1. EPR Spectra of anion radicals formed in the electrochemical reduction of some quinoxaline 1,4-dioxides

The anion radicals formed in the electrochemical reduction of 2,3-dimethyl-quinoxoline and its N,N′-dioxide, the 6-methoxy and 6-chloro derivatives of the N,N′-dioxide, and the preparation quinoxidine in DMF were studied. Polarographic analysis showed that the first step in the reduction is a one-electron step and is reversible for all of the compounds except quinoxidine. However, the EPR spectra of the primary anion radicals were recorded only for 2,3-dimethylquinoxaline and its 1,4-dioxide. For the remaining compounds we were able to obtain anion radicals of only the reduction products, the structures of which in a number of cases were established from the EPR spectra. Translated from Khimiya Geterotsiklicheskikh Soedinenii, No 3, pp. 376–380, March, 1980.     

Titanium(III) chloride and electrochemical reduction of pyrazine,quinoxaline and triazine derivatives and of their salts

The reduction of pyrazine, quinoxaline and triazine derivatives by titanium(III) chloride leads to di- or tetrahydrogenated compounds. High yields of tetrahydro compounds are also obtained through the reduction of quinoxalinium salts. These results are compared with those obtained by electrochemical reduction.     

Photochemical vs. electrochemical electron-transfer reactions. one-electron reduction of aryl halides by photoexcited anion radicals

Electrochemical reduction of aryl halides generally leads to expulsion of halide ion. The product aryl radical is unavoidably further reduced. In contrast, reduction of aryl halides by photoexcited anion radicals may be stopped at the aryl radical stage owing to the bimolecular nature of electron-transfer reactions. We have tested this hypothesis by photoinducing electron-transfer from anthraquinone anion radical to several aryl halides. For each halide it was possible to trap the corresponding radical by anthracene forming stituted 9-phenylanthracenes.     

Absorption spectra of nitro derivatives. 2-alkylamino-4-nitropicolines and their N-oxides

The UV spectra of 15 2-N-substituted 4-nitropicolines and their N-oxides in ethanol have been determined. The spectra of all investigated compounds with the exception of 2-alkylnitramino-4-nitropicolines contain three main bands: two absorption bands are attributed to ^* excitation of the -electron of the aromatic system. The third ICT in the longest wavelength region is assigned to the 4-nitro group via the pyridine ring. The bands in the spectra of 3-methyl derivatives are characterized by smaller intensity than 5-methyl derivatives due to the disturbance of the mutual electronic interaction of the substituent by the stericortho effect.Department of Organic Chemistry, University of Economics, PL-53-342 Wroclaw, Poland. Published in Khimiya Geterotsiklicheskikh Soedinenii, No. 6, pp. 774–779, June, 1998.     

Free radicals of electrochemical reduction of derivatives of 3-nitro-1,4-dihydropyridines

By means of ESR, under conditions of eletrochemical generation of particles, the formation of four types of secondary free radicals has been confirmed in the process of electrochemical reduction of molecules of N-unsubstituted derivatives of 3-nitro-1,4-dihydropyridine in dimthylformamide — specifically, a dianion radical of the molecule of the original compound; an anion radical of the corresponding isomeric 4,5-dihydropyridine; a radical of the nitroalkane type; and in addition, for the compound substituted with a nitrobenzene group; a free radical with the nitrobenzene structure. Methods for synthesis of the individual compounds are described, and a scheme is presented for the mechanism of their electrochemical conversion.Latvian Institute of Organic Synthesis, Riga LV-1006. Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 3, pp. 355–364, March, 1995. Original article submitted February 21, 1995.     

Detection of nitrosofuran anion radicals in the electrochemical reduction of 5-nitrofuran. EPR investigations and quantum-chemical calculations

Previously unknown radicals that are associated with the 5-N-furylhydroxylamine-5-nitrosofuran redox system were detected by EPR spectroscopy in the electrochemical reduction of 5-nitrofuran. The electron structures of the free-radical products of the reduction of 5-nitrofuran were calculated by the INDO method. On the basis of the hyperfine structures (hfs) of the EPR spectra and the results of quantum-chemical calculations it was concluded that 5-nitrosofuran anion radicals and the nitrosofuran dimer were recorded.Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 9, pp. 1178–1183, September, 1989.     

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.     

Chemistry of heterocyclic N-oxides and related compounds 13. Acylamination of pyridine N-oxide by aniline,p-anisidine,and their N-p-tosyl derivatives

Acylamination of pyridine N-oxide by aniline, p-anisidine, and their tosyl derivatives was carried out in an alkaline medium in the presence of p-tosyl chloride. The reaction proceeds selectively with the formation of the corresponding 2-(N-p-tosyl) anilino- and 2-p-(N-p-tosyl)anisidino-pyridines. The reaction products were converted by acid hydrolysis into the corresponding 2-anilinopyridines.For communication 12, see [1].Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 7, pp. 936–938, July, 1986.     

Structures and reactivities of 2-methyl- and 2-benzyl-3-hydroxypyridines and their N-oxides

It was shown by UV spectroscopy that the introduction of CH₃ and CH₂Ph groups into the 2 position of 3-hydroxypyridine and its N-oxides does not have a substantial effect on the acidbase transformations of these compounds. The effect of methyl and benzyl groups on the electronic and energy characteristics of 3-hydroxypyridine and its N-oxide was studied by means of perturbation theory within the framework of the Hückel MO method. The reactivity indexes (RI) of 2-methyl- and 2-benzyl-3-hydroxypyridines and their N-oxides were calculated, and the RI were correlated with the experimentally observed regularity of the orientation of electrophilic substitution in the indicated compounds.     

Free radicals in the electrochemical reduction of certain mononitro and dinitro derivatives of pyridine

In the primary process of electrochemical reduction of substituted 3-nitropyridines in dimethylformamide, their anion radicals are formed. This also takes place in the reduction of 3,5-dinitropyridines and 3-nitropyridines with a nitrophenyl substituent at position 2 or 4. For these dinitro derivatives, however, secondary free radicals are formed as well; in a basic medium, these are the products of reduction of the corresponding Meisenheimer complexes. Serving as the reaction center for electroreduction is the nitro group on the pyridine ring, not the group on the phenyl ring. For the mononitropyridines and dinitropyridines that were studied, free radicals of the nitropyridine type are formed as the primary species. The structure of the primary and secondary free radicals was established by analysis of the hyperfine structure of their ESR spectra.Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 8, pp. 1079–1087, August, 1993.     

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.     

Synthesis of nitro derivatives of 2-alkylamino-4-nitropicolines and their N-oxides

2-Alkylamino-4-nitro-3- or-5-methylpyridines, 2-alkylnitramino-4-nitro-3- or-5-methylpyridines, and their N-oxides were synthesized in good yields. Some properties of the products are reported.Department of Organic Chemistry, University of Economics, PL-53-342 Wroclaw, Poland. Published in Khimiya Geterotsiklicheskikh Soedinenii, No. 5, pp. 652–656, May, 1998.     

Synthesis of dihalopicoline N-oxides and their 4-nitro derivatives

Three aminohalo-substituted - and -picolines, six dihalo-substituted - and -picolines, six dihalo-substituted - and -picoline N-oxides and six respective dihalo-4-nitropicoline N-oxides were synthesized in excellent yields. Some properties of the products were reported.Academy of Economics, Department of Organic Chemistry, PL 53-423, Wroclaw. Published in Khimiya Geterotsiklicheskikh Soedinenii, No. 10, pp. 1366–1371, October, 1996. Original article submitted November 15, 1995.     

Electrochemical and ESR studies on the reduction of some indolinone derivatives and of their N-oxides in DMF

2-Phenyl-3-oxo-3H-indole 4 and 3-imino-, 3-iminoxy-, derivatives, the corresponding N-oxide 3, and 2-phenyl- 5 and 2-(2'-pyridyl-) isatogen 6 have been reduced in DMF and in DMF in presence of proton donors, and followed by polarography, controlled potential coulometry, cyclic voltammetry, UV, and ESR. In aprotic media, compounds 1, 4, 5 and 6, are reduced via anion radical intermediates.     

Tautomerism of phenylazo derivatives of 3-hydroxypyridine and their N-oxides

Azophenol-quinone hydrazone tautomerism was detected for phenylazo derivatives of 3-hydroxypyridines and their N-oxides. The absorption maxima of the phenylazo derivatives of 3-hydroxypyridine have a greater (than their N-oxides) tendency to be shifted to the longer-wavelength region.Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 6, pp. 810–812, June, 1973     

Free radicals from electrochemical reduction of hexahydroquinolones and hexahydroisoquinolones

Electrochemical reduction of 3,5-diethoxycarbonyl- and 3,5-diacetyl-2,6-dimethyl-1,4-dihydropyrdine, hexahydroquinol-5-one, and hexahydroisoquinol-8-one, which contain dihydropyridine moieties, produces free radicals detectable by EPR only for the most easily reduced hexahydroisoquinol-8-ones. The hyperfine structure of the EPR spectra is determined and is consistent with that for the 4,5-dihydropyridine moiety.Latvian Institute of Organic Synthesis, Riga LV-1006, Latvia; Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 7, pp. 924–928, July, 1999.