Radical anion reactions

Now, we may consider in detail the mechanism of oxygen radical production by mitochondria. There are definite thermodynamic conditions, which regulate one-electron transfer from the electron carriers of mitochondrial respiratory chain to dioxygen these components must have the one-electron reduction potentials more negative than that of dioxygen Eq( 02 /02]) = —0.16 V. As the reduction potentials of components of respiratory chain are changed from 0.320 to +0.380 V, it is obvious that various sources of superoxide production may exist in mitochondria. As already noted earlier, the two main sources of superoxide are present in Complexes I and III of the respiratory chain in both of them, the role of ubiquinone seems to be dominant. Although superoxide may be formed by the one-electron oxidation of ubisemiquinone radical anion (Reaction (1)) [10,22] or even neutral semiquinone radical [9], the efficiency of these ways of superoxide formation in mitochondria is doubtful. 

Radical anion reactions have been treated less thoroughly than radical cations, although the same principles should apply. Bauld et al. [13] discussed the benzocyclobutene to o-quinodimethane (3 4) rearrangement because... 

At low pH values, dimerization must involve the combination of two neutral carbon radicals since the same ( ) / meso ratio is obtained as from the photochemical reaction of the carbonyl compound in methanol [26], a process which also involves neutral radicals. The switch in isomer ratio to that characteristic of alkaline media occurs in the region of pH close to the value of pKj for the neutral radical. Dimerization then occurs in a fast reaction between the radical-anion and the neutral radical. In strongly alkaline solutions where the pH pK the major reactive species formed at the potential of the first reduction wave is the radical-anion. Reaction between two radical-anions is relatively slow due to coulorobic repulsion so that dimerization in strongly alkaline solution still occurs by reaction... 

It is useful to briefly discuss some of the common and, perhaps, less common experimental approaches to determine the kinetics and thermodynamics of radical anion reactions. While electrochemical methods tend to be most often employed, other complementary techniques are increasingly valuable. In particular, laser flash photolysis and photoacoustic calorimetry provide independent measures of kinetics and thermodynamics of molecules and ion radicals. As most readers will not be familiar with all of these techniques, they will be briefly reviewed. In addition, the use of convolution voltammetry for the determination of electrode kinetics is discussed in more detail as this technique is not routinely used even by most electrochemists. Throughout this chapter we will reference all electrode potentials to the saturated calomel electrode and energies are reported in kcal mol. ... 

As mentioned above, in an earlier work, Nohl et al. [9] suggested that neutral ubisemi-quinone reduced dioxygen to superoxide (this suggestion was dropped in subsequent studies of these authors). Although the participation of neutral semiquinone in the reduction of dioxygen is impossible, the observation of these authors might be interpreted as the support of a role of ubihydroquinone in mitochondrial superoxide production. If neutral semiquinone is indeed formed in mitochondria via the protonation of semiquinone radical anion (Reaction... 

Although the redox potential of-OH is very high [E( OI I/OE I ) = +1.9 V (Klan-inget al. 1985) E(-OH, H+/H20) = 2.73 (Wardman 1989)], direct ET is rarely observed in OH-reactions, and where it occurs intermediate complexes are likely to be involved. For example, in its reaction with thiocyanate, where the final product is the three-electron bonded dirhodane radical anion [reaction (31) for other three-electron bonded systems, see Chaps. 5 and 7], a similar three-electron bonded intermediate might precede ET [reactions (29) and (30)]. 

Hydroxyl radicals react with many halide (pseudohalide) ions at close to diffusion-controlled rates thereby forming a three-electron-bonded adduct radical [e.g., reaction (1) k = 1.1 x 1010 dm3 mol-1 s 1 Zehavi and Rabani 1972], These adducts may decompose into OH" and the halide (pseudohalide) radical which then complexes with another halide (pseudohalide) ion yielding the dihalogen radical anion [reactions (2) and (3) k2 = 4.2 x 106 s"1 k3 1010 dm3 mol"1 s"1 for resonance Raman spectra of such intermediates, see Tripathi et al. 1985]. 

The reduction of disulfides is also only given by the hydroxymethyl radical anion [reaction (18)] while the hydroxymethyl radical itself is practically unreac-tive (Akhlaq et al. 1989). 

Thy and Ura behave like typical carbonyl compounds, and the first intermediate is a radical anion [reaction (163), in the case of Thy/Thd] which is in equilibrium with its O-protonated conjugate acid [equlibrium (164)]. 

From an extended study on the sequence selectivity of UV-induced cleavage of dsODNs (Table 12.12) it has been concluded that an ET to neighboring bases must occur [reaction (40)] followed by a subsequent competition between electron backdonation [reaction (41)], decay of the 5BrUra radical anion [reaction (42)] and hole transfer [reaction (43) Chen et al. 2000. 

Solvation of thiolates is similarly low in both protic and dipolar aprotic solvents because of the size and polarisability of the large weakly basic sulfur atom, so is unlikely to contribute appreciably to the observed solvent effect. The intermediate nitro radical anion is stabilised by H-bonding in a manner which retards its dissociation in the SrnI mechanism (upper equation in Scheme 10.35). In contrast, the electron flow in the direct substitution at X (lower equation in Scheme 10.35) is such that solvation by methanol promotes the departure of the nucleofuge. In summary, protic solvation lowers the rate of the radical/radical anion reactions, but increases the rate of the polar abstraction yielding disulfide. 

A similar hybrid type of radical/anionic reactions can be effected, when manganese metal, activated by catalytic amounts of lead dichloride and trimethylchlorosi-lane, is employed instead of zinc, which makes the original process synthetically more reliable and attractive by reducing the amounts of reagents (RX and ketone) needed to a 1.5 molar excess over the alkenes (Scheme 6.36) [57]. 

In most experiments and applications with titanium dioxide photocatalysts, molecular oxygen is present to act as the primary electron acceptor. Usually the electrons trapped as Ti(III) are transferred to dioxygen adsorbed at the semiconductor surface yielding peroxyl radical anions (reaction (7.16)) [16]. 

In the case of bisulfite, the primary propagation reaction involved the perox-ymonosulfate radical anion reaction with HSCV as follows ... 

Scheme 5. Examples of sequential anionic/radical/ anionic reactions promoted by Smij.

A solvated electron may be regarded as the simplest radical anion. The chemistry of solvated electron reactions is qualitatively similar to radical anion reactions but the physical properties of electron solutions are very complicated [167]. They are not suitable as a model of radical anions. 

The rate determining step for the disappearance of that remarkably stable (US.. SU) radical anion is the protonation of both radical anion [reaction (42)] and the free thiolate [reaction (43)] by reaction withHX... 

In the presence of oxygen, MeS02Cl establishes equilibrium with superoxide radical anion [reaction (49)] ... 

Early workers [103] detected benzilic acid formed during the reduction of benzophenone in dimethylformamide in the presence of carbon dioxide. The carbon dioxide radical anion system is known to have E" = —2.2V (vs. SCE) [104] and will thus not be formed in preference to the ketone radical anion. Reaction occurs through trapping of aromatic carbonyl radical anions by carbon dioxide, and this has been developed into a convenient synthesis of aryllactic acids. The modern technological process uses constant current conditions. On a small scale, a divided cell with mercury cathode has been used to obtain benzilic acids from substituted benzophenones and carbon dioxide in 70-90% yields [105] and to convert 4-isopropylacetophenone to the corresponding phenyllactic acid in 85% yield [106]. On a technical scale, these reactions are best carried out in an undivided cell using a lead cathode and a sacrificial aluminum anode with dimethylformamide as solvent... 

An interesting example of a radical anion reaction is the action of oxygen negative ion formed in chemical ionization in a mass spectrometer [109] ... 

Lithiating agent S.5.2.3.5 Li naphthalide radical anion Reaction with RX 5.5.2.2.1 C, H,LiN2... 

In the presence of other molecular species, a chain reaction involving conversion of a primary radical to a secondary species has sometimes been observed for example, the p-methylphenoxyl radical from HRP/H2O2 oxidation of p-cresol has been shown [121] to oxidize ascorbate to the corresponding radical anion. Reactions of this kind can be extremely effective. 

C.L. Greenstock, G.W. Ruddock (1976). Determination of superoxide (02 ) radical anion reaction rates using pulse radiolysis. Int. J. Radiat. Phys. Chem., 8, 367-369. 

Thymine and uracil behave similar to typical carbonyl compounds, i.e. the first intermediate is a radical anion [reaction (55)] which is in equilibrium with its oxygen-protonated conjugated acid [reaction (57)]. The other functional groups, especially the second carbonyl function, withdraw electron density, and hence the p a values of these oxygen-protonated radical anions are much lower (thymine p a = 6.9 [52]) than those of the corresponding radicals from simple carbonyl compounds. However, the C(6)-protonated isomers [cf. reaction (58)]... 

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