Aminium cation radicals

Formation of an intimate ion pair of OH " and aminium radical cation was also proposed for the intermediate step before deprotonation. The presence of the above radical was verified through UV analysis of the polymer formed with the characteristic band on the end group. Through chromatographic analysis of the TBH-DMT reaction products, H2O was detected as the above mechanism proposes after deprotonation. 

Rosenblatt etal have examined the effect of structure and isotopic substitution upon the permanganate oxidation of some alky famines (Table 4). The isotope effect of 1.84 is considered to be sufficiently low to be compatible with aminium radical-cation formation, and it is felt that, while C-H cleavage is significant for oxidation of primary amines, the dominant mode of oxidation of tertiary amines is electron-transfer, e.g. 

The microscopic reverse of Eq. (2), the protonation of free radicals (such as, for example, aminium radicals) offers an additional access to aminium radical cations [57-60] (Eq. 3). 

An anodic azacyclization, producing tropane-related 11-substituted dibenzo[a,d]cycloheptimines 123, was recently developed by Karady et al. [136, 137]. This two-electron process is initiated by anodic oxidation of the O-substituted hydroxylamine 119 in nucleophilic solvent. It is proposed that the first one-electron oxidation leads to the aminium radical cation 120 which adds rapidly to the double bond. The electron-rich carbon radical 121 is readily oxidized to the carbocation 122. Selective nucleophilic attack on 122 from the less hindered exo-side yields the 11- substituted product 123. Depending on the... 

The X-ray crystal structure of the hexafluoroantimonate salt of 1,4-diithin radical cation stabilized by bicyclo[2.2.2]octane annelation revealed a planar ring and was in agreement with theoretical calculations. Tertiary aminium radical cations underwent facile 5-exo-cyclization to give distonic 2-substituted pyrrolidinium radical cations. 

A more detailed evaluation of the diverse structures proposed for the secondary species goes beyond the scope of this review. We mwely emphasize that the ESR results provide detailed evidence for the nature of the radical center, but fail to elucidate the cationic site. The identity of this center is left to secondary considerations or speculation. We also note that any alternative structure has the virtue of not contradicting the ab irutio calculations the potential c ture of chloride ion has precedent in the nucleophilic substitution at a cyclopropane carbon (see Section 7). Another type of ring-opened structure has been postulated as an intermediate in the aminium radical cation catalyzed rearrangement of l-aryl-2-vinylcyclopropanes (see Section 5). 

A similar pattern of reactivity has been observed by Burrows and coworkers for the reaction between A -acetyllysine methyl ester (Lys) and dG. This reaction was studied in order to gain an understanding of structural aspects of DNA-protein cross-links (DPCs). These cross-links are regarded as a common lesion of oxidative damage to cells, but remain, from a chemical point, a poorly understood DNA lesion. As pointed out by Burrows, oxidation of protein-DNA complexes should occur preferentially at the primary amines since these sites have a lower oxidation potential (1.1 V vs. NHE, pH 10) than G. While protonation of the primary amine inhibits the oxidative process, transient deprotonation of a lysine residue would give rise to a lysine aminyl radical (or aminium radical cation). Using... 

Radical cations can be generated by many chemical oxidizing reagents, including Brpnsted and Lewis acids, the halogens, peroxide anions or radical anions, metal ions or oxides, nitrosonium and dioxygenyl ions, stable aminium radical cations, semiconductor surfaces, and suitable zeolites. In principle, it is possible to choose a reagent with a one-electron redox potential sufficient for oxidation-reduction, and a two-electron potential insufficient for oxidation-reduction of the radical ion. 

Flash photolysis and laser flash photolysis are probably the most versatile of the methods in the above list they have been particularly useful in identifying very short-lived intermediates such as radicals, radical cations and anions, triplet states, carbenium ions and carbanions. They provide a wealth of structural, kinetic and thermodynamic information, and a simplified generic experimental arrangement of a system suitable for studying very fast and ultrafast processes is shown in Fig. 3.8. Examples of applications include the keton-isation of acetophenone enol in aqueous buffer solutions [35], kinetic and thermodynamic characterisation of the aminium radical cation and aminyl radical derived from N-phenyl-glycine [36] and phenylureas [37], and the first direct observation of a radical cation derived from an enol ether [38],... 

A rich variety of chemical oxidizing reagents have been applied for the generation of radical cations. The principal reagent types include Bronsted and Lewis acids the halogens certain peroxide anions or radical anions numerous metal ions or oxides nitrosonium and dioxygenyl ions stable organic (aminium) radical cations ... 

Electron transfer sensitized irradiation of acyclic dienes gives rise to mixtures of [4 + 2] and [2 + 2] dimers for example, an 8 1 mixture was obtained from 2,4-dimethyl-l,3-pentadiene [137]. On the other hand, [4 + 2] dimers were obtained exclusively upon oxidation with aminium radical cation salts [138]. This difference has led some workers to question the key role of electron transfer in these reactions... 

Aminium radical cations and aminoalkyl radicals have substantially different spin density distributions and, therefore, substantially different hyperfine coupling (hfc) patterns. Aminium radical cations have appreciable proton hfcs only in the position adjacent to the nitrogen center whereas the neutral aminoalkyl radicals have sizable hfcs in both the a- and (3-positions. CIDNP effects induced in these species are expected to reflect these differences. 

The polarization observed in the photoreaction of anthraquinone is also consistent with the concept of two consecutive intermediates [178]. Anthraquinone is a poorer oxidizing agent than benzoquinone so that the corresponding radical ion pair is less stable. Consequently, the aminium radical cation decays faster and does not contribute appreciably to the vinylamine polarization. 

In summary, these results constitute strong evidence for the two-step reaction sequence. They require that the deprotonation of the aminium radical cation be competitive on the CIDNP timescale i.e. surprisingly fast since it involves a carbon acid. The results delineate the fate of the amine derived intermediates with particular clarity, since they are observed directly for amine derived products. The conclusions based on the above CIDNP results were confirmed by time resolved optical spectroscopy in a variety of systems [179-182]. However, in essentially all these systems the reaction progress is monitored by following the complementary spectra of the acceptor derived radical intermediates, such as ketyl, semiquinone, stilbene, or thioindigo radical anions. 

In general, the increased efficiency of C—C bond cleavage observed for these substrates can be ascribed to two features a weakened C—C bond in the radical cation due to the presence of the second functional group and the greater acidity of the N-H or O —H functions compared to the a-C—H acidity of the aminium radical cation. 

Ring-opened cyclopropane radical cations have also been postulated to account for the stereochemistry of the aminium radical cation catalyzed rearrangement of l-aryl-2-vinylcyclopropanes [225]. These systems, of course, contain substituents that may veil the true nature of the cyclopropane radical cations by delocalizing spin and charge. In addition, we note that the experimental findings allow some latitude in their interpretations. 

ESR spectroscopies. In fact, the addition of one equivalent of 22a-c to an acetonitrile solution of aminium salt causes the replacement of the ESR signal of the aminium radical cation with a broad singlet signal shifted at a higher field (Fig. 3). 

Trisubstituted Nitrogen Oxidations and Aminium Radical Cation Deprotonations... 

The enzyme cytochrome P450 metabolizes tertiary amines in the liver by a one-electron transfer mechanism. An electron is transferred from N to a heme group featuring an Fe=0 bond to give an aminium radical cation and the radical anion [Fe]-0-. An a-hydrogen atom is then abstracted by the oxygen-based radical to give an iminium ion. Flydrolysis of the iminium ion affords a secondary amine and an aldehyde, both of which are further oxidized into water-soluble compounds and then excreted. The oxidation of tertiary amines to secondary amines can also be executed in the laboratory. 

Scheme 4. Detection of enol ether radical cations via aminium radical cations...

Thus, the types of reactions favored by amino radicals depend to a significant degree upon the extent with which the lone electron pair is associated with a proton, with a Lewis acid, or with an electron-withdrawing group. Clearly, aminium radical cations 2 (Scheme 1), metal-complexed aminyl radicals 3, amidyl radicals 4, sulfo-namidyl radicals 5 and cyanamidyl radicals 6 are electrophilic in nature. On the other hand, dialkylaminyl radicals 1 have been shown to display nucleophilic character... 

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