π-Electronic structure of the nitrofuran system

Anion radicals of 2-vinylene derivatives of 5-nitrofuran were obtained by electrochemical generation. The hyperfine structure (hfs) of the ESR spectra of these anion radicals indicates delocalization of the unpaired electron over their entire π-electron system. The effect of the vinylene grouping on the distribution of the unpaired electron in the anion radicals and on the distribution of the unpaired electron in the vinylene grouping itself was examined on the basis of the hfs constants. It was found that the vinylene grouping by localizing more than 10% of the density of the unpaired electron on itself reduces, by a factor of 1.4, the effect of substituents in the 2 position of 5-nitrofuran on the distribution of the spin of the unpaired electron in the nitrofuran framework.     

Esr and polarography of nitroazoles. 4. Polarographic study of nitrobenzimidazoles

The electrochemical reduction of 2-substituted 5(6)-benzimidazoles has been studied with the aid of classical polarography and cyclic voltammetry in acetonitrile. The influence of the substituents in position 2 on the magnitudes of the half-wave potentials of the first stage is exerted by induction and resonance mechanisms with approximately the same contributions, and that on the magnitudes of the half-wave potentials of the second stage predominantly by the resonance mechanism. The possibilities of taking the radical-stabilizing factor into account in the correlations are discussed. In order to study the 2-substituted 5(6)-nitrobenzimidazole series, their pK _a values in acetonitrile have been determined by potentiometric titration.For communication 3, see [1].Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 9, pp. 1246–1251, September, 1984.     

Nitroazines. 4. Polarographic behavior of 5-nitropyrazolo[3,4-b]pyridines and the esr spectra of their electrochemically generated free radicals

1-Methyl- and 1-phenyl-substituted derivatives of 5-nitropyrazolo[3,4-b]pyridines are reduced at a dropping mercury electrode in a single one-electron wave with the formation of radical anions in interval of potentials from –0.94 to –1.06 V. Compounds unsubstituted at the N(₁) nitrogen atom give an additional polarographic wave at E_1/2 = –1.4 V due to the reduction of the deprotonated form (anion). The potentials and HFS constants of the ESR spectra of the corresponding electrochemically generated free radicals are given.For Communication 3, see [1].Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 5, pp. 689–693, May, 1985.     

Polarographic reduction of adenine and its 9-(α,ω-dihydroxy-2-alkyl) derivatives

The polarographic reduction of adenine and its 9-(,)-dihydroxy-2-alkyl) derivatives on a short-period mercury electrode in strongly acidic media is determined by the rate of diffusion of the corresponding cations and, at pH values that exceed the pK_a values, by the kinetics of surface protonation of the uncharged molecules; this was proved by the dependence of the limiting current on the height of the mercury column, the temperature, and the depolarizer concentration. The polarographic reduction of adenine, adenosine, 9-(,-di-hydroxy-2-alkyl)adenines, and the corresponding phosphates in acidic media proceeds with the loss of sic electrons per molecule. The similar trends of the change in the half-wave potentials and the numerical pK_a values is associated with the fact that the site of protonation is simultaneously the site of nucleophilic attack by the electron during electrical reduction.Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 12, pp. 1675–1678, December, 1977.     

Voltamperometry of 1,4-dihydropyridine derivatives

Thirty-nine -substituted 1,4-dihydropyridine derivatives were subjected to electrochemical oxidation on a rotating platinum microelectrode in acetonitrile, and the E_p (E_1/2) values were measured. A satisfactory linear correlation between the E_p and Taft. ^* constants of the substituents in the position was observed. A linear correlation of the E_p values with the ⁰, - and ^– constants also exists.See [1] for communication I.Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 11, pp. 1525–1529, November. 1975.     

Free radicals in electrochemical reduction of 3,5-dicarbethoxy-1,4-dihydropyridines with nitrophenyl groups in positions 2, 4, and 6

In the electrochemical reduction of 2,6-bis- and 2,4,6-tris(nitrophenyl) derivatives of 3,5-dicarbethoxy-1,4-dihydropyridine, in the first stage, one of the para-nitrophenyl groups in position 2 or 6 of the heterocycle is reduced. Free radicals have been obtained and identified, the primary species being ion radicals of the nitrophenyl type. The presence of the heterocycle in the molecule of the 1,4-dihydropyridine derivative stabilizes secondary free radicals of the nitrosophenyl type. In the process of electrochemical reduction, no evidence has been found of any intramolecular transfer of electrons or protons from the dihydropyridine part of the molecule to the nitrophenyl groups. Derivatives of 2,6-bis(p-nitrophenyl)-3,5-dicarbethoxy-1,4-dihydropyridine have been synthesized, and the oxidation and methylation of these derivatives have been studied.Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 4, pp. 481–487, April, 1991.     

Polarographic investigation of the mobility of halogen atoms in halogen-substituted components of nucleic acids

In order to establish quantitatively the reducibility of halogen-substituted components of nucleic acids, the half-wave potentials (E_1/2) of their electrochemical reduction have been determined. The electrochemical reduction of all compounds takes place in the interval from –0.5 to –1.8 V and consists in the splitting off of the halogen atom (iodine, bromine) from the given molecule. The iodine derivatives are reduced much more readily (500–800 mV) than the bromine derivatives.Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 10, pp. 1404–1408, October, 1992     

Free radicals in the electrochemical reduction of derivatives of 3,5-dinitro-1,2-dihydropyridine

The characteristics (potentials, limiting currents, reversibility) of the electrochemical reduction of 3,5-dinitro-1,2-dihydropyridines were determined by cyclic voltammetry and polarography. It was established by ESR that the free radicals formed in these processes have a radical-dianion structure in the case of both the N-unsubstituted and the N-substituted dihydropyridines. The hyperfine coupling constants of the interaction between the unpaired electron and the nuclei of the atoms at various positions of the heterocycle were determined.Latvian Institute of Organic Synthesis, Riga LV-1006. Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 2, pp. 212–218, February, 1997.     

Electrochemical reduction of 4-(nitrophenyl)-1,2- and 4-(nitrophenyl)-1,4-dihydropyridines and the ESR spectra of the obtained free-radical particles

In the course of electrochemical generation the intermediate reaction products (free radicals of the nitro-and nitrosophenyl type, which appear on the cyclic voltammetric curves) were identified by ESR. The N-substituted derivatives are characterized by reduction of the dihydropyridine ring. The 4-nitrophenyl derivatives are characterized by the absence of intramolecular electron transfer during electrochemical reduction. In the case of the corresponding derivatives of 1,2-dihydropyridine intramolecular transfer of electrons and protons is possible under these conditions. Combined schemes of the primary and secondary chemical reactions involved in the electrochemical reduction of the investigated compounds are presented. It was established that the substances investigated with reference to the mechanism of the electrochemical transformations include the antihypertensive nifedipine (corinfar, fenigidine).Latvian Institute of Organic Synthesis, Riga LV-1006. Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 2, pp. 219–233, February, 1997.     

Mechanism and products of electrochemical reduction of 4-nitrophenyl-2,6-dimethyl-3,5-dicyano-1,4-dihydropyridines in dimethylformamide

Free anion radicals of the nitrobenzene and nitrosobenzene type, which were identified by EPR spectroscopy, are formed in the electrochemical reduction of isomeric 4-(nitrophenyl)-2,6-dimethyl-3,5-dicyano-1,4-dihydropyridines on mercury and solid electrodes. Reduction of the dihydropyridine ring is observed only for N-substituted p- and m-nitrophenyl derivatives of 1,4-dihydropyridine. An intermediate with a 2-pyridonemethide structure was identified; a set of primary and secondary chemical reactions that are associated with the electrolytic reduction of the investigated compounds is presented.Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 11, pp. 1494–1505, November, 1988.