O. Manoušek

Electrochemical behaviour of 2,2′-dinitrodiphenylsulphide and its cyclization reactions initiated by an electrode reduction

Abstract In supporting electrolytes containing 50% ethanol, 2,2′-dinitrodiphenylsulphide(I) is reduced at mercury electrodes to 2,2′-di(hydroxylamino)diphenylsulphide(III) and after protonation to 2,2′-diaminodiphenylsulphide. In a chemical follow-up reaction III undergoes an intramolecular disproportionation and yields finally the thiadiazepine(IV). This product, however, can be easily reduced at the given potential to the dihydrothiadiazepine(V). This mechanism has been confirmed by dc polarography, cyclic voltammetry, controlled-potential coulometry and controlled-pontential preparative electrolysis. The yield in the electropreparation of IV is better than in a chemical procedure. This holds true in particular for slightly acidic, neutral and slightly alkaline solutions with 50% ethanol or for a 0.5 M LiCl solution in anhydrous ethanol. At pH 4.6 III is very stable and can be obtained in a high yield as a valuable intermediate for further chemical reactions.

Electrochemically initiated intramolecular cyclization of 2-nitro-2′-isothiocyanatobiphenyl

Abstract 2-Nitro-2′-isothiocyanatobiphenyl was prepared as a new substance, suitable for the investigation of an electrochemically initiated intramolecular cyclization. If it is reduced at a mercury cathode in acidic media an acid catalysed intramolecular follow-up reaction occurs which leads to the formation of 6-mercaptodibenzo ( d, f )-(1,3)-diazepin-5-oxide. The rate of the follow-up reaction decreases in neutral or alkaline media and the reduction of the isothiocyanato group also takes place. The main product is here 5,6-dihydrobenzo( c )cinnoline.

Competition between different electrolytical reduction mechanisms in 2,3,5,6-tetrafluoro-1,4-dicyanobenzene

Abstract The electrochemical reduction of 2,3,5,6-tetrafluoro-1,4-dicyanobenzene (I) proceeds via a radical anion which may further react either electrochemically or chemically. Only one nitrile group is affected. In a sequence of reactions either a − CH2NH2 group (n=4) is formed or a fission of the C-CN bond occurs (n=2). The third possibility is a dimer formation which in this case does not play an important role. The first two mechanisms compete and their rates are a function of proton availability, of the concentration of the non-aqueous co-solvent and of the electrode potential. The results were obtained by d.c. polarography, discontinuous square-wave voltage techniques, cyclic voltammetry, and constant-potential coulometry and microcoulometry. The electrochemical properties of I were compared with those of 1,4-dicyanobenzene (II), 1,3,5-tricyanobenzene (III) and 4-cyanopyridine (IV).

Electrolytical reduction of maleinamide and fumaramide of 2-nitrobiphenyl-2′-amine at mercury electrodes

Abstract The role played by geometric isomerism in the intramolecular cyclization reaction initiated by the electrode reduction of maleinamide and fumaramide of 2-nitrobiphenyl-2′-amine was studied at mercury electrodes. The cyclization reaction leading to the formation of benzo( c )cinnoline occurs with the former substance whereas only a reduction of the nitro group and of the CC double bond could be observed with the latter.

Fission of Activated Carbon—Nitrogen and Carbon—Sulphur Bonds. XI. Polarographic Reduction of Substituted Benzenesulphonamides

Substituted benzenesulphonamides bearing strongly electron-attracting substituents can be reduced at the dropping mercury electrode to ammonia, sulphur dioxide and monosubstituted benzene. The products have been identified in a controlled potential electrolysis using a mercury pool electrode. A pH-independent, two-electron wave is observed at pH 7 to 12. Fission of the C—S bond occurs before that of the S—N bond. The reaction represents a kind of aromatic SN2 substitution with the —SO2NH2 grouping as a leaving group. The substituent effects on half-wave potentials at pH 9·3 for derivatives I–XIX were examined. The reduction is strongly facilitated by conjugation with acceptor groups in the para-position; this effect is even stronger than would be expressed by \(\sigma {}_p^ - - x\) constants.

Fission of activated carbon-nitrogen and carbon-sulphur bonds. XII. Polarographic reduction of substituted benzonitriles bearing in para -position a carbonyl group in acidic media

4-Cyanoacetophenone and 4-cyanobenzaldehyde are reduced in acid media in a four-electron step followed by a one-electrone reduction of the protonated carbonyl group. In the four-electron step the cyano group is reduced in a protonated species to methylenamino grouping. 4-Cyanobenzophenone is reduced in acid media in two two-electron steps. In the first step which is split into two one-electron waves, the carbonyl group is reduced. In the consecutive two-electron step 4-cyanobenzhydrol is further reduced.

Fission of Activated Carbon-Nitrogen and Carbon-Sulphur Bonds. X. Polarographic Reduction of Substituted Methyl Phenyl Sulphones

Reduction of twenty-six meta- and para-substituted methyl phenyl sulphones at the dropping mercury electrode and at a mercury pool electrode was investigated and by identification of reduction products was decided which of the two functional groupings attached to the benzene ring is reduced. With most derivatives bearing an electron attracting group as a substituent, the methyl sulphonyl group leaves as methane sulphinate anion. A pH-independent, two-electron wave is observed between pH 7 and 14. The reaction represents a kind of aromatic SN2 substitution and is strongly facilitated by conjugation in the transition state with an acceptor group in the para- position. The effect of para-substituents is even greater than expressed by the values of \(\sigma {}_p^ - - x\) substituent constants.

Quantitative Treatment of Substituent Effects in Polarography. VI. Polarographic Reduction of Benzonitriles m- and p-Substituted with Electronegative Groups

Benzonitriles bearing a COO−, COOR or CN group in the meta- or para-position are reduced at pH > 8 in one two-electron step in which cyanide ions are the leaving group. Effects of substituents can be expressed by Hammett equation: ΔE 1/2 = ϱ Xσ− X. The activation by electron attracting substituents is for benzonitriles less important than for aryl alkyl sulphones and benzenesulphonamides. The transition state in the benzonitrile reduction is probably more symmetrical.

POLAROGRAPHY OF PYRIDOXINE AND SOME OF ITS DERIVATIVES

Polarographic curves for pyridoxol, 4-methoxymethyl-5-hydroxymethyl-3-hydroxy-2-methylpyridine, pyridoxamine, pyridoxamine-5-phosphate, 4-pyridoxthiol, bis-4-pyridoxyldisulphide, pyridoxal, pyridoxal-5-phosphate, pyridoxaloxime and pyridoxaloxime-5-phosphate in buffered media were described and discussed. Reduction of the C—O, C—N(+) and C—S bonds in pyridoxol, pyridoxamine and pyridoxthiol has been proved. The reactivity of these bonds in polarographic reductions decreases in sequence: C—SH < < C—N(+)H3 < C—OCH3, C—OH. The effect of esterification by phosphoric acid in position 5 in pyridoxal, pyridoxamine and pyridoxaloxime and the participation of the phosphoric acid grouping in acid-base reactions was shown. Two types of polarographic dissociation curves of dibasic acids were demonstrated. Most of the acid-base reactions encountered in this group of substances involve not only hydroxonium but also other protondonors.

Polarographic Reduction of 2-(2-Methyl-3-Hydroxy-5-Hydroxymethyl-4-Pyridyl)Thiazolidine-4-Carboxylic Acid

In the connection with the systematic polarographic study of pyridoxine derivatives1 the polarographic behaviour of the condensation product of pyridoxal with cystein, i.e. of 2-(2-methyl-3-hydroxy-5-hydroxymethyl-4-pyridyl)thiazolidine-4-carboxylic acid2 (I) was studied. The waves of this substance were interpreted using comparison with waves of pyridoxine derivatives and by the means of controlled potential electrolysis.