Pyrazine cation

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). 

The results of analyses of the ESR spectra of various reaction systems are summarised in Table II. It was shown that all spectra have in common the splittings arising from two equivalent nitrogens (about 8.2 G) and four equivalent protons (about 3.0 G), and additionally from an even number of equivalent protons with different splitting constants. These assignments let to the reasonable assumption that the radical products are N,N -disubstituted pyrazine cation radical derivatives, as shown in Fig. 4. This assumption was strongly supported by the fact that the hyperfine structure as well as the g-value of the ESR... 

Figure 4. N,N -Disubstituted pyrazine cation radicals with assignments for hyperfine structures of the ESR spectra in the Maillard reaction mixtures.

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. 

Direct homolytic carboxylation of pyrazine cation has been described. A solution of ethyl pyruvate and hydrogen peroxide prepared at — 10° with a solution of aqueous ferrous sulfate and pyrazine in aqueous sulfuric acid gave 2-ethoxycarbonyl-pyrazine and diethoxycarbonylpyrazine (1358). 

Homolytic acylation of the pyrazine cation with aldehyde radicals have been reported (616). A mixture of pyrazine, acetaldehyde, aqueous acetic acid, sulfuric acid, r-butyl hydroperoxide, and ferrous sulfate gave 2,5-diacetylpyrazine, and similar reactions occurred with propionaldehyde and benzaldehyde (616). 

Covalent hydration of the pyrido[3,4-h]pyrazine cation has been studied by UV97 and l3C NMR spectroscopy.5... 

Interestingly, a conical intersection very similar to that of the S i(n7r ) and S 2(7T7r ) neutral excited states has been found for the n and n hole states of the pyrazine cation.A linear vibronic-coupling model has been constructed for the and states of the pyrazine cation employing many-body Green s function methods for the calculation of the vibronic-coupling parameters. The ah initio calculated photoelectron spectrum of the Ag n ) and states is compared in Fig. 7 with the... 

Electrochemical processes involving pyrazine (pz) and monoprotonated pyrazine cation (pzlf adsorbed on a silver surface have been investigated by Surface-Enhanced Raman Scattering (SERS). The investigation of the faradaic and non-faradaic behaviour of pz adsorbed on silver by SERS is a good example of the application of this technique to the study of solid-liquid interfaces. Electrochemical SERS can be used to infer the orientation of the adsorbed molecule. The results presented in this work indicate that pz adsorbs end-on via the lone pair electrons on the nitrogen, and this orientation is not potential dependent. 

Adsorption of Monoprotonated Pyrazine Cation on SUver Electrodes. Pyrazine is a very weak base (pKai = 0.65 (12)), and hence the monoprotonated pz spectrum was obtained in a 1 M HCIO4 + 1 M KBr solution. Figure 8 shows spectra of pzlT adsorbed on a silver electrode at two different potentials. One band at ca. 990 cm is observed for an applied potential of -310 mV vs. SCE. A broad envelope containing two bands (one at ca. 1015 cm and another at ca. 1030 cm, corresponding to the modes number 1 and 12 in Wilson s notation, respectively) appears as the applied potential becomes more positive (see the spectrum at -170 mV presented in Figure 8). The forms of these two normal vibrational modes are presented below (13) ... 

In this section we discuss very briefly a few other examples of vibronic coupling in polyatomic molecules. The pyrazine cation has been treated along similar lines as the 5i -S2 excited manifold of the neutral species. Its X Ag ground state and the A Big first excited state contain a vacancy in the n and n molecular orbitals, respectively. Therefore, they differ from the S and S2 states basically in that the electron is ionized out of these two orbitals rather than promoted to the n virtual orbital. The analysis shows that the conical intersection occurring between the X-A potential-energy surfaces is essentially a replica of that in the S1-S2 electronic manifold of neutral pyrazine. This interesting structural relationship shows that the shape of the surfaces is governed by the occupied rather than the virtual molecular orbitals in this system. 

The vibronic coupling model has been applied to a number of molecular systems, and used to evaluate the behavior of wavepackets over coupled surfaces [191]. Recent examples are the radical cation of allene [192,193], and benzene [194] (for further examples see references cited therein). It has also been used to explain the lack of structure in the S2 band of the pyrazine absoiption spectrum [109,173,174,195], and recently to study the photoisomerization of retina] [196],... 

Protonation of pyrido[2,3-f ]pyrazine occurs normally without covalent hydration, although the 2-hydroxy derivative did show such behaviour (63JCS5737). The pyrido[3,4-f)]pyrazine parent base does show the phenomenon, although the exact structure of the covalent hydrate seemed to be in doubt between protonated (392) and (397). The issue was resolved in favour of the former by NMR (79JHC301, 75AG356). The 3-hydroxy derivative also shows hydration effects, as does the 7-amino cation (63JCS5166). 

Pyrazino[l,2-a]pyrazine, octahydro-spectra, 3, 340 Py razino[2,3- h]pyrazine applications, 3, 368 cations... 

Pyrazino[2,3-fc]pyrazine, decahydro-as vulcanization accelerator, 3, 368 Pyrazino[2,3-fc]pyrazine, 2,3-dimethyl-cations... 

Thiazine has been formulated as 1 rather than 2 because it does not form a sulfonamide under Hinsberg conditions. Symmetrical azines can form only one classical monocation, e.g., pyrazine forms 3. The nonclassical cation 4 has been postulated for pyridazine, but there is no compelling evidence in its favor. 

Pyrazin-2-one (124) has been shown to exist predominantly as such by comparison of its ultraviolet spectrum with those of the fixed alkylated derivatives and by its infrared spectrum. The pK measurements support this conclusion but cannot yield quantitative results since cations of a common type are not formed. ... 

Pyrazine-2-thione (213) and quinoxaline-2-thione (214) probably exist in the thione form since their ultraviolet spectra are different from those of the 2-methylthio analogs. The basicity of quinoxaline-2-thione is 1.4 pK units less than that of 2-methylthio-quinoxaline, and the ultraviolet spectra of the cations are dissimilar. Presumably quinoxaline-2-thione and its 2-methylthio derivative do not form similar cations (215, P = alkyl, H), and it would appear that either the thione gives the cation 216 or the 2-thioether gives the cation 217. Similar considerations apply to pyrazine-2-thione. 

Cobalt trifluoride fluorination corresponds to the electron-transfer mechanism via a radical cation. RF groups attached to the ring enhance the stability of intermediate dienes and monoenes. Perfluoroalkyl pyridines, pyrazines, and pyrimidines were successfully fluorinated but pyridazines eliminated nitrogen. The lack of certain dienes was attributed to the difference in stability of FC=C and RFC=C and steric effects [81JCS(P1)2059]. 

Pentadienyl radical, 240 Perturbation theory, 11, 46 Propane, 16, 165 n-Propyi anion conformation, 34 n-Propyl cation, 48, 163 rotational barrier, 34 Propylene, 16, 139 Protonated methane, 72 Pyrazine, 266 orbital ordering, 30 through-bond interactions, 27 Pyridine, 263 Pyrrole, 231... 

E.s.r. showed that, X. ray irradiation of tetraalkyldiphosphine diphosphides gave phosphoranyl radicals with t.b.p. structures (39).114 A structure has been assigned to phosphiny1hydrazy1s (40). The dimethy1 ami no radical was particularly persistent.115 The e.s.r. parameters of the electrogenerated pyrazine radical cations (41) have been recorded.116 The spectra of a stable furanyl phosphate radical adduct117 and a phenalene radical anion which involves injection of spin density into half an attached cyclophosphazene ring,11 are reported. 

Bis-imidoyl chloride 187 reacts with A -methyl imidazole 188 to give bis-cationic diimidazo[l,2- 2, T-c]pyrazine 189 (Equation 46) <2006T731>. The charges are believed to be delocalized over the ring system. 

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