Anion-Radical Basicity

Product 34 predominates in the polar aprotic solvent (acetonitrile), while in the polar protic solvent (methanol) products 35 are formed preferentially. The different products are caused by the relative rate of deprotonation against desilylation of the aminium radical, that is in turn governed by the action of enone anion radical in acetonitrile as opposed to that of nucleophilic attack by methanol. In an aprotic, less silophilic solvent (acetonitrile), where the enone anion radical should be a strong base, the proton transfer is favoured and leads to the formation of product 34. In aprotic solvents or when a lithium cation is present, the enone anion radical basicity is reduced by hydrogen bonding or coordination by lithium cation, and the major product is the desilylated 35 (Scheme 4). 

Let us now consider another organic species, such as a sulphone ArS02R known to be irreversibly reduced less easily than pyrene. The basic mechanism for its cathodic reduction has already been presented (reactions 3-6). It is necessary, however, to assume here that the chemical degradation of the anion radical when produced in solution is at least reasonably fast. 

The values of finK and ogek° obtained from the various experiments were then averaged and the results are shown in Table 3.3. Both the /toe values are close enough to 0.5 to support the basic argument that the first step, the formation of the anion radical, is rate-limiting. 

The pKa of the "OH radical is 11.9. The basic form is O ", which predominates at PH 12. Von Sonntag and coworkers14 found that the absorption at 310 nm of pulse radiolysis of pH = 13 N2O saturated solution of 1,4- or 1,3-cyclohexadiene indicates that 0 " anion radical only abstracts hydrogen atoms but does not add to the double bond. 

As noted in Section 2.2.5, the effect of dimerization may also be seen on the second wave, the wave that corresponds to the reduction of the radicals formed at the first wave. The example presented in Figure 2.35 shows the cyclic voltammetry of benzaldehyde in basic ethanol.26 The second wave represents the reduction of the benzaldehyde anion radicals formed at the first wave that have escaped dimerization. In other words, Scheme 2.29 should be completed by Scheme 2.30. 

Proton landing defines the basicity of anion-radicals. This landing assumes 1 1 stoichiometry with respect to an anion-radical and a proton donor molecule. For example, in the reaction of the naphthalene anion-radical (CjoH ) with methanol, this 1 1 stoichiometry should result in the formation of 50 50% mixture of naphthalene (CjoHj) and dihydronaphthalene (CioHjo). 

Neutral organic molecules can also be one-electron donors. For example, tetracyano-quinodimethane gives rise to anion-radical on reduction with 10-vinylphenothiazine or N,N,N, N -tetramethyl-p-phenylenediamine. Sometimes, alkoxide or phenoxide anions hnd their applications as one-electron donors. There is a certain dependence between carbanion basicity and their ability to be one-electron donors (Bordwell and Clemens 1981). 

Handy et al. 2006). Anion-radicals are bases and their basicity is freqnently very high. When the anion-radicals are formed in alkylimidazolinm ionic liqnids, there is some danger of their nndesir-able protonation. 

It is the position ortho to the maximum number of substituents that is most electron rich in an anion-radical of a starting aromatic compound. Being a less basic species, anion-radicals exhibit a more selective primary isotope effect than their more basic (and therefore... 

The absorption spectral properties of the neutral and anionic forms are quite different as shown in Fig. 1. Due to the rapid dismutation of flavin radicals to form an equilibrium mixture with the hydroquinone and oxidized forms of the flavin, special procedures must be employed to measure the spectral properties of free flavin radicals. Nearly quantitative amounts of anion radical can be formed in aprotic solvents under basic conditions Alkylation of the N(5) position of the flavin hydroquinone followed by oxidation results in nearly quantitative formation of the... 

Stabilization of a radical anion of humic acid may be caused by an adsorption effect. Bijl (3) observed that solid barium hydroxide octahydrate turned blue when placed in a solution of quinhydrone the blue solid was highly paramagnetic. Under the conditions we used for preparing these salts, insoluble sodium humate (with a large surface area) could have stabilized the anion radical by adsorption from the basic solution. Weiss and McNeil (18) observed a similar phenomenon with base soluble xanthenes, and proposed that biradicals may be formed in such a system. His compounds, however, do not appear to have the structural requirements to satisfy such a stabilized system. The recent report by Weber (29) on the spin content increase associated with the basification of a naphthoquinone-naphthohydroquinone system seems to parallel our observations quite closely. 

The basicity of anion radicals therefore consists of proton landing. The proton landing assumes 1 1 stoichiometry with respect to an anion radical and a proton-donor... 

Another example involves alkylation of the lithium salt of the anthracene anion radical by 2-octyl fluoride (Herbert et al. 1985). The akylation does not occur in dimethylfor-mamide (which strongly solvates Li+), whereas it is facilitated and becomes a quantitative reaction in diethyl ether. Coordination of the type (>CH—F " Li+), which is possible in a slightly basic solvent such as diethyl ether, seems to be the decisive factor in the reaction. 

The selectivity of the trap towards hydroxyl radicals was demonstrated by several control experiments using different radicals, showing that the formation of the respective hydroxylation product, 5-hydroxy-6-0-zso-propyl-y-tocopherol (57), was caused exclusively by hydroxyl radicals, but not by hydroperoxyl, alkylperoxyl, alkoxyl, nitroxyl, or superoxide anion radicals. These radicals caused the formation of spin adducts from standard nitrone-and pyrroline-based spin traps, whereas a chemical change of spin trap 56 was only observed in the case of hydroxyl radicals. This result was independent of the use of monophasic, biphasic, or micellar reaction systems in all OH radical generating test systems, the trapping product 57 was found. For quantitation, compound 57 was extracted with petrol ether, separated by adsorption onto basic alumina and subsequently oxidized in a quantitative reaction to a-tocored, the deeply red-colored 5,6-tocopheryldione, which was subsequently determined by UV spectrophotometry (Scheme 23). 

It is now well established that a variety of organic molecules such as polynuclear aromatic hydrocarbons with low ionization energies act as electron donors with the formation of radical cations when adsorbed on oxide surfaces. Conversely, electron-acceptor molecules with high electron affinity interact with donor sites on oxide surfaces and are converted to anion radicals. These surface species can either be detected by their electronic spectra (90-93, 308-310) or by ESR. The ESR results have recently been reviewed by Flockhart (311). Radical cation-producing substances have only scarcely been applied as poisons in catalytic reactions. Conclusions on the nature of catalytically active sites have preferentially been drawn by qualitative comparison of the surface spin concentration and the catalytic activity as a function of, for example, the pretreatment temperature of the catalyst. Only phenothiazine has been used as a specific poison for the butene-1 isomerization on alumina [Ghorbel et al. (312)). Tetra-cyaonoethylene, on the contrary, has found wide application as a poison during catalytic reactions for the detection of active sites with basic or electron-donor character. This is probably due to the lack of other suitable acidic probe or poison molecules. 

Several anion radicals have been found to undergo protonation on carbon by water. Steady-state esr studies on electron adducts in water have shown that the adducts of acrylate and acetylene-dicarboxylate protonate on carbon rapidly whereas the adducts of fumarate and maleate do not (Neta and Fessenden, 1972). A more recent study by pulse techniques has shown that the differences between the various adducts are not qualitative but present differences in the rate of protonation. It has been found that the acid forms of the acrylate electron adduct protonate slowly on carbon whereas the basic form reacts much more rapidly [reaction (80)]... 

Azine anion-radicals are highly basic and undergo N-protonation in the presence of proton sources. The resultant neutral 1-hydropyridinyl radicals are discussed subsequently (Section III,A,2). 

Conditions under which anion-radicals are commonly made, e.g., reaction with an alkali metal in an ethereal solvent, are strongly basic. Azoles without a nitrogen substituent in general, therefore, lose the proton from the heteroatom before the electron is added and, when the latter is added, the product is a dianion radical. Conversely, on oxidation of an azole to give a cation-radical, if the heteroatom is unsubstituted, the proton may be lost, yielding a neutral azolyl radical. Thus, from azoles various radicals may be formed dianion, anion, neutral, or cation-radicals according to the nature of the substituent at N. 

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