potential of —0.350 V for the 0N(S03)2270N(S03)23 couple in alkaline media. This potential was derived from potentiometric titrations of this electrochemically reversible couple by Aoyagui and Kato, who also noted that for H0N(S03)22 the pKt = 12.0 (20). These results have been confirmed in a voltammetric investigation (255). 

The two-site jump model appears to work well for this polymer. However, it should be noted that the density of transitions is large here due to the larger spin quantum number of deuterium (/= 1), the fact that there are three of them in the isotopically substituted polymeric radical, and that the coupling constant for each deuterium is smaller by a factor of 6.4 compared to the protonated radical. Coupling these facts to the visual fitting process, these fits may not be unique. In fact, when the same model is applied to the temperature dependence of the protonated PMMA spectra (Fig. 14.2), reasonable visual fits could not be obtained with this model. Deuteration of the... 

Pyridinyl radicals are weakly basic, and disappear in aqueous solution in a reaction which involve either dimerization or disproportionation in the reaction of a radical with a protonated radical. The consumption of protons in disproportionation reactions depends upon the stability of the initial product, varying from one to two. 

The basicity of pyridinyl radicals is of interest since some pyridinyl radicals react with one another in an acid catalyzed reaction (sect. 4.4). The absorption spectra of radicals generated from the pyridinium ion by pulse radiolysis in aqueous solution at different pH values allow the evaluation of the basicity of the radicals 97,98) I jjg Qf protonated form of l-methyl-3-carbamidopyridinyI radical, (CONHj) ), is 1.43 the protonated radical has alsorption maxima at 3(X) nm and 440 nm, at somewhat longer wavelengths than those for the unprotonated radical at 280 nm and 420 nm. The nicotinamide adenine dinucleotide radical (NAD ) has a pK, of 0 or less, with a shift in absorption maximum due to protonation being observed only in 1.5 M HCIO4. 

Fig. 15-6. The energy diagrams of a one-proton radical pair in zero field (left) and in the presence of a weak ( al) magnetic field (right). The energy separations are illustrated in unit of h.. (Reproduced from Ref. [14] by permission from Taylor Francis Limited)...

A) Magnetic field dependence of the singlet recombination yield (proton radical pair for various rate constant (fc4i), calculated using Eq. (15-19). (B) Properties of the... 

Fig. 15-7(A) shows the MFEs on the singlet recombination yield (

Scheme 19. Gassman s proton/radical cation dichotomy.

There has been some discussion about the detailed mechanism of the reduction of aromatic nitro compounds. In acid solution the slow step in the four-electron reduction has been found [96,98] to be the uptake of the second electron, the reduction of ArN02H to the dihydroxylamine, ArN(OH)2. According to some authors, ArN02H [96,98,99] is formed on protonation of the radical anion, but there are also strong proponents for its formation by electron transfer to a preprotonated nitro compound [100,101]. In water the pKa of the protonated radical anion, ArN02H, for most aromatic nitro compounds, is between 2 and 4 [102]. 

Nitrones have been used to indicate the presence of short-lived radicals, as the addition of a radical to a nitrone forms a radical which is stable enough for an esr-investigation [96]. Reduction of a nitrone as A-/-butylphenylnitrone in DMF yields the Schiff base, which is further reduced. In the presence of a proton donor the protonated radical anion from the Schiff base may add to the nitrone to a stable radical, so it is recommended not to have a proton donor present when using nitrones for investigation of the formation of radicals during a reduction. 

We consider here the detailed consequences of the reduction in aprotic solvents, two sequential steps, of azobenzene and analogous aromatic azo compounds [12-15]. The dianion is considerably more basic than the radical anion, and the dianion is only long-lived in very dry nonacidic solvents [14-16]. Both the dianion and the radical anion derived from azobenzene and substituted azobenzenes have been used as EGBs. The anion resulting from protonation of the dianion is less basic (by several pATa units) than the dianion itself but more basic than the radical anion [15]. Using the dianion as EGB may therefore result in mono- or diprotonation, depending on the strength of the acid. The radical anion leads directly to hydrazobenzene due to the further reduction of the protonated radical anion (Scheme 3) and fast protonation of the more basic anion [pATa(Ph-NH-NH-Ph) =26.1, pATa(Ph-NH-N -Ph) 31, pAa(Ph-NH-N -Ph) < 20]. 

However, the diprotonated cation 126 formed at higher acid concentrations cacid > 11.0 M is also oxidized to the protonated radical cation 1261, which follows the same mechanism of decomposition, giving protonated 2-bromo-6-nitro-p-benzoquinone (130) as the final product. 130 can be further reduced to 131, as observed on CV curves recorded in solutions with higher acid concentrations. 

Sulfonamides have been studied for cathodic and anodic activity at rotating disk electrodes of gold and platinum in 0.1 M H2SO4 and 0.1 M Na2C03 solutions. All the sulfonamides display anodic activity. The free (protonated) amino group of the sulfonamides is oxidized by a one-electron reaction to the protonated radical cation imine (73). 

Irradiation of 3-methylthiophene in hexafluoropropan-2-ol with added methanesulfonic acid brings about single electron-transfer oxidation and this yields a species that has been identified as the Z>w-protonated radical-cation of (289). Irradiation of 2-acetyl-5-iodothiophene leads to cleavage of the C-I bond and the resultant radical adds to acrylonitrile to afford the adduct (290) in moderate yield. Reactions of this type have also been described for iodofuran and iodopyrrole derivatives and in the latter case 4,5-diiodo-l//-pyrrole-2-carboxaldehyde reacts photochemically with thiophene to give... 

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