Cations deprotonation

A number of papers have reported studies on pyrimidine radical cations. 1-Methylthymine radical cations generated via a triplet-sensitized electron transfer to anthraquinone-2,6-disulfonic acid were detected by Fourier transform electron paramagnetic resonance (FTEPR). The parent 1-methylthymine radical cation, and its transformation to the N(3)-deprotonated radical cation, were observed. Radical cations formed by addition of HO and POs" at C(6) were also detected depending on the pH. Similarly, pyrimidine radical cations deprotonated at N(l) and C(5)-OH were detected from the reaction of 804 with various methylated pyrimidines." These radicals are derived from the initial SO4 adducts of the pyrimidines. Radical cations of methylated uracils and thymines, generated by electron transfer to parent ions of... 

In such hydrogen fluoride free media, which stabilize aryl cations to a lesser extent than 70% hydrogen fluoride/pyridine mixture, aryl cation deprotonation does not occur very rapidly. On the other hand, the fluoride nucleophilicity increases with the pyridine content thus, very high fluorodediazoniation yields can be reached in hydrogen fluoride free media, provided that the diazotization step is conducted under strictly controlled conditions.40,45 67 Illustrative results, obtained by the following general procedure, are listed in Table 2 and compared to those resulting from other conditions when available.40... 

The mechanism of the reaction of thiophene with a variety of radicals as a function of pH has been studied using ESR (81JCS(P2)207). Attack by -OH at pH 6 proceeds by direct addition with a preference to add to the a-position the ratio of (226) to (227) is 4 1. At low pH the (3-adduct easily loses OH- to form the thiophene radical-cation, which may undergo rehydration. In the case of 2-methyIthiophene the radical-cation deprotonates to give the thenyl radical this is reminiscent of the electrochemical oxidation (Section 3.14.2.6). The radical-cations are also formed by direct electron abstraction from the thiophene substrates by chlorine anion-radicals. At pH >6, (226) starts disappearing with formation of ring-opened products (Scheme 61). 

The latter result is particularly interesting because it contrasts so vividly with the chemistry observed in homogeneous solution. With a homogeneously dispersed single electron photoacceptor, 1-methoxynaphthalene gives the same product as that formed in Ti02, but 1-methylnaphthalene gives a completely different product, that derived from exclusive side chain activation by radical cation deprotonation, eq. 94 (290) ... 

When 2-benzopyrylium cations have a substituent with a fairly mobile hydrogen atom (a-alkyl group or heteroatom group in any other position of the cation), deprotonation of such a substituent occurs even under mild conditions by an acid-base interaction as the primary step (Section III,A). Although deprotonation in both cases leads to compounds whose structures can be depicted by two resonance formulas, either with charge separation (betaines) or without (anhydrobases), on discussing products of C-deprotonation, the term (and the corresponding formula ) anhydro-base is more often used, whereas products of O-deprotonation are called betaines. ... 

Pyrimidines. Photoexcited anthraquinone-2,6-disulfonate undergoes ET with Thy and its methyl derivatives as indicated by Fourier transform EPR (Geimer et al. 1997). These pyrimidine radical cations deprotonate at N( 1) thereby giving rise to the corresponding N-centered radicals [reaction (6)]. 

From a pulse radiolysis study on the S04 -induced reactions of Thd (Deeble et al. 1990), it has been concluded that the pKa of the Thd radical cation (deprotonation at N(3)) should be near 3.5, i.e. close to that at N( 1) in Thy. It is noted that also in the parent, Thy, the pKa values at N( ) and at N(3) are quite close. A Fourier-transform EPR study using photoexcited anthraquinone-2,5-disulfonic acid to oxidize Cyt and IMeCyt shows that the radical cation of the former de-protonates rapidly at N(l) while that of the latter deprotonates at the exocylic amino group (Geimer et al. 2000). The EPR evidence for the formation of heteroatom-centered radicals by S04 in its reactions with some other pyrimidines (Bansal and Fessenden 1978 Hildenbrand et al. 1989 Catterall et al. 1992) is in agreement with a marked acidity of such radical cations. It is re-emphasized that this conclusion does not require that radical cations are formed in the primary step. 

Pyrimidines. Reaction of Thy with photoexited menadione or its electrochemical oxidation yields mainly to the N(l)-C(5)-linked dimer (Hatta et al. 2001). This can be accounted for if the precursor radical cation deprotonates at N( 1) (see above). For this N(l)-centered radical a second mesomeric form with the spin at C(5) can be written. Head-to-tail recombination leads to the isopyrimi-dine-type dimer [reaction (12)]. Isopyrimdines are unstable (see below) and add rapidly water [reaction (13)]. This dimer is also formed in the reaction with S04 , albeit with a lower yield. 

Guanine is the most easily oxidize DNA base. This means that holes, created at random sites, will move around until encountering a guanine. In order to be stably trapped on guanine, the cation will have to deprotonate. The site of deprotonation has only recently been determined. EPR/ENDOR results predicted a cation deprotonated at the exocyclic amine G(N2-H) while model calculations predicted a cation deprotonated at N1 G(N1-H). ... 

Benzylic deprotonation is often an inefficient process. It may be more important than it would appear from the end products, however, since radical cation deprotonation followed by reduction of the radical and reprotonation may regenerate the starting material. This mechanism has been proposed to explain the inefficiency of some PET alkylations [68]. In suitable models such a process has been revealed, e.g. deuterium incorporation at the bis-benzylic position in 2-(4-methoxyphenyl)-2-phenylethyl methyl ether and cis-trans isomerization in 2-methoxy-l-(4-methoxyphenyl)indane (but not in the corresponding 3-methoxyphenyl derivatives) [204], as well as deconjugation of 1-phenylalkenes to 3-phenylalkenes in the presence of 1,4-dieyanobenzene, biphenyl (as a secondary donor) and a hindered pyridine as the base [205]. Deprotonation of N,N-dimethylaniline has likewise been observed (Scheme 38) [206-207],... 

Trisubstituted Nitrogen Oxidations and Aminium Radical Cation Deprotonations... 

Formation of a-carbon radicals either from cation decomposition or through abstraction of H by OH- is considered here because it can be construed formally as loss of an electron followed by proton transfer. If the cation deprotonates from the peptide chain by transfer to components of high proton affinity then the a-carbon radical is formed directly. Abstraction by OH in fluid systems gives the same result. 

FIGURE 12.72 Loss of the pyrophosphate gives an aUyhc cation that can be attacked by a molecule of 3-methyl-3-buten-l-ol pyrophosphate to give a new cation. Deprotonation gives geranyl pyrophosphate. 

Rgs. 14.31 and 14.32). Perhaps a related technique will work here. Indeed it does, as the addition of a small amount of the catalyst AlBr3 or AICI3 to a mixture of benzene and isopropyl bromide or chloride results in the formation of isopropylbenzene (Fig. 14.35). The mechanism should be easy to write now. A complex is first formed between isopropyl bromide and AlBr3. This complex can be attacked by benzene to give a resonance-stabilized cyclohexadienyl cation. Deprotonation of the intermediate cyclohexadienyl cation gives isopropylbenzene. 

The acylium ion can then add to a benzene ring to generate an intermediate cyclo-hexadienyl cation. Deprotonation by a Lewis base completes the reaction and produces... 

By far the most important reaction in this chapter is electrophilic aromatic substitution by a variety of electrophiles (E ). This reaction involves the initial formation of a resonance-stabilized, but not aromatic, cyclohexadienyl cation. Deprotonation regenerates an aromatic system (Fig. 14.119). 

FIGURE 19.34 Under acidic conditions the enol is formed and then reacts with iodine to give the resonance-stabilized cation. Deprotonation leads to the Ot-iodide. 

Electrochemical and Electron paramagnetic resonance (EPR) studies have been focused on neutral carotenoid radicals. These species are derived from carotenoid radical cations deprotonation (Focsan et al., 2015). At lower radical concentration this behaves like a scavenger for reactive species (single oxygen, hydroxyl or peroxyl radicals) (Foot, 1976). Although the antioxidant activity of carotenoids is higher than that of a-tocopherol, a modest contribution (less than 5%, due to modest concentration levels of carotenoids in oil) to the total activity is expected (Muller et al., 2011). 

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