Substituted amino radicals reactions

The thermochemistry of substituted amino radicals may best be determined by the method employed for alkyl radicals—namely, measurement of the activation energy, Ej, for the forward Reaction 1, the enthalpy increase being aH. If we... 

Association with Radicals, Including Dimerization. Little evidence exists for the monoalkyl-substituted amino radicals, although this would be expected by analogy with alkyl radicals. In the pyrolysis of dimethyltriazene the two association reactions... 

Decomposition. Just as alkyl and alkyloxy radicals can decompose to give a stable double-bonded molecule and either a hydrogen atom or methyl radical— Reactions 23 and 24—so we would expect decomposition of alkyl-substituted amino radicals to take place. 

An interesting method for the substitution of a hydrogen atom in rr-electron deficient heterocycles was reported some years ago, in the possibility of homolytic aromatic displacement (74AHC(16)123). The nucleophilic character of radicals and the important role of polar factors in this type of substitution are the essentials for a successful reaction with six-membered nitrogen heterocycles in general. No paper has yet been published describing homolytic substitution reactions of pteridines with nucleophilic radicals such as alkyl, carbamoyl, a-oxyalkyl and a-A-alkyl radicals or with amino radical cations. 

The use of free-radical reactions for this mode of ring formation has received rather more attention. The preparation of benzo[Z)]thiophenes by pyrolysis of styryl sulfoxides or styryl sulfides undoubtedly proceeds via formation of styrylthiyl radicals and their subsequent intramolecular substitution (Scheme 18a) (75CC704). An analogous example involving an amino radical is provided by the conversion of iV-chloro-iV-methylphenylethylamine to iV-methylindoline on treatment with iron(II) sulfate in concentrated sulfuric acid (Scheme 18b)(66TL2531). 

Oxidative Animation of Nitrones to a-Amino-Substituted Nitroxyl Radicals Similar to the oxidative methoxylation reaction, oxidative animation of 4H -imidazole TV-oxides, in amine saturated alcohol solutions, give stable nitroxyl (282), nitronyl nitroxyl (283), imino nitroxyl (284) and (285) radicals with the amino group at the a-carbon atom of the nitroxyl group (Scheme 2.107) (520, 521). The observed influence of substituents on the ratio of animation products at C2 and C5 atom is close to the ratio observed in the previously mentioned oxidative methoxylation reaction. It allows us to draw conclusions about the preference of the radical cation reaction route. 

Elastin-mimetic protein polymers have been fabricated into elastic networks primarily via y-radiation-induced, radical crosslinking of the material in the coacervate state [10]. Although effective, this method cannot produce polymers gels of defined molecular architecture, i.e., specific crosslink position and density, due to the lack of chemoselectivity in radical reactions. In addition, the ionizing radiation employed in this technique can cause material damage, and the reproducibility of specimen preparations may vary between different batches of material. In contrast, the e-amino groups of the lysine residues in polymers based on Lys-25 can be chemically crosslinked under controllable conditions into synthetic protein networks (vide infra). Elastic networks based on Lys-25 should contain crosslinks at well-defined position and density, determined by the sequence of the repeat, in the limit of complete substitution of the amino groups. 

Radicals can be either reduced (to anions or organometallics) or oxidized to cations by formal single electron transfer (Scheme 11).50 Such redox reactions can be conducted either chemically or electro-chemically51 and the rates of electron transfer are usually analyzed by the Marcus theory and related treatments.50 These rates depend (in part) on the difference in reduction potential between the radical and the reductant (or oxidant). Thus a species such as an a-amino radical with high-lying singly occupied molecular orbital (SOMO) is more readily oxidized, while a species such as the malonyl radical with a low-lying SOMO is more readily reduced. The inherent difference in reduction potential of substituted radicals is an important control element in several kinds of reactions. 

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... 

Flash photolysis of a substituted hexaarylbiimidazole and reactions of the imi-dazolyl radical. Coraor, G. R., Riem R. H. MacLachlan, A. Urban, E. J. (Exp. Stn., E. I. DuPont de Nemours, and Co., Wilmington, Del.). J. Org. Chem. 1971, 36 (16), 2272-5. (Eng). The rate of reaction of 2-(o-chlorophenyl)-4,5-diphenylimida-zolyl radicals (L with additives was studied in various solvents. Evidence based on measured rate consts., including kinetic D isotope effects, prove that the rate detg. step in the reaction L + aromatic amine is an electron change reaction at the amino N, while in the reaction L + hydroquinone the rate-detg. step is H abstraction. 

Indoles can be used as radical acceptors instead of 63 [120, 121]. Simple and twofold reactions giving either 3-alkylindoles [120] or l,l-bis(3-indolyl)alkanes [121] were observed in 16-72% and 54-90% yield, respectively. In both methods the indole is subject to radical addition in 3-position. The resulting a-amino radical undergoes a further oxidation and deprotonation to the 3-substituted indole. In the case of twofold additions, the second indole unit is introduced by a subsequent polar Friedel-Crafts-type alkylation. 

A wide variety of a-trifluoromethyl-substituted amino acids are now available from the reaction of 5-fluoro-4-trifluoromethyl-l,3-oxazoles with allylic alcohols and benzyl alcohols. The reaction sequence involves a low-temperature Claisen rearrangement or a radical 1,3-benzyl shift from oxygen to carbon, respectively [88AG(E)848 89S850] (Scheme 86). 

A study of the effect of the structure and size of the amine radical (butylamine, dibutylamine, hexylamine, piperidine, aniline) on its reactivity in the nucleophilic replacement of tosyloxy groups in cellulose tosylate has shown (29) that this reactivity depends primarily on the basicity of the amine. TTiis assumption was confirmed by a study of the interaction of cellulose toxylates of different degrees of substitution (DS from 0.3-1.7) wit iminodiacetic and anthranilic acids 30, 31). TTie degree of substitution of the reaction products with respect to the N-carboxyalkyl(aryl)amino groups was found to be lower than that of the corresponding N-alkyl(aryl)araino derivatives. 

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 this section, reactions are featured that involve the ortho trapping of an iminium ion generated oxidatively without the abstraction of H by the adjacent group. (In principle, this process could also involve an a-amino radical). Some years ago, we demonstrated that Mn02 oxidation of o-substituted /-anilines can lead to intermediates that can be trapped by appropriate ortho nucleophiles [68JCS(C)1722 69CI(L)443]. Thus o-amino- and o-carboxyphenyldialkylamines were cyclized to give... 

Over half of the remaining market for products used in the processing of rubber is made up of antioxidants, antiozonants and stabilizers, either amino compounds or phenols. Aniline is used to manufacture vulcanization accelerators, antioxidants and antidegradants. Of the latter, several are A-substituted derivatives of p-phenylenediamine and octyl dipheny-lamine. Diphenylamines terminate free-radical reactions by donating the amino hydrogen, and are used to protect a wide range of polymers and elastomers. Many synthetic rubbers incorporate alkylated diphenylamine antioxidants. Other antioxidants include aryl amine resinous products from, e.g. condensation of aniline and acetone in the presence of... 

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