André Darchen

Mécanisme de réduction des mononitrobenzènes en milieu aqueux: Formation des produits secondaires lors des électrolyses

Abstract In acidic solutions, the cathodic reduction of substituted mononitrobenzenes involves the non-reducible intermediary N,N-dihydroxylphenylamine (Ar-N(OH)2). During the macroelectrolysis, the nitrosobenzenes formed by dehydration of the dihydroxylamines couple with the phenylhydroxylamines (Ar-NHOH); the resulting azoxy compounds precipitate or are reduced to the corresponding azo or hydrazo derivatives.

Mecanisme e.c.e. de reduction du para-dinitrobenzene en para-nitrophenyhydroxylamine

In acidic solutions, at low temperature, the first cathodic reduction of p-dinitrobenzene involves the intermediate N-p-nitrophenyl-N,N-dihydroxyamine (O2NC6H4N(OH)2). The dehydration of this dihydroxyamine leads to the p-nitrosonitrobenzene (O2NC6H4NO) which is reduced to the N-p-nitrophenylhydroxylamine (O2NC6H4NHOH). The interposed chemical reaction of this e.c.e. mechanism involves acid-base catalysis.

Substitutions selectivesde carbonyle dans des complexes polynucleaires: IV. Regioselectivite sous activation thermique ou electrochimique pour un complexe binucleaire du fer non-symetrique

Abstract Controlled replacement of carbon monoxide by trimethylphosphite in μ-(alkoxymethanethione- C , S )-μ-(methylthio)1,1,1,2,2,2-hexacarbonyldi complexes (I) is studied using thremal-activation or electron-transfer catalysis. The thermal reaction is not selective and leads to a mixture of mono- and di-substituted complexes II and III, respectively. The monosubstitution is regiospecific with P(OMe), coordinated to Fe(1) that is also linked to the carbene ligand. Electrochemical studies show an enhanced reactivity for the ligand substitution catalyzed by electron transfer. Controlled potential electrolysis of I in the presence of P(OMe) 3 follows an ECE mechanism selectively affording monosubstituted products; the major complex II is accompanied by complex IV in which P(OMe) 3 is bound to Fe(1) but occupies the axial position in contrast to complex II where P(OMe) 3 is equatorial. The isomerization IV → II establishes that IV is not the primary product of the thermal reaction but is the kinetic product when the reaction is promoted by electron transfer. Under thermal activation the ligand substitution may operate via reaarangement of an intermediate between the phosphite and the carbene ligand.

Etude electrochimique de la labilite des complexes paramagnetiques η5-cyclopentadienyle η6-arene fer(I). Reaction d'echange de l'arene par le cyclopentadiene

The decomposition of neutral electrogenerated η5-cyclopentadienyl-η6-arene iron(I) is investigated. The cyclic voltammetry shows that the cationic precursors η5-CpFeII+-η6-Ar are reduced in a one-electron transfer step followed by a chemical step which involves coordination of solvent molecules. The increasing stability of η5-C5H5FeI-η6-C6H6 follows the order of solvent used: acetonitrile or pyridine < dimethylformamide < acetone or methylene chloride. In acetonitrile, the fast replacement of C6H6 by donor ligands occurs via a postulated intermediate, the 17-electron complex η5-C5H5FeI(CH3CN)2. Without donor ligands decomposition of η5-CpFeI-η6-Ar leads to the corresponding ferrocenes, Cp2FeII. In the presence of cyclopentadiene the electrochemical activation allows replacement of the arene group of η5-CpFeII+-η6-Ar by a cyclopentadienyl ligand.

Reduction electrochimique regio- et stereospecifique de cetones et d'acides, activee par un greffon organo-fer cationique en α du groupement reductible☆

Abstract Ketones and acids derived from the η5-CpFe+-η6-arene series in which the function is attached to the ring are conveniently transformed into secondary alcohols or pinacols and to primary alcohols by cathodic reduction on mercury. The electroreduction is activated by the cationic organoiron group and is regiospecific on the functional group rather than on the ring, contrary to the reduction with chemical reagents. The electroreduction of α-oxotetramethylene-cyclopentadienyliron+-η6-benzene specifically occurs from the exo side, giving only the endo alcohol.

Cationic (η5-cyclopentadienyl)(η6-arene)iron(II) catalyst for the electroreduction of nitrate ion

The 19-electron complex radicals resulting from the one-electron reduction of the cationic precursors [(η5-cyclopentadienyl)(η6-arene)FeII]+ quickly reduce nitrate ions to ammonia in aqueous solutions.

Substitution selectives de carbonyles dans des complexes polynucleaires V. Etudes Structurales des composes de mono-et disubstitution de co par P(OMe)3 dans un complexe binucleaire non symetrique

Abstract The complexes μ-(alkoxymethanethione- C , S -μ-(methylthio)-1-trimethylphosphite-1,1,2,2,2,-pentacarbonyldiiron (II) are obtained by controlled replacement of CO by P(OMe) 3 in the hexacarbonyl precursors I. The structure of the complex IIa (alkoxy-adamantylmethoxy) has been determined by single-crystal X-ray diffraction. The structure shows that P(OMe) 3 has replaced one carbonyl on Fe(1) and occupies an equatorial position placing it in cis relationship with the carbene C(1) and the two carbonyls coordinated to Fe(1). Knowledge of the structures I and II allows one to elucidate their 13 C NMR spectra in the carbene and carbonyl regions. For instance 2 J and 3 J coupling constants and chemical shifts for these ligands can be proposed on the basis of bond lengths and on the dynamic behavior of compounds I and II for which the fluxionality is limited to the ligands coordinated to Fe(2). These data allow one to propose the structure of the bis-phosphite complexes III and of hte isomeric monophosphite IVa, obtained when the monosubstitution is carried out by electron transfer catalysis.

Electron transfer catalysis of the regioselective replacement of CO by P(OMe)3 in dissymmetrical diiron hexacarbonyl complexes

Abstract Substitution of carbon monoxide by trimethylphosphite in two dissymetrical diiron hexacarbonyl complexes of known structure has been studied using thermal activation or electron transfer catalysis. The same regioselectivity for the monosubstitution is observed in both cases.

Electron transfer catalysis of arene deco-ordination in the cationic (η5-cyclopentadienyl)(η6-arene)iron(II) complexes

Replacement of the arene group in [Fe(η5-C5H5)(η6-C6H6–n-Men)]+(n⩽ 4) by three P(OMe)3 ligands is carried out under mild conditions by reductive electron transfer catalysis in acetonitrile and gives the free arene and [(η5-C5H5)Fe {P(OMe)3}3]+ in good yields.

Stepwise electrochemical catalysis of three successive replacements of the carbonyl group by trimethyl phosphite in a tri-iron cluster complex

The electrochemically catalysed replacement of the carbonyl ligand by P(OMe)3 in the tri-iron cluster Fe3S2CO8L (L = C3H2S2) proceeds stepwise and leads to the clusters Fe3S2CO8–nL[P(OMe)3]n(n= 1, 2, or 3).