R. Hazard

Stereochemistry of electroreductions of bromocyclopropanes: 1-asymmetric electrochemical synthesis by reduction at a mercury cathode in the presence of adsorbed alkaloïds

Abstract The electrochemical behaviours of 1-bromo-2,2-diphenylcyclopropane carboxylic acid, its methyl ester and 1,1-dibromo-2,2-diphenylcyclopropane are investigated in the presence of strongly adsorbed alkaloids: yohimbine, emetine, brucine, strychnine and methylstrychninium cations. The polarographic study evidences the existence of interactions between the alkyl bromides and nitrogen cations; these interactions make easier the 2 e cleavage of the carbon-halogen bond. Whatever the alkaloid used, rather poor optical yields are obtained after electroreduction of monobromo compounds. On the contrary, notably optically active products are obtained from the dibromide derivative, but only when the inductor can act as a protonating species; 45% optical yield can be achieved in the presence of emetine. The mechanism of asymmetric electrochemical synthesis is interpreted in terms of (i) preferential presentation of one of the two stereotopic faces of the substrate at the mercury cathode, made chiral by the adsorption of the alkaloid and (ii) protonation by the acidic form of the inductor of the carbanion resulting from a 2 e reduction.

Electrochemical reduction of substituted pyridazines: a new access to activated pyrroles

Abstract The electrochemical two electron reduction of pyridazines, substituted by electron withdrawing groups, primarily lead to their corresponding 1,2-dihydro-derivatives. Depending on the nature of the ring substitution, these intermediates can either rearrange into 1,4-dihydro-pyridazines, or undergo electrochemical reduction to give rise to activated pyrroles by a ring contraction reaction with extrusion of nitrogen. Another way of access to the latter has been achieved by a disproportionation reaction of 1,2-dihydro-pyridazines, leading directly to the expected pyrroles and recovery of 50% of pyridazines.

Activated pyrroles from pyridazines: nitrogen extrusion by electroreduction

Abstract The bielectronic electrochemical reduction of pyridazines, substituted by electron-withdrawing groups, leads to their corresponding 1,2-dihydro derivatives. Depending on the nature of the ring substitution, these intermediates can either rearrange into 1,4-dihydropyridazine isomers or be further electrochemically reduced into activated pyrroles.

Aspect stereochimique de l'electroreduction de cyclenes actives. II. Reduction sur cathode de mercure des dicarboxy - 1,2 cyclopentene et cyclohexene et de leurs diesters methyliques: mecanisme et stereochimie☆

The mechanism of the 2e electrochemical reduction of 1,2-dicarboxycyclopenten I and cyclohexen II has been established from a polarographic study. —In stronly acidic media, the first electronic transfer occurs at the cationic species H3X+, resulting either from a monopreprotonization at the H2X diacid (A1 wave) or a dipreprotonization at the HX− anion (A2 wave). —In less acidic media, in a sharp pH interval, a wave (B), corresponding to the reduction of H2X, resulting from a preprotonization at HX− is observed. —In the case of the compound I, one observes for 5 < pH < 7, a wave (C) due to the reduction of HX−. The polarograms of the methylated esters I′ and II′ show a single 2e wave; E12 is pH independent. Analysis of the mixtures of 1,2-dicarboxycyclans obtained after preparative electrolysis, seems to indicate that the stereochemical course of the reduction depends essentially on the nature of the diffusing species (sudden change of the cis-trans isomers ratio occurs according to an A1 or A2 way of reduction). In the case of the diesters, the main product obtained after the electrolysis is always the cis dihydrogenated compound. An interpretation is suggested.

On the electrochemical behaviour of Nifedipine and related compounds: instability of the reduction products in protic media

Abstract The electrochemical behaviour of nifedipine and of the analogous 3,5-dicarbethoxy-2,6-dimethyl-4(2-nitrophenyl)-1,4-dihydropyridine has been investigated in protic media. The nitroso derivative (2e − reduction) can be prepared in a redox flow cell; whatever the acidity of the supporting electrolyte, it is unstable and rearranges into indole ( VIII ). The amine (6e − reduction) is obtained by electrolysis at very negative potential, in an acetic buffer; in more acidic media, transformation takes place into 3-carboalcoxy-2-methyl quinoline ( VI ). The phenylhydroxylamine (4e − reduction) is prepared by controlled potential electrolysis in an acetic buffer; in more acidic media, disproportionation occurs giving a mixture of VIII and VI . A similar behaviour is observed for the N -methyl derivative of nifedipine; however, the corresponding hydroxylamine does not disproportionate in acidic medium but gives rise to the N -oxide derivative of the quinoline ( VI ).

Electrochemical reduction of o-nitrophenylthioacetic derivatives. Production of 2H-1,4-benzothiazines

Comparative studies of the electrochemical reduction of 2-(o-nitrophenylthio)-acetic acid and its methyl ester in aqueous media of different acidity show that reduction of the ester in an ammoniacal buffer is the best adapted for electrosynthesis of 4-hydroxy-2H-1,4-benzothiazin-3(4H)-one. The electrochemical behaviours of the corresponding phenylhydroxylamines and azoxy compounds are also investigated.

Mechanism of the electroreduction of aliphatic nitro compounds. Preparation of N-hydroxypyrrolidinones by reduction of γ-nitroesters

Abstract N-Hydroxypyrrolidinones can be prepared by electroreduction of γ-nitroesters, in very acidic or weakly basic media. In weakly acidic media, non-electroactive oximes are obtained simultaneously with the expected heterocycles. From experimental observations, a general scheme is proposed for the reduction of an aliphatic nitro group; the formation at the cathode of a two-electron intermediate, different from the nitroso compound, is taken into account.

Preparation de pyrrolines N-oxydes, pyrrolines et pyrrolidines par reduction electrochimique de γ-nitrocetones

Abstract Judiciously selected conditions of controlled potential electrolysis, at a mercury cathode in aqueous organic media, of γ-nitroketones result in quantitative yields of 1-pyrroline 1-oxides, pyrrolines or pyrrolidines.

Stereochemical aspect of the electroreduction of activated cyclens— III. Study of 2,3-dicarboxy (and dicarbethoxy) indens' reduction mechanism; stereochemistry, kinetic study of the double bond migration

Abstract The two-electron reduction mechanism of the title compounds has been established by means of polarographic measurements showing, on the other hand, an equilibrium between 2,3- and 1,2- disubstituted forms. Kinetic study of this equilibrium allows the knowledge of the first acidity constants of the two diacidic isomers Electrolysis results confirm that, for this series of compounds, the stereochemistry of the obtained dihydrogenated cyclane depends fundamentally on the nature of the diffusing species: (i) when the latter is a monocarboxylate (case of the diacid, pK 1 2 ), a very small amount (5 per cent) of cis cyclan is obtained. (ii) in the case of a dicarboxylate ion (diacid, pH > pK 2 ), the yield of the cis isomer is about 15 per cent. (iii) if a molecule diffuses, the yield of cis cyclan is maximum but the protonation of the final carbanion interferes kinetically.

Electrochemical Synthesis and Properties of an Isoxazolone.

Synthese de (dimethyl-3,3 phenyl-4) oxazolidone-5 par reduction electrochimique du (methyl-3 nitro-3 phenyl-2) butyramide