Pyrazines free-radical

Pyrazine is chlorinated at 400°C to give a mixture of mono-, di-, tri- and tetra-chloropyrazines, presumably by a free radical mechanism. 

Namiki and Hayashi (46) recently summarized their theory of formation of intermediate free radicals, N, N -disubstituted pyrazine cation radicals, in an early stage of the Maillard reaction (cf. 47). 

As described above, the free radical products are considered as N,N -disubstituted pyrazine cation radical products. However, formation of such pyrazine derivatives in the Maillard reaction is unknown, and little work has been done on the synthesis of N.N -diethylpyrazinium salt (8), the one—electron oxidation product of the N, N -diethylpyrazine cation radical. The... 

Formation of novel free radical products at an early stage of the Maillard reaction was demonstrated by use of ESR spectrometry. Analyses of the hyperfine structures for various sugar-amino compound systems led to the conclusion that the radical products are N,N -disubstituted pyrazine cation radicals. These new pyrazine derivatives are assumed to be formed by bimolecular condensation of a two-carbon enaminol compound involving the amino reactant residue. The presence of such a two-carbon product in an early stage reaction mixture of sugar with amine was demonstrated by isolation and identification of glyoxal dialkylimine by use of TLC, GLC, NMR, MS and IR. 

Lipid-Protein-Carbohydrate Interactions. Evidence for such complex interaction was recently reported by Huang et al (36) who observed that the addition of corn lipids to zein and corn carbohydrates enhanced the formation of alkylpyrazines, indicating that lipid-derived free radicals may accelerate the rate of Maillard reactions. Two of the alkylpyrazines, identified in such mixtures after heating for 30 minutes at 180°C, have 5-carbon alkyl substitution at the pyrazine ring and could only be explained by interaction of lipid or lipid decomposition products. These authors suggested that condensation of amino ketones, formed by protein-carbohydrate interaction, may yield 3,6-dihydropyrazine which would in turn react with pentanal, a lipid oxidation product, to form 2,5-dimethyl-3-pentylpyrazine. 

Organic Bridging Ligands.—Experiments with a pyrazine-2-carboxylate complex have given kinetic evidence of free-radical intermediates, exemplifying the chemical mechanism of electron transfer. In equation (56) and (57), HL = pyrazine-2-carboxylic... 

Thus, ketone enolates easily substitute chlorine in position 2 of the electrophilic nucleus of pyrazine (1,4-diazabenzene), and even in the dark, the reaction proceeds via the Sj l mechanism (Carver et al. 1981). It is expected that the introduction of the second chlorine in the ortho position to 4-nitrogen in the electrophilic nucleus of pyrazine promotes the ion-radical pathway even more effectively. However, 2,6-dichloropyrazine in the dark or subjected to light reacts with the same nucleophiles by Sr.,2 and not S nI mechanism (Carver et al. 1983). The authors are of the opinion that two halogens in the pyrazine cycle facilitate the formation of a-complex to the extent that deha-logenation of anion-radicals in solution and a subsequent nucleophilic attack of free pyrazine radical become virtually impossible. Thus, the reaction may either involve or exclude the intermediate a-complex, and only special identification experiments can tell which is the true one. 

The product of reaction of pyrazinecarboxamide with Cr + is a dark green complex of chromiumCiii) and the pyrazinecarboxamide radical ion, first characterized in 1965 (see ref. 118 and references cited therein). Evidence has now been presented to show that in reactions with various cobalt(ni) complexes, the effective reducing agent is, not the radical-ion complex but free Cr + present in equilibrium with it (Pz = substituted pyrazine) ... 

In 2008, Itami reported a transition-metal-free C-H arylation of simple azines such as pyrazines, pyrimidines, pyridazines, and quinoxalines with aryl iodides under the influence of KOt-Bu. Unfortunately, this reaction showed no regios-electivity due to its radical reaction pathway (e.g., the ratio for the arylation of... 

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