Liliana Rampazzo

Stereoselective reduction of meso- and dl-1,2-dibromo-1,2-diphenylethane at the mercury electrode

Summary Meso- and dl-1,2-dibromo-1,2-diphenylethane are both reduced to ∼100% trans-stilbene at the mercury electrode in DMF containing 0.1 M Et4NClO4. Polarographic, coulometric and controlled-potential experiments as well as u.v. spectra substantiate a two-electron process giving the trans-olefin and Br− ions. Reduction performed in the presence of weak or strong proton donors gave the same results as in the absence of acids. Arguments in favour of a “synchronous concerted” mechanism are discussed.

Electrochemistry of 9,9′-spirobifluorene derivatives: 2-acetyl- and 2,2′-diacetyl-9,9′-spirobifluorene. Preparation of stereoisomeric 2,3-bis(9,9′-spirobifluoren-2-yl)butane-2,3-diols

2-Acetyl- and 2,2′-diacetyl-9,9′spirobifluorene 1 and 2 were studied by cyclic voltammetry in dimethylformamide. The corresponding anion radicals show remarkable persistency in aprotic DMF. The (apparent) standard potentials are E°=–1.77 V (SCE) and E°=–1.75 V for the (quasi-reversible) reduction of 1 and 2 to the anion radicals, respectively. Preparative electrolysis of 1 in DMF–Et4NClO4(0.1 mol dm–3), with excess acetic acid as proton donor, furnished alcohol 3 and the diastereoisomeric pinacols 5 and 6, which were isolated and characterized. The diastereoisomeric excess, de, as evaluated (NMR) on the electrolyzed solution was only slightly in favour of the (±) compound. Spectroscopic properties of compounds 1–6 are, inter alia, the 13C NMR chemical shift for the spiro-carbon at δ= 65.9 (TMS), and the fragmentation patterns in the mass spectra, with the 100% relative abundance of the molecular-ion M˙+ in the case of the aromatic ketones 1 and 2. Some comments on the influence of conformations of 5 and 6 and of the presence of an intramolecular hydrogen-bond in some intermediates and products are also presented.

Electrochemical reduction of halogen containing compounds at a mercury cathode: Fluoroacetic, difluoroacetic, trifluoroacetic acids and corresponding ethyl esters in dimethylformamide

Summary The electrochemical behaviour of fluoroacetic ethyl ester (I), fluoroacetic acid (II), difluoroacetic ethyl ester (III), difluoroacetic acid (IV), trifluoroacetic ethyl ester (V) and trifluoroacetic acid (VI) in DMF-Et4NClO4 solutions on a mercury cathode has been studied. All compounds except I are reducible. The esters show a one-electron, diffusion controlled, polarographic wave, with E1/2=−2.56 V (III) and E1/2=−2.36 V (V), with cleavage of the C-F bond. The reduction mechanism is discussed: electron transfer via the carbonyl function is suggested. In fluoroacetic acid (I) the hydrogen of the nearly undissociated molecule is reduced, E1/2=−2.29 V. In the case of difluoroacetic acid, two waves are observed, E1/2=−1.74 V and E1/2=−2.16 V respectively, the first one being kinetic in nature. Controlled potential experiments demonstrate that the first wave corresponds to the reduction of solvated proton, whereas the second wave is caused by the hydrogen reduction from the undissociated acid. Trifluoroacetic acid behaves as a moderately ‘strong’ acid in DMF and only the reduction wave of solvated proton is observed, E1/2=−1.48 V.

Polarographic and voltammetric reduction of some aliphatic α-bromoesters

Abstract The electrochemical reduction of CH 3 CH(Br)COOEt (I), CH 3 CH 2 CH(Br)COOEt (II) and (CH 3 ) 2 CBrCOOEt (III) has been studied at mercury and glassy carbon cathodes in dimethylformamide-Et 4 NClO 4 . Polarographic and cyclic voltammetric data in dry DMF show two reduction waves (peaks) for I and II and a single wave (peak) for III either on mercury or on glassy carbon. The effect of the concentration of the ester on the current function values and on the coulometric n app values, as well as the proton donor effects are described. At the level of the first wave of I and II and of the single wave of III, there is a smooth transition from a two-electron process at the low concentrations to a one-electron process at the high concentrations. From preparative controlled-potential electrolyses at E =−1.5 V (first wave) in dry DMF at Hg, dimeric products were isolated, e.g. from I, meso - and dl -diethyl-2,3 dimethylsuccinate, together with meso - and dl -2,2′-oxybisdiethylpropionate; from II, meso - and dl -diethyl-2,3 diethylsuccinate; and from III, diethyl tetramethylsuccinate together with one unexpected isomeric product identified as EtOOCCH(CH 3 )CH 2 C(CH 3 ) 2 COOEt. Possible mechanisms are discussed.

Electrochemistry of 9,9' spirobifluorene derivatives : electrosynthesis of stereoisomeric 2,3-bis(2'-acetyl-9,9'-spirobifluoren-2-yl)butane-2,3-diols and of 1-(2'-acetyl-9,9'-spirobifluoren-2-yl)ethanol and redox properties of polyacetylated spirobifluorenes

Abstract 2,2′-diacetyl-9,9′-spirobifluorene 3a, an axially dissymmetric compound, when dissolved in virtually aprotic solvent DMF, is characterised by several reduction peaks in the voltammetric experiments, using a glassy carbon electrode; the first two electron transfers are reversible, with standard potentials E10 = −1.75 V and E20 = −1.90 V vs sce (saturated calomel electrode) respectively. A third reduction step occurs with Ep = −2.40 V, at v = 50 mV −1 . The first two steps are pertinent to the formation of the anion radical A.− and of the dianion diradical −.A−. of 3a (A) respectively. Controlled potential electrolysis of 3a on a rvc electrode in DMF-Et4NClO4 (0.1 M), with acetic acid 1:1 mol 3a/mol acid added, furnishes the two diastereomeric keto-alcohols 4 and the six diastereomeric keto-pinacols 5–7. This result is obtained if the electrolysis is stopped after 1 F/mol 3a was passed. If the electrolysis is continued until both CO groups are reduced, a mixture of diastereomeric pinacols 8 is obtained. Spectroscopic properties of compounds 5–7 are sensitive to the various spatial arrangements in the isomers. Some aspects of the redox properties of 3a, as related to the spiro-structure of the molecule, are also discussed. Redox properties of tri- and tetra-acetylated spirobifluorenes 3b and 3c in virtually aprotic conditions, as determined through cyclic voltammetry, are in turn related to those of the diacetylated derivative 3a: compounds [3a, b, c] are the first examples of spirocompounds bearing multiple redox-active substituents.

Polarographic reduction of oxygen in formamide

Summary The polarographic reduction of oxygen in 0.1 F KCl-formamide solutions shows two irreversible waves of equal height with half-wave potentials of −0.14 V and −0.92 V ( vs . internal Hg pool), respectively. The first reduction process leads to H 2 O 2 ; the polarographic diffusion coefficient for oxygen in formamide solutions has been evaluated. Experimental results suggest that oxygen reduction in KCl-formamide solution is similar to that in aqueous KCl solution.

Electrochemistry of organic acceptor compounds: aromatic anhydrides

Abstract The electrochemistry of some aromatic anhydrides (A; I-IX) is presented. Voltammetric experiments on I-IX at a glassy carbon electrode in dry dimethylformamide + 0.1 M Et 4 NClO 4 were used to assign the relative electron-acceptor properties of the anhydrides concerned and to reveal compounds for which persistent radical ions A − and dianions A 2− can be prepared in the solvent system used. Some results of large-scale controlled potential electrolyses of tetraphenylphthalic anhydride (IV) at a mercury pool electrode at an applied potential E of −2.2 V in the presence of weak proton donors are also reported. Phthalide (X) can be isolated among the products, albeit in a very low yield. Quenching of the electrolysed solution with a strong acid leads to dimeric lactones XI and XII.

Electrochemical screening for electron acceptors: aromatic esters

Abstract Voltammetric experiments on some aromatic esters, A, at a glassy-carbon electrode in dimethylformamide are used to study the relative electron-acceptor properties of the molecules investigated and to detect compounds for which the corresponding radical ions A.− and dianions A2− are persistent in the solvent system used. The molecules chosen include the trimethylbenzene tricarboxylates (I)-(III), tetramethylbenzene tetracarboxylate (IV), methyl-1-naphthalene carboxylate (V) and the dimethylnaphthalene dicarboxylates (VI) and (VII), as well as the methylanthracene carboxylates (VIII) and (IX). Esters (II), (III), (IV) and (VII) provide examples of two-step reductions of A, A + e−A.-, A.− + e−A2−, which are reversible even at low potential scan rates. A larger equilibrium constant for the reaction 2A. A + A2− is observed in molecules where steric interactions inhibit some CO2Me groups from being coplanar with the aromatic nucleus. The electron affinity of dimethyl-1,8-naphthalene dicarboxylate is higher than the E.A. of 2,6-naphthalene dicarboxylate. Further consideration of the voltammograms has led us to regard esters (I)-(IX) as each having some distinctive feature in its electrochemical behaviour, particularly where chemical reactions following electron transfer are thought to occur.

Current-time curves on the dropping mercury electrode calculated for interactions between the adsorbed substance and the depolarizer

Abstract Current-time curves for polarographic reduction in the presence of substances adsorbed on the electrode (and not reducible) in a particular case of specific interaction between the depolarizer and the above-mentioned substances are calculated, on the basis of the following assumptions: (I) the interaction takes place only in a layer of thickness I adjacent to the electrode (o . For λ I the current presents a maximum followed by an extended minimum; for λ → ∞ the current, after the maximum, tends asymptotically to zero. For a fixed value of the thickness, l, and for sufficiently high values of the rate constant of the interaction, k, the current, after the maximum, diminishes rapidly and reaches zero more quickly as k increases. These features can be found in the literature.

Benzocyclobutadiene dimer from the electrochemical reduction of α,α,α′,α′-tetrabromo-o-xylene

Abstract Electrochemical reduction (c.p.e.) of α,α,α′,α′-tetrabromo-o-xylene in DMF-0.1 M Et4NClO4 at a Hg cathode, E=−1.40 V vs. SCE, yields 6a, 10b-dihydro-benzo[a] biphenylene, i.e. the rearrangement product of Diels-Alder benzocyclobutadiene dimer. Besides the dimer, a polymer and traces of tri-o-xylylene and tetra-o-xylylene, m/e=306 and m/e =408, are present. Coulometric measurements indicate a four-electron process, with the elimination of four Br−ions/molecule of the starting material. Polarographic measurements on the tetrabromide give two waves with E′1/2=−0.29 V and E″1/2=−1.22 V and a poorly resolved wave with E″1/2=−2.1V. C.p.e. of α,α,α′,α′-tetrabromo-o-xylene at −0.7 V gives 1,2-dibromo-benzocyclobutene (u.v., n.m.r.) and polymeric material. The stoichiometry of this process, involving 3 electrons/molecule of tetrabromide and the release of 3 Br− ions/ molecule, suggests two parallel routes for the reduction of α,α,α′,α′-tetrabromo-o-xylene at −0.7 V. Since the tetrabromide is an equilibrium mixture of two conformers, which do not interconvert readily, each path can be ascribed to a single conformer.