Protonated amino radicals

The reductive couphng of imines can follow different pathways, depending on the nature of the one-electron reducing agent (cathode, metal, low-valent metal salt), the presence of a protic or electrophihc reagent, and the experimental conditions (Scheme 2). Starting from the imine 7, the one-electron reduction is facihtated by the preliminary formation of the iminiiim ion 8 by protonation or reaction with an electrophile, e.g., trimethylsilyl (TMS) chloride. Alternatively, the radical anion 9 is first formed by direct reduction of the imine 7, followed by protonation or reaction with the electrophile, so giving the same intermediate a-amino radical 10. The 1,2-diamine 11 can be formed from the radical 10 by dimerization (and subsequent removal of the electrophile) or addition to the iminium ion 8, followed by one-electron reduction of the so formed aminyl radical. In certain cases/conditions the radical 9 can be further reduced to the carbanion 12, which then attacks the... 

The mechanism for the photoreaction between 133 and cyclohexene can be summarized as in Scheme 8. The initiating electron transfer fluorescence quenching of 133 by cyclohexene resulted in the formation of an w-amino radical-radical cation pair 136. Proton transfer from the 2-position of the cyclohexene radical cation to the nitrogen atom of the a-amino radical leads to another radical cation-radical pair 137. Recombination of 137 at the radical site affords the adduct 134, while nucleophilic attack at the cation radical of 136 leads to another radical pair 138 which is the precursor for the adduct 135. 

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. 

The stability of the semireduced Eosin radical produced by reduction with triethanolamine is similar to that reported for the radical in aqueous solution [209], Our results indicate that the lifetime is substantially shorter when it is generated by reduction with triphenylamine. We attribute this difference to the ability of the triethanolamine radical cation to form the a-amino radical by loss of a proton, as shown below, thus preventing the reaction of the radical by reverse electron transfer (Eq. 18). 

It has been suggested in the literature that the a-amino radical is the species that initiates polymerization [210], This view is supported by our observation that, in spite of the relatively high quenching rate constant of Eosin triplet by triphenylamine (Table 5), the system Eosin-triphenylamine does not sensitize the photopolymerization of multifunctional acrylates. Thus, it is necessary that the amine contains a hydrogen at the a-carbon to be released as a proton after oxidation of the amine by the dye triplet. This deprotonation prevents the back electron transfer and forms a carbon radical that is sufficiently long-lived to be captured by the monomer. 

Grollmann U, Schnabel W (1980) On the kinetics of polymer degradation in solution, 9. Pulse radiolysis of polyethylene oxide). Makromol Chem 181 1215-1226 Hamer DH (1986) Metallothionein. In Richardson CC, Boyer PD, Dawid IB, Meister A (eds) Annual review of biochemistry. Annual Reviews, Palo Alto, pp 913-951 Held KD, Harrop HA, Michael BD (1985) Pulse radiolysis studies of the interactions of the sulfhydryl compound dithiothreitol and sugars. Radiat Res 103 171-185 Hilborn JW, PincockJA (1991) Rates of decarboxylation of acyloxy radicals formed in the photocleavage of substituted 1-naphthylmethyl alkanoates. J Am Chem Soc 113 2683-2686 Hiller K-O, Asmus K-D (1983) Formation and reduction reactions of a-amino radicals derived from methionine and its derivatives in aqueous solutions. J Phys Chem 87 3682-3688 Hiller K-O, Masloch B, Gobi M, Asmus K-D (1981) Mechanism of the OH radical induced oxidation of methionine in aqueous solution. J Am Chem Soc 103 2734-2743 Hoffman MZ, Hayon E (1972) One-electron reduction of the disulfide linkage in aqueous solution. Formation, protonation and decay kinetics of the RSSR radical. J Am Chem Soc 94 7950-7957... 

Neutral aminyl radicals (I), also referred to as amino radicals, can be considered to be nucleophilic species whereas aminium cation radicals (II), metal complexed aminyl radicals (III), and amidyl radicals (IV) are electrophilic in nature. Greater utility has been observed with electrophilic nitrogen radicals than with neutral aminyl radicals (71 SI). Aminyl radicals are easily protonated with Br0nsted acids to give aminium cation radicals and readily complex with Lewis acids to form radicals III therefore, control of the reaction conditions is critical to ensure that reactions of interest are occurring from only one species. 

Under irradiation with light (A > 290 nm), tertiary amines are able to transfer one of the electrons of the lone pair on nitrogen to a molecule to produce a cation radical that loses a proton a- to nitrogen to form an a-amino radical.280... 

In the PET reactions of ketone-amine pairs, proton transfer from the amine cation radicals to the ketyl radical anions is exceptionally efficient and radical pair formation (thus coupling or reduction) dominates other possible reaction modes (Eqs. 33, 34). In general coupling of the a-amino radical formed by proton transfer from initially formed amine radical cation ketyl radical anion terminates the reaction process. Early examples of ketone-amine photoreactions were reported by Cohen [170] and have been updated recently [10, 11]. Detailed mechanistic studies of these reactions have been discussed by Peters [171]. 

The decomposition by the iron(II) salt of hydroxylamine-O-sulfonic acid in the presence of formamide, alkylformamides, or A -alkylacetamides provides the amino radical cation +NH3 which, by hydrogen abstraction, generates carbamoyl and a-Wamidoalkyl radicals. Both kinds of radicals selectively attack protonated quinoxaline. 

Cyclopropyl-substituted ketones are suitable substrates for generating distonic radical anions from ketyl radical anions. A series of cycloalkanone substrates with unsaturated side-chains, to trap the primary radical formed after cyclopropylcar-binyl ring opening, has been investigated (Scheme 31) [118, 119]. For the first electron-transfer step triethylamine is used as electron donor. The reaction sequence is terminated by proton or hydrogen transfer from the solvent or the a-amino radical formed after deprotonation of the amine radical cation. 

The rate of initial electron transfer from A,7V-dimethylaniline to [Fe(phen)3] + is diffusion-limited. This is followed by the rate-determining proton transfer from the radical cation to pyridine to give the deprotonated a-amino radical which is rapidly oxidized by a second equivalent of [Fe(phen)3] + to yield the product iminium ion. Kinetic isotope effects [kii/kjf) for the proton transfer were determined from the J3/tfo ratios of the products derived from p-substituted A-methyl-A-trideuteromethylanilines. The k /kx) value first increases and then decreases with increasing pAa of p-substituted A,A-dimethylaniline. Such a bell-shaped isotope effect profile is typical of proton-transfer reactions [82, 85]. The maximum kn/fco value is determined as 8.8 which is much larger than the corresponding value for the demethylation of the same substrate by cytochrome P-450 (2.6) [79]. 

In the primary reductive step, oxygen from air or Fe(III) ions function as electron acceptor. The radical cation l+ , generated in the oxidative counterpart, loses a proton, affording the strongly reducing a-amino radical (Figure 10), which injects an electron into the conduction band of CdS. Hydrolysis of the resulting iminium salt leads finally to the secondary amine. Repetition of this multi-step reaction affords the completely demethylated product 2. 

Tertiary ammonium radicals are also acidic so, if (x-CH protons are present in the amine, deprotonation of these intermediate products stabilizes the a-amino radicals... 

Carbonate radical is generated by the reaction of OH radical with carbonate ion and bicarbonate ion [reaction (19)(20)], so this experiment was done under N20 saturation[reaction (9)]. Carbonate radical has an absorption peak at 600 nm. As well as hydrated electron and sulphate radical, the rate constant of the reaction of carbonate radical with polymer chains[reaction (21)] can be calculated from estimating the slope of the pseudo first-order decay rate of the absorbance at 600 nm against polymer concentration. Then, the rate constants with CM-chitin and CM-chitosan, CM-cellulose were determined as (3.9 6.4)x 105[MXs l](Figure 8). These values are lower than the value of OH radical and sulphate radical, and so this shows carbonate radical is less oxidative than OH radical and sulphate radical. Focusing the rate constants of CM-chitosan, the value at around pH 9.5 is lower than over pH 10. This is because of pKa of amino group, protonation and unprotonation. For a weak reactivity of carbonate radicals, it can be assumed that carbonate radical have a selectivity attacking polymer chains. 

Iminium salt 35 was reduced in anhydrous acetonitrile in the presence of at least 1 equiv of camphorsulfonic acid (CSA) (Scheme 52) [127]. In this way, pyrrolidine 36 was obtained in good yield. The cyclization presumably involves a N-protonated unsaturated a-amino radical, while the non-protonated form gives less cyclization and more reduction of the C=N bond. 

Alkyl radicEils arising from rearrangements of amino radical cations have also been successfully used for the homolytic alkylation of protonated heteroaromatic bases 13.39) (Schemes 10 and 11)... 

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... 

Proton ENDOR is a useful probe of the origin of the EPR signals from organic-based radicals. In a series of cw ENDOR studies including samples with isotopically labeled amino acids, proton ENDOR was used to identify the amino acid radical in cytochrome-c peroxidase (CcP) as a tryptophan species. Similarly, isotopic labeling was used to identify the tyrosyl... 

For practical applications, initiator systems functioning on the basis of dye reduction are most important. Scheme 10.7 illustrates how free radicals are formed with the aid of a co-initiator of the tertiary amine type. In this case, the amino radical cation, formed by electron transfer, loses a proton to give an a-aminoalkyl radical, which initiates the polymerization. 

Protonated Amino Radicals. The reac.tion of hydroxyl amine with TiCl, in aqueous, acidic methanol results in the formation of the simplest protonated amino radical, NH,T. This radical added readily to butadiene and to simple olefins to form products which were the result of coupling of the intermediate B-aminoalkyl radicals ( ). The addition reactions of protonated di-alkylamino radicals were described in a series of elegant papers by Neale and his co-workers (5-10). The radicals were generated from the appropriate N-chloroamines in presence of the unsaturated system. The most effective acid solvent combination was found to be 4M sulfuric acid in glacial acetic acid, but other acid/solvent combinations were also used. The reactions proceeded by chain mechanisms, which were initiated by light or by... 

The reaction was most effective when X was an electron withdrawing substituent, but only because this suppressed the competing ionic chlorination of the double bond. In some cases, the ionic chlorination was the predominant reaction. Protonated amino radical addition to unsubstituted olefins, acetylenes and al-lenes, however, gave acceptable yields of products, particularily in the case of the sterically unhindered substrates. The products obtained from the acetylenes were the a-chloroketones or aldehydes. 

One interesting aspect of the reactivity of protonated amino radicals is that the reactivity toward unsaturation is much higher than toward allylic abstraction. There is essentially no competing allylic abstraction even in very favorable cases [7). 

Intramolecular addition of protonated amino radicals has been examined by several groups. The following reaction has been carried out using silver perchlorate in acetone, ferrous sulfate or acidic (4M H2SO4) conditions (11). The yields varied. 

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