Hydrogen bonds anion-radicals

In dry MeCN, n = 0.6 was found for 51, but the value increased to 0.94 on addition of water up to 10% [13]. For 52a in very dry MeCN, the mechanism of the reductive dimerization was examined and the experimental results were interpreted as an RS mechanism under these conditions [125]. However, addition of water increased the rate of reaction considerably, and in the presence of water the kinetic measurements were in accord with the RR mechanism [126]. In DMF, addition of water also accelerates the dimerization process for 52 [7,10], similar to what is observed for many monoactivated alkenes (Sec. II.A.7). The accelerating effect of water in DMF on the dimerization of 52a has been studied in greater detail [15]. On the basis of a reaction order in water close to 1, it was suggested that the dimerization reaction takes place between a free radical anion and a hydrogen-bonded radical anion [15]. The involvement of hydrogen bonding between radical anions and water may also account for the low activation energies found for the reductive dimerization of 52a in MeCN [126] and in DMF [10] (see Table 11). 

S. Sinnecker, E. Reijerse, F. Neese and W. Lubitz, Hydrogen bond geometries from paramagnetic resonance and electron-nuclear double resonance parameters Density functional study of quinone radical anion-solvent interactions, J. Am. Chem. Soc., 2004, 126, 3280. 

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. 

More generally, double bonds between two carbons or one carbon and a heteroatom, possibly conjugated with other unsaturated moieties in the molecule, are eligible for two-electron/two-proton reactions according to Scheme 2.20. Carbonyl compounds are typical examples of such two-electron/two-proton hydrogenation reactions. In the case of quinones, the reaction that converts the quinone into the corresponding hydroquinone is reversible. With other carbonyl compounds, the protonation of the initial ketyl anion radical compete with its dimerization, as discussed later. 

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). 

Addition of 0- to double bonds and to aromatic systems was found to be quite slow. Simic et al. (1973) found that O- reacts with unsaturated aliphatic alcohols, especially by H-atom abstraction. As compared to O, HO reacts more rapidly (by two to three times) with the same compounds. In the case of 1,4-benzoquinone, the reaction with O consists of the hydrogen double abstraction and leads to the 2,3-dehydrobenzoquinone anion-radical (Davico et al. 1999, references therein). Christensen et al. (1973) found that 0- reacts with toluene in aqueous solution to form benzyl radical through an H-atom transfer process from the methyl group. Generally, the O anion-radical is a very strong H-atom abstractor, which can withdraw a proton even from organic dianions (Vieira et al. 1997). 

Trifluoroethanol (TFE, CE3CH2OH) also demonstrates high H-bond activity. The dyad system in which a radical and electron-donor parts are linked directly undergoes intramolecular electron transfer on substitution of TEE for toluene as a solvent. The transition was interpreted as a marked effect of hydrogen bonding (or reversible protonation) of the anionic R-0 structure with TFE (Nishida et al. 2005). Scheme 5.17 depicts this transition. 

As mentioned earlier in Chapter 5, there are ion-radicals capable of forming hydrogen-bond complexes with neutral molecules. Such complexation significantly changes the redox potential comparatively to that of an initial depolarizer. Of most importance is that the formation of ion-radicals is a reversible process. In other words, the redox-switched effect operates in this host-gnest systems. Scheme 8.5 illnstrates the effect realized in the systems of ferrocene/ferrocenium (Westwood et al. 2004) and of nitrobenzene/the nitrobenzene anion-radical (Bn et al. 2005). 

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