Radical Protonated

Alonso et al. (2005) described anion-radical proton abstraction from prochiral organic acids. If the anion radicals were formed from homochiral predecessors, asymmetric deprotonation can be reached. However, low reactivity of the anion radical is required Slow proton transfer, that is, high activation energy of the reaction discriminates well between diastereoselective transition states. 

Some other interesting observations regarding free radicals in these systems are noteworthy. In many instances, multiple conformations of radicals are found at lower but not higher temperatures. This indicates that the radicals exist in shallow energy wells at low temperature this phenomenon was observed very early, in the 4 K ENDOR investigation of radical formation in amino acids.23 Unlike the process in DNA. In which it is well understood that the thymine anion radical protonates at C6 to form T(C6)H-, in the crystalline state there is a not clear link between pyrimidine electron adducts and H-addition radicals. We finally note that a deuterium isotope effect of protonation/deprotonation processes was found in cytosine.HCl and 2 -deoxycytidine.HCl, as evidenced by a lower propensity for these processes to occur in partially deuterated systems than in predated ones. 

Medium acidity is not essential for the deprotonation of the toluene cation radical, but medium basicity provokes the abstraction of the cation radical proton. Although this effect is obvious, special experiments were undertaken to define it more accurately (Bernstein 1992 and references therein). The studies show that at least three water molecules are re-... 

The electronic nature of a nitrogen centered radical, dictated by reaction conditions and/or the radical precursor employed, is crucial to the mode of reaction, to the ability to undergo efficient intramolecular cyclizations or intermolecular additions, and to the products isolated from the radical reaction. The types of radicals discussed in this review include neutral aminyl radicals, protonated aminyl radicals (aminium cation radicals), metal complexed aminyl radicals, and amidyl radicals. Sulfonamidyl and urethanyl radicals are known (71S1 78T3241), but they are not within the scope of this chapter. 

The metal-ammonia reduction proceeds by addition of an electron to the alkyne to form a radical anion, followed by protonation to give a neutral radical. Protons are provided by the ammonia solvent or by an alcohol added as a cosolvent. Addition of another electron, followed by another proton, gives the product. 

At the time Steenken s review article was written, the radical anion of guanine had not been fully studied in aqueous solution [5], This was considered in a later study which showed that radical anion G — rapidly protonates at N3 or N7 followed by tautomerization to give a radical protonated at C8 G(C8+H) [51], Many of the solid-state studies of guanine derivatives report these H-addition radicals even at low temperatures [37, 52],... 

The dark green complex [Co PhB(Bu lm)3 NHBu ] can further react with a radical proton abstractor (a phenol radical) to form the corresponding cobalt(Ill)-imido complex in a redox reaction that is reported to model a cracial step in the oxidation of water to elemental oxygen in the catalytic system of photosystem II [406] (see Figure 3.131). A similar iron(III) complex exists as well [415]. 

Neutral aminyl radicals bearing electron-withdrawing tV-substituents (for example, amidyl radicals), or aminium radicals (protonated aminyl radicals) and metal ion complexed aminyl radicals are more reactive due to the lower nonbonding electron density on nitrogen, and consequently the diminished repulsive interaction with the it electrons of the alkene4,6. 

It is clear from Table 7 that most of the phenoxyl radicals protonate on the oxygen to form phenol radical cations with pX a values about —1 to —2, i.e. 10 units lower than the pX a values of the parent phenols. Because of the strong acidities involved and the choice of the appropriate acidity functions, the pATa values are not as accurate as those measured under milder conditions (pH 2-12). There is no simple correlation between the pX a values and the a substituent constants. This is not surprising, since the a constant reflects the electron distribution in the molecule while the value depends on the electron distribution in the radical, which is different from that in the parent molecule. There appears to be some correlation between the effect of substituents on the pATj values and their effect on the spin density distribution in the radical, but not all the substituents... 

By comparison with phenoxyl radicals, the isoelectronic anUinyl radicals protonate much more readily pATa values in the range 4-7 have been reported for the equilibria between various anilinyl radicals and their corresponding aniline radical cations 45i... 

Radical proton abstraction catalyzed by transition metals involves strong KIE (kH/kD = 5 to 7). When the transition state is relatively dissymmetric, the KIE value is decreased to 2 or 3. 

By using incipient wetness impregnation of the solid samples with a solution of organic radical species such as TEMPO or TOTAPOL, surface enhanced NMR spectroscopy by DNP can be carried out to allow fast characterization of functionalized solid surfaces. Polarization is transferred from the radical protons of the solvent to the rare NMR active nuclei at natural abundance on the surface. Lesage et al. [48] have applied surface enhanced NMR spectroscopy by DNP to... 

The reactions of polyaromatic hydrocarbons (PAHs) in DDAB bicontinuous microemulsions followed the ECE (electrochemical-chemical-electrochemical) mechanism at low scan rates [81,82]. The three-step ECE process consists of electron transfer to the PAH to form an anion radical, protonation of the anion radical to yield a neutral radical,... 

Fig. 37 EPR spectra of semiquinone radical—protonated histidine interactions. Adapted with permission from [245]. Copyright 2008 American Chemical Society...

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