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Deprotonation aminium radical reactions

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. [Pg.232]

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... [Pg.187]

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. [Pg.172]

Deprotonation at the a-carbon of aminium radicals produced in SET reactions... [Pg.1055]

The photoreduction of 9,10-anthraquinone-1,5-disulfonate by 2,2,6,6-tetramethyl piperidine in aqueous media has been studied in the nanosecond and microsecond time domains by use of time-resolved optical and ESR measurements [171]. Electron transfer from the amine to the excited state of the anthraquinone derivative occurs with a rate constant of 5.7 x 10 m s . The aminyl radicals formed via deprotonation of the aminium radicals are long-lived (ca 0.5 ms), because the steric hindrance of these radicals slows down recombination reactions. The aminyl radicals formed in these systems have been characterized by ESR. [Pg.1066]

Studies using isotopic labeling as well as mechanism based on inactivation such as 4-alkyldihydropyridines and cycloalkylamines have supported the view that the first step involves an electron-transfer process (path a. Scheme 23) [18, 184-186, 211]. Deprotonation of the resultant aminium radical would yield the a-aminoalkyl radical. The formation of the dealkylated amine and carbonyl derivative has been proposed to occur via a second electron transfer to the enzyme and a nonenzymatic hydrolysis of the imine formed. In the P-450 catalyzed reaction, however this process is proposed to occur via a radical recombination process to yield a carbinolamine (99) which then decomposes to the dealkylated amine and the corresponding carbonyl derivative. Evidence for this was obtained by the incorporation of label from 02, into the carbonyl derivative [212-214]. [Pg.1073]

Synthesis of oxazabicycloalkanes and related products was achieved by a one pot electron-proton-electron (EPE) transfer mediated reactions of the amine moiety [317]. Here the iminium cation is formed from the second electron oxidation of the a-aminoalkyl radical, generated via the a-deprotonation of the planar aminium radical owing to their low ionization potential. The iminium cation thus formed can... [Pg.1099]

Okazaki O, Guengerich FP. Evidence for specific base catalysis in M-dealkylation reactions catalyzed by cytochrome P450 and chloroperoxidase. Differences in rates of deprotonation of aminium radicals as an explanation for high kinetic hydrogen isotope effects observed with peroxidases. / Biol Chem 1993 268 1546-52. [Pg.509]

Watanabe et al. (134) and Miwa et al. (135) have suggested a single-electron transfer mechanism for the enzymic N-demethylation reaction based on the small kinetic isotope effect, since the deprotonation of the a-hydrogen of the aminium radicals prepared by several means is known to proceed with smaller values (134, 135). The oxidation of cyclopropylamine by P-450 has been shown to afford products believed to be derived from the aminium radical intermediate... [Pg.445]

Direct Measurement of Deprotonation Rates Relative Rates of Aminium Radical a-CH Deprotonation Based on Photoproduct Distributions Desilylation Reactions of a-Trialkylsilylaminium... [Pg.2065]

A more thorough evaluation of the dynamics of tertiary aminium radical a-CH deprotonation reactions has come from relatively recent studies employing stopped-flow, time-resolved laser flash photolysis, and electrochemical techniques. In one effort, Dinnocenzo and Banach examined acid-base reactions of the stable aminium radical 3, prepared by SET oxidation of N,N-dianisyl-N-methylamine (Scheme 5), Using stopped-flow kinetic methods, these workers showed that the bimolecular rate constants for a-deprotonation of 3 by four quinuclidine bases 4 in MeCN are (1) in the range of 1 x 10 to 6 x 10 M ... [Pg.2068]

Information about the relative kinetic acidities of aminium radicals has also come from a number of studies probing product distributions of SET-promoted photochemical reactions. One of the earhest investigation in this area was conducted by Cohen and co-workers, in which product distributions of SET-induced photoreactions of unsymmetrically substituted tertiary amines were used to gain information about how alkyl substituents affect the rates of aminium radical a-CH deprotonation. Irradiation of solutions of 4-benzoylbenzoic acid containing a variety of tertiary amines leads to formation of dealkylation products (Scheme 9). In these processes, the initially formed aminium radical 10 transfer a proton to the 4-benzoylbenzoic acid anion radical to generate an a-amino radical, which then donates an electron to another 4-benzoylbenzoic acid molecule. Hydrolysis of iminium ions 11, formed in this manner, then gives carbonyl and secondary amine products. The results of these studies qualitatively indicate that dealkylation occurs preferentially at the less alkyl-substituted a-carbon. [Pg.2071]

Detailed investigations of the SET photochemistry of tertiary amine-stilbene systems by Lewis and co-workers - have elucidated a number of factors that govern the relative rates of a-deprotonation reactions of aminium radicals. As shown in Scheme 10, photoreactions of stilbene with unsymmetricaUy substituted tertiary amines in acetonitrile proceed via pathways in which contact ion radical pairs (CIRP), generated by excited-state SET, undergo proton transfer to yield radical pair precursors of stilbene-amine photoadducts. By careful analysis of the product distributions, Lewis has determined the relative kinetic a-CH acidities of the intermediate aminium radicals (Table 101.4). [Pg.2072]

Laser flash photolysis techniques have been employed to evaluate the dynamics of decarboxylation reactions of cation radicals derived from a-aminocarboxylates. > In one report, variously substituted aminium radicals 47 were generated by laser flash excitation of anilinocarboxylates 46 (Scheme 21) in MeCN solutions containing the acceptor, 1,4-dicyanobenzene. These transients undergo fast, first-order decay by a pathway involving loss of carbon dioxide. The rate constants for decarboxylation were found to be in the range of 8 x 10 to 4 x 10 s . In addition, the rates show the same dependence on nitrogen, a-alkyl and a-phenyl substituents, as do the related a-CH deprotonation, a-desilylation, and retro-Aldol cleavage reactions. [Pg.2080]

As with arene-amine radical ion pairs, the ion pairs formed between ketones and amines can also suffer a-deprotona-tion. When triplet benzophenone is intercepted by amino acids, the aminium cation radical can be detected at acidic pH, but only the radical formed by aminium deprotonation is detectable in base (178). In the interaction of thioxanthone with trialky lamines, the triplet quenching rate constant correlates with amine oxidation potential, implicating rate determining radical ion pair formation which can also be observed spectroscopically. That the efficiency of electron exchange controls the overall reaction efficiency is consistent with the absence of an appreciable isotope effect when t-butylamine is used as an electron donor (179). [Pg.277]

The one-electron oxidation of a secondary amine results in the formation of a secondary aminium ion which on deprotonation gives an aminyl radical (Scheme 1). The nature of the final products derived from these intermediates dqiends very much on the structure of the substrate and the reaction conditions. If the amine has a hydrogen atom on the a-carbon atom the major products usually result from deprotonation at this a-position. With aromatic secondary amines, products can result from coupling of the delocalized radicals at a ring carbon atom. The formal dimerization of aminyl radicals shown in Scheme 21 is therefore not often a useful method of preparation of hydrazines. Nickel peroxide has been used to oxidize diphenylamine to tetraphenylhydrazine in moderate yield, and other secondary arylamines also give... [Pg.745]

Hydrogen abstractions in ketone/amine and quinone/amine systems continue to attract the attention of CIDNP spectroscopists [94] despite the facts that the application of CIDNP to these reactions dates back to 1974 [95] and that the basic mechanism — electron transfer from the amine DH followed by deprotonation of the resulting aminium cation DH + to give an a-aminoalkyl radical D" has already been cleared up in those early investigations [46]. CIDNP spectroscopy is very well suited to probe the microscopic details of such reactions that involve more than one radical intermediate. Polarizations can arise in both DH + and D, but the spin density distributions of these two radicals differ strongly. Hence, the polar-... [Pg.123]

Amines form relatively stable radical cations, and back electron transfer often overrides chemical reactions, except in the presence of good electrofugal groups in the a—position. Nevertheless, a-deprotonation (or alternatively a proton or hydrogen transfer process within an exciplex) may be fast enough to cause addition to unsaturated esters and ketones, " aromatics, and arylalkenes." " Deprotonation and further oxidation of some tertiary amines appear to produce aminium salts that are then aUylated by aUyl radicals simultaneously produced by fragmentation of allylsilane. Both processes are brought about by photosensitized electron transfer." ... [Pg.102]


See other pages where Deprotonation aminium radical reactions is mentioned: [Pg.2067]    [Pg.229]    [Pg.1058]    [Pg.1071]    [Pg.1075]    [Pg.279]    [Pg.279]    [Pg.181]    [Pg.2066]    [Pg.2068]    [Pg.2071]    [Pg.2075]    [Pg.2075]    [Pg.2078]    [Pg.124]   


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Aminium radical

Reactions deprotonation

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