André Tallec

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.

Electrosynthesis and redox behavior of vinylogous TTF displaying strong conformational changes associated with electron transfers

Abstract The redox properties of a series of substituted vinylogous tertrathiafulvalenes (TTF) prepared by oxidative coupling of 1,4-dithiafulvenes (DTF) have been investigated in acetonitrile and dichloromethane. The different steps of the electrodimerization mechanism have been characterized: fast electron transfer, coupling between two cation-radicals and slow deprotonation. Through the substituent choice of DTF, it is possible to control the relative stabilities of the different redox species of the electrogenerated vinylogous TTF. According to the nature and position of the substituent, the structural changes induced by the steric interactions lead to a compression of potential (where the second electron is easier to remove than the first one), or on the contrary to a large increase of the separation between the first and second oxidation potentials (by comparison with similar molecules without steric hindrance). Density functional modeling calculations and detailed analysis of the electrochemical behavior have been used to rationalize the substituent effect. A good agreement with the occurrence of an EE mechanism in which the electron transfer is concerted with the conformation changes is found. The inner reorganization energies are low (0.35–0.45 eV) allowing a fast passage between the different conformations during the electron transfers.

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.

Regioselective electroreduction of 6H-1,3-thiazines, comparison with chemical reductions. I: Investigation of 2-phenyl-6H-1,3-thiazines

The electrochemical reduction of 2-phenyl-6H-1,3-thiazines carried out at a mercury cathode in acetate buffer + ethanol (1:1) has been studied : - In the case of compounds monoactivated at carbon 5 (R5 = CHO or COCH3) either hydrodimers (resulting from coupling at C-2), 3,6-dihydro-2H-1,3-thiazines (reduction of the imine bond) or tetrahydrothiazines can be obtained. - Diactivated 6H-1,3- thiazines (R4 = CO2Et, R5 = CHO or COCH3) successively lead to 5,6-dihydro-4H-1,3-thiazines (reduction of the ethylenic bond) and tetrahydrothiazines. - The reduction of a 6H-1,3-thiazine bearing only one withdrawing group at carbon 4 gives rise to a ring opening. At the same time, the action of various chemical reducing agents has been examined : the reduction of the imine bond is performed using NaBH3CN while NaBH4 or LiAlH4 leads to the reduction of the substituents on the heterocyole.

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.

Dication charge transfer salts of TTF vinylogues

TTF vinylogues were electrochemically synthesized and dication charge transfer salts were isolated. Important conformational changes between the neutral and oxidized states were observed by X-ray structure analysis.

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 ).

Cyclocondensation reactions from electrogenerated halomalonate carbanions—compared reactivity of bromo and chloro anions and influence of the supporting-electrolyte cation

Monohalomalonate carbanions, generated by the 2e− reduction of X2C(CO2Et)2(X = Cl or Br) react with Michael acceptors, giving cyclocondensation products (cyclopropane or dihydrofurane derivatives). The chloro carbanion appears to be more reactive; the reactivity of the bromo carbanion strongly depends on the supporting-electrolyte cation.

Cathodic synthesis of powerful electron π-donors including dithiadiazafulvalenes

Intramolecular coupling of bis(2-ethylthio-1,3-thiazolium) salts upon electrochemical reduction leads to dithiadiazafulvalenes (DTDAF). These novel π-electron donor molecules show extremely high donating properties compared to their sulfur analogues, tetrathiafulvalenes (TTF). In addition we report that a DTDAF precursor substituted by a dithiafulvenyl group undergoes dimerization upon oxidation to afford vinylogous TTF. Cathodic coupling of the corresponding tetra(2-ethylthio-1,3-thiazolium) salt leads to a new donor including two different donor families: DTDAF and vinylogous TTF.