Franco Marcuzzi

Electrochemical reduction of carbonyl compounds in aqueous media in the presence of β-cyclodextrin. Reduction of acetophenone and p-methoxyacetophenone in alkaline solution

Abstract The electrochemical reduction of acetophenone, 1, and 4-methoxy-acetophenone, 2, has been investigated in aqueous alkaline solutions, on mercury electrode, in the absence and in the presence of β-cyclodextrin (β-CD). Macroelectrolyses afforded mixtures of d,l and meso pinacols (head-to-head dimers) as preferential products and, in the case of 1, also head-to-tail dimers. These dimers showed some optical activity (enantiomeric excess of ca 24% for each diastereoisomer) when obtained by reduction in the presence of β-CD, whereas d,l pinacols did not. The same results were obtained by reduction of pre-formed 1:1 complexes with β-CD. A mechanism involving dimerization of radical anions is proposed for the formation of both type of dimers, and the asymmetric induction observed in the head-to-tail dimerization products is attributed to chiral effects exerted by β-CD on the guest molecules in this particular process.

Dicyanomethylene derivatives of squaric acid: electrochemical behaviour and ESR investigation

The electrochemical behaviour of mono- and bisdicyanomethylene-substituted squaric acid dianion was investigated in dimethylformamide and acetonitrile on a Pt electrode. In cyclic voltammetry the oxidation of these dianions involves two successive, chemically reversible, one-electron processes, affording the corresponding radical anions and neutral species. The redox potentials are strongly dependent upon the number of dicyanomethylene substituents. Whereas the neutral species are stable only in the time scale of the voltammetric experiments, the radical anions can be accumulated by macroscale electrolysis. The ESR spectra of these radicals are reported. The monosubstituted dianion is protonated by electrochemical oxidation of hydrogen, whereas the disubstituted ones are not. Compounds obtained from the disubstituted dianions and the methyl viologen dication are also described.

Electrochemical behaviour of 1,2-dihydroxycyclobuten-3,4-dione in dimethyl formamide

Abstract The electrochemical behaviour of 1,2-dihydroxycyclobuten-3,4-dione (squaric acid, H 2 SQ) in dimethylformamide was investigated. On a Pt electrode, the reduction involves two successive, chemically reversible, one-electron processes, affording hydrogen and the basic species HSQ − and SQ 2− respectively. The oxidation of SQ 2− thus formed occurs in two consecutive, chemically reversible, one-electron steps, affording the radical anion SQ .− and the neutral species SQ°; the neutral species SQ° is stable in the time-scale of the voltammetric experiments only. In contrast, the chemically irreversible oxidations of H 2 SQ and HSQ − appear as more complicated processes. The oxidation of hydrogen in the presence of the electrogenerated basic species was also studied. A comparison with the results obtained on an Hg electrode is made and the occurrence of a different voltammetric behaviour, as regards the reduction process, is discussed briefly.

Oxidative dimerisation of some stabilised carbanions: a mechanistic investigation

Abstract The electrochemical oxidation of carbanions (p-CH3C6H4)Z1Z2C− (1−, Z1=Z2=CN; 2−, Z1=CN, Z2=COOEt; 3−, Z1=Z2=COOEt) was studied in CH3CN+n-Bu4NBF4 by experimental and simulated voltammetry, macroscale electrolyses and product analyses, with the aim of investigating the mechanism of the dimerisation process. The carbanions, generated by cathodic reduction of the conjugated acids, show a chemically irreversible oxidation peak whose position reflects the relative basicity of the anions (3−>2−>1−). Kinetic data deriving from peak potential analysis indicate that neutral radicals are generated at the electrode, and that a fast radical–radical coupling (DIM1 mechanism) is by far the most effective dimerisation process for all the substrates. Accordingly, the corresponding homodimers are formed quantitatively after exhaustive, one-electron macroscale electrolyses, also when the oxidation is performed in the presence of a second anion, oxidisable at more positive potentials. However, at high anion to radical ratio, a significant contribution of a radical–anion coupling (DIM2) can be involved when the anion is a good nucleophile. The position of the voltammetric oxidation peak of 3− is strongly affected by the presence of alkaline cations, which cause a shift of the wave towards more positive potentials (Li+>Na+>K+>NR4+) and also a decrease of the rate of the heterogeneous electron transfer in the same order. Both these effects can be explained by ion pairing.

Stable sulfinic acids from the ene reaction of methyl-substituted allenes with sulfur dioxide

Abstract Allenes 2,4-dimethyl-2,3-pentadiene and 3-methyl-1,2-butadiene in liquid sulfur dioxide undergo ene addition with the solvent giving the stable 3-(2,4-dimethyl-1,2-pentadienyl) and 2-(3-methyl-1,2-butadienyl) sulfinic acids.

The crystal structure of the 1:1 inclusion complex of β-cyclodextrin with squaric acid

The crystal and molecular structure of the 1:1 inclusion complex of beta-cyclodextrin (cyclomaltoheptaose) with squaric acid (3,4-dihydroxycyclobutene-1,2-dione) was determined by X-ray diffraction. The complex crystallizes in the monoclinic P2(1) space group and belongs to the monomeric cage-type, characterized by a herringbone-like packing motif. Co-crystallized water molecules are present on seven sites, of which six are fully occupied. The guest molecule is placed inside the beta-cyclodextrin cavity, perpendicular to the plane defined by the glycosidic O-4n atoms, and held in place by direct and water-mediated hydrogen bonds mainly involving symmetry-related beta-cyclodextrin molecules. The accommodation of the planar guest molecule into the beta-cyclodextrin cavity determines a significant distortion of the latter from the sevenfold symmetry.

High selectivity in the catalytic hydrogenation of acyl-substituted pyridines in alkaline solution: effect of complexation by β-cyclodextrin

Abstract Acetyl- and benzoyl-substituted pyridines are reduced in two consecutive steps, with hydrogen in aqueous alkaline solution in the presence of Pd/C under very mild conditions, to the corresponding alcohols and hydrocarbons in high yields. The complexation with β-cyclodextrin affects the reaction rate in an unexpected manner.

An “indirect” self-protonation process in the electrochemical reduction of phenyl-substituted indenes

In a recent paper [l] we described the electrochemical reduction of a series of self-protonating indenes, i.e. indenes bearing at least one acidic hydrogen at C(1). The overall process implies a two-electron, two-proton reduction of l/3 of the indene (IndH), the remaining 2/3 acting as a proton donor; the reduction involves the C(2)-C(3) double bond of the five-membered ring exclusively and affords the corresponding indans (IndH,):

Electrochemical reduction of activated carbon–carbon double bonds. Part 2. Mechanism and stereochemistry of the reduction of self-protonating indenes

The reduction at a mercury electrode in dimethylformamide of five variously substituted indenes bearing at least one (acidic) hydrogen at C(1) was carried out, under self-protonation conditions and in the presence of phenol as a proton donor. Reduction occurred exclusively at the C(2)–C(3) double bond of the five-membered ring and afforded the corresponding indans of various configurations, with a marked predominance of the cis isomer, under kinetic control. The stoicheiometry was in agreement with a two-electron, two-proton reduction process involving all indene in the case of the presence of phenol or 1/3 indene under self-protonation conditions, the remaining 2/3 acting as a proton donor. Under both conditions direct protonation of the radical anion rather than the dianion formed by disproportionation was observed. Voltammetric and kinetic behaviour of the indenes was found to be dependent on the structure of the substrates, with differences in rate constants of 3–4 orders of magnitude; the balance between the acidity of the indene and the basicity of the corresponding radical anions was discussed. The stereochemistry observed was rationalised in terms of steric effects on the direction(s) of protonation of the intermediates.

Electrochemical reduction of phenyl-substituted cyclopentadienes : a case of an 'indirect father-son' self-protonation process

Abstract The electrochemical reduction of 1,2,3,4-tetraphenyl-1,3-cyclopentadiene and 1,2,3,4,5-pentaphenyl-1,3-cyclopentadiene has been investigated in DMF. At low temperatures (≤−30°C) and under anhydrous conditions cyclic voltammetry experiments indicate that both substrates are reducible in two successive, chemically reversible, one-electron steps, affording the corresponding radical anions and dianions. More complex voltammetric behaviour is detected at higher temperatures, when the radical anions are protonated by the substrate itself, giving rise, via the so-called ‘father–son’ self-protonation process, to further reducible compounds generating additional basic intermediates. In exhaustive electrolyses, 1/3 of the starting substrate is converted into dihydroreduction products and 2/3 into its conjugate bases. On the other hand, the quantitative formation of the dihydroreduction products is observed when exhaustive electrolyses are carried out in the presence of phenol, which acts as the proton donor instead of the substrate. Under voltammetric conditions, however, evidence of deprotonation of the substrate by the phenoxide anions thus generated is obtained, indicating the occurrence of an ‘indirect father–son’ self-protonation process, which takes place since phenol is kinetically more acidic, but thermodynamically less acidic, than the substrate itself. The mechanisms of decay of the radical anions are proposed on the basis of the comparison of experimental and simulated voltammetric data; this approach allowed also the determination of the characteristic kinetic rate constants, some of which are strongly dependent on the structure of the substrate, and of the importance of the homoconjugation reaction involving phenol and phenoxide anion. The stereochemistry of the dihydroreduction process and the voltammetric behaviour of the corresponding products are considered also, even in relation to the voltammetric behaviour of the starting substrates.