Free radicals in electrochemical reduction of 1-(nitrophenyl)-3,5-dicarbethoxy-4-phenyl-1,4-dihydropyridines

In the primary electrochemical reduction of 1-(nitrophenyl)-3,5-dicarbethoxy-4-phenyl-1,4-dihydropyridine in DMF, free radicals of nitrophenyl type are formed; these are the products of a one-electron reduction of cathode-protonated molecules of the original compound. In alkaline DMF, where cathodic protonation of the initial compound is retarded, union-radicals of the starting material are generated in addition, together with p-nitrophenol free radicals.Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 11, pp. 1498–1505, November, 1992.     

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.     

Electrochemical behaviour of 1-benzyl-3-carbamoylpyridinium chloride,a NAD+ model compound

In aqueous media, 1-benzyl-3-carbamoylpyridinium ion undergoes an initially reversible one-electron reduction to produce a free radical, which irreversibly dimerises to a 4,4′-linked diastereoisomeric pair. At more negative potential, the free radical undergoes a further one-electron reduction to a mixture of 1,6- and 1,4-dihydropyridine derivatives. At sufficiently positive potential the dimers can be oxidised back to pyridinium cation. The dimeric products are strongly adsorbed on the electrode surface, also at potential values where 2e products are formed, and this is a preparative drawback for the dimers and the dihydropyridines. Such problems can be overcome by the addition of benzene and of surface-active substances respectively. A detailed mechanism of the electrochemical reduction of 1-benzyl-3-carbamoylpridinium ion is proposed.     

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.     

Electronic structures and reactivities of derivatives of 1,4-dihydropyridines. 2. 1-Methyl-3-carboxyamido-1,4-dihydropyridine and its cation radical

The three-dimensional and electronic structures of 1-methyl-1,4-dihydronicotinamide and its cation radical were calculated within the MINDO/3 approximation. The results are compared with the physicochemical properties of 1,4-dihydropyridine derivatives.See [22] for Communication 1.Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 1, pp. 58–62, January, 1988.     

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.     

Oxidation of 1,4-dihydropyridines

The rate constants for the oxidation of 17 mono- and polynuclear 1,4-dihydropyridine derivatives with chloranil were determined. The constants obtained were compared with the results of fermentative and electrochemical oxidation.See [1] for communication I.Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 5, pp. 691–694, May, 1973.     

Synthesis of 1,4-dihydropyridines having an N-alkylpyridinium substituent at the 4-position and their affinity towards liposomal membranes

There has been synthesized a series of 1,4-dihydropyridines having an N-alkylpyridinium substituent at position 4 with a varying length of hydrocarbon chain. Their affinity to model (liposomal) membranes has been studied. It was found that this affinity increased with lengthening of the hydrocarbon chain on the Npyridinium substituent at the 4-position of the 1,4-dihydropyridine ring. However, lengthening of the hydrocarbon chain in the 3,5-ester groups of the 1, 4-dihydropyridine ring led to a decrease in the binding to the liposome when a 4-(N-hexadecylpyridinium) substituent was present.Latvian Institute of Organic Synthesis, Riga LV-1006. Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 8, pp. 1112–1117, August, 1995. Original article submitted June 6, 1995.     

Synthesis and three-dimensional structure of 3,5-dichloro-3,4,5,6-tetrahydropyridines

The reaction of N-chlorosuccinimide with 2,6-dimethyl-3,5-dimethoxycarbonyl-4-(2-difluoromethoxyphenyl)-1,4-dihydropyridine (phoridone) has been studied. Depending on the amount of chlorinating agent, 3,4,5,6-tetrahydropyridines with different degrees of chlorination were obtained. The three-dimensional structure of 3,5-dimethoxycarbonyl-4-(2-difluoromethoxyphenyl)-2-methyl-6-chloromethylene-3, 5-dichloro-3,4,5,6-tetrahydropyridine has been studied.Latvian Institute of Organic Synthesis, Riga LV-1006. Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 8, pp. 1118–1123, August, 1995. Original article submitted July 28, 1995.     

Reaction of derivatives of 1,4-dihydropyridine with the peroxynitrite anion

The reaction of derivatives of 1,4-dihydropyridine (1,4-DHP) with peroxynitrite anion is an oxidation reaction; no products of nitration were detected. It was found that some 1,4-DHP increase the lifetime of the peroxynitrite anion and the yield of hydroxyl radical in decomposition of the peroxynitrite anion.Latvian Institute of Organic Synthesis, Riga LV-1006. Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 3, pp. 355–357, March, 1998.     

High-pressure synthesis of carbamates by the carbonylation of aromatic nitro compounds in cyclohexanol

A study was carried out on the high-pressure carbonylation of nitrobenzene and 3-chloronitrobenzene by CO in cyclohexanol in the presence of PdCl₂-FeCl₃-pyridine with the formation of cyclohexyl-N-phenylcarbamate and cyclohexyl-N-3-chlorophenylcarbamate in 90–95% yield and nitro compound conversion of 95–99%. The feasibility of the repeated use of this catalyst was demonstrated.Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 8, pp. 1885–1887, August, 1991.     

Free radicals in the electrochemical reduction of 1,2-dihydro-3-nitropyridine derivatives

The formation of primary radical anions of N-substituted 1, 2-dihydro-3-nitropyridines was confirmed by the ESR method under conditions of electrochemical generation of free radicals, and their structure was established. The causes of the different ability of 1,2- and 1,4-dihydro-3-nitropyridines to form free radicals during electrochemical reduction that are stable enough to be studied by the ESR method are examined.Latvian Institute of Organic Synthesis, Riga LV-1006.Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 9, pp. 1222–1227, September, 1996. Original article submitted May 20, 1996.     

Sterically crowded chelate complexes of copper(II) with phenoxazinyl ligands: ESR study

A copper complex with 1-H-1-oxo-2,4,6,8-tetrakis(tert-butyl)phenoxazinyl free radical ligands was synthesized, and its magnetic properties and ESR spectra were measured at 77–290 K. It was shown that a ground electron state with one unpaired electron is typical of this complex at 77–290 K. The magnetoresonance parameters of this complex are characterized by ag-factor value (1.978) that is rather unusual for copper-containing compounds and by hyperfine coupling due to Cu nuclei. The considerable broadening of ESR spectral lines with increasing temperature is probably not due to the formation of a quartet state (S=3/2). A model of electron states of the compound investigated was developed in the framework of ligand field theory, which made it possible to explain the pattern of the ESR spectra, which were untypical of copper complexes.Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 5, pp. 894–899, May, 1995.This study was financially supported by the International Science Foundation (Grant NRT 000).     

Novel electrochemical heterogeneous catalytic reactor for organic pollutant abatement

Taking nitrobenzene as a model, electrochemical heterogeneous catalytic degradation of organic pollutant is investigated using a novel electrochemical heterogeneous catalytic reactor coupled by three-dimensional electrode with heterogeneous catalytic reactor. The nitrobenzene removal efficiencies considerably depend on the applied potential, initial pH, and the concentration of the supporting electrolyte. The results show that, with catalyst the nitrobenzene removal efficiencies are higher than that of control (without catalyst). The efficiency of ferrous catalyst is the best for nitrobenzene removal especially. It is clear that the reaction of electro-oxidation is promoted by the catalyst. A possible mechanism of nitrobenzene degradation is deduced based on products. Published in Russian in Elektrokhimiya, 2007, Vol. 43, No. 3, pp. 313–318. The text was submitted by the authors in English.     

Out-of-Plane Distortions of Chlorinated Nitrobenzene Radical Anions

According to UHF/INDO calculations of the model conformations of the chlorinated nitrobenzene radical anions, the rotation of the nitro group relative to the plane of the benzene ring is accompanied by its pyramidal deformation caused by the pseudo-Jahn–Teller effect. The degree of the structural distortions of the chlorinated nitrobenzene radical anions depends on the arrangement of the chlorine atoms in the benzene ring and on the solvent, increasing from DMF to its mixtures with water. The isotropic hyperfine coupling constants and their dependence on the water content in a binary mixture of solvents are interpreted for a number of chlorinated nitrobenzene radical anions.     

Hydrogen peroxide formation in the oxidation of carbonyl-containing compounds at β-CH bonds

The concepts of the selective inhibition of reactions with the participation of HO ₂^· radicals by nitrobenzene in accordance with a cyclic mechanism were supported using cyclohexanol oxidation as an example. It was demonstrated that the nitroxyl radical generated from nitrobenzene selectively reacts with HO₂ ^· to form hydrogen peroxide and nitrobenzene. A decrease in the rates of oxidation of esters, carboxylic acids, and ketones with specially chosen structures in the presence of nitrobenzene, as well as the detection of H₂O₂ and corresponding ,-unsaturated compounds among the reaction products, indicated that the degradation of -peroxyl radicals of the above compounds occurred under conditions of the liquid-phase oxidation of organic substances that result in the formation of the HO₂^· radical and an unsaturated compound.Translated from Kinetika i Kataliz, Vol. 45, No. 6, 2004, pp. 814–820.Original Russian Text Copyright © 2004 by Nepomnyashchikh, Nosacheva, Perkel.     

ESR spectra of free radicals of 3,3-disubstituted 1,2,3,4-tetrahydropyridines produced during electrochemical reduction

The polarographic properties (potentials, number of electrons, reversibility) of the electrochemical reduction of 3,5-diethoxycarbonyl-3-(p-nitrobenzoyl)-6-(p-nitrophenyl)-1,2,3,4-tetrahydropyridine and its derivatives in dimethylformamide have been determined. In the course of the electrochemical generation, ESR spectra of primary radical anions of p-nitrobenzoyl structure were obtained along with ESR spectra of nitrophenyl-p-substituted free radicals formed as a result of further reduction.Latvian Institute of Organic Synthesis, Riga LV-1006. Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 8, pp. 1103–1110, August, 1998.     

Cathodic conversion of polytetrafluoroethene a the presence of electron carriers

The conversion of polyretrafluoroethene (PTFE) at a three phase interface (M), PTFE-cathode-solution with simultaneous electrochemical and chemical (naphthalene anion radical) reduction has been studied. The influence of naphthalene concentration on the extent of this transformation is extreme as a result of specific processes at the TPI.Institute of Bioorganic Chemistry and Petrochemistry, Ukraine National Academy of Sciences. Ukraine, 252660 Kiev-94, I Murmanskaya Ulitsa. Translated from Teoreticheskaya i Éksperimental'naya Khimiya, Vol. 32, No. 1 pp. 40–42, January–February, 1996. Original article submitted May 12, 1995.     

Volt-amperometry of 1,4-dihydropyridine derivatives

It was found that compounds of the 3,5-dicarbonyl-1,4-dihydropyridine series are oxidized on a rotating platinum microelectrode in acetonitrile at potentials that are accessible for electrochemical investigation (from 0.8 to 1.4 V relative to a saturated calomel electrode). The electrochemical oxidation potentials of 41 compounds were determined, and the influence of electronic effects of substituents introduced into various positions of the dihydropyridine ring on E_p and E_1/2 was revealed. The potentials obtained were compared with the peculiarities of the chemical and enzymatic oxidation of-the corresponding compounds.Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 1, pp. 84–87, January, 1972.     

Photoinduced transformation of α,β-epoxyketones to β-hydroxyketones by Hantzsch 1,4-dihydropyridine

Irradiation (λ >300 nm) of Hantzsch 1,4-dihydropyridine with aromatic α,β-epoxyketones in acetonitrile selectively breaks the Cα---O bond of the epoxides giving the corresponding β-hydroxyketones in excellent yields.