Mario Emilio Cardinali

Electrochemical behaviour of 1-benzyl-3-carbamoylpyridinium chloride, a NAD+ model compound

Abstract In aqueous media, 1-benzyl-3-carbamoylpyridinium ion undergoes an initially reversible one-electron reduction to produce a free radical, which irreversibly dimerises to a 4,4′-linked diastereoisomeric pair. At more negative potential, the free radical undergoes a further one-electron reduction to a mixture of 1,6- and 1,4-dihydropyridine derivatives. At sufficiently positive potential the dimers can be oxidised back to pyridinium cation. The dimeric products are strongly adsorbed on the electrode surface, also at potential values where 2e products are formed, and this is a preparative drawback for the dimers and the dihydropyridines. Such problems can be overcome by the addition of benzene and of surface-active substances respectively. A detailed mechanism of the electrochemical reduction of 1-benzyl-3-carbamoylpridinium ion is proposed.

Electrochemical behaviour of 3-iminoindolenine derivatives in dimethylformamide: II. Reduction of 2-phenyl-3-aryliminoindolenine-1-oxide

Summary The electrochemical behaviour of substituted 2-phenyl-3-aryliminoindolenine 1-oxides (I a-f ) in DMF solution was studied by polarography, cyclic voltammetry, controlled-potential coulometry and u.v spectroscopy. In aprotic medium (DMF with KClO 4 as supporting electrolyte) compounds (I a-f ) are reduced at a mercury dropping electrode in two one-electron steps. The potentials of the first reversible step have been correlated with the Hammett constants. In the presence of a protonating agent such as 3,4-dimethylphenol or water the first wave grows in height at the expense of the second until with sufficient protonating agent only one wave of two-electron height is observed. On addition of a stronger protonating agent such as benzoic, monochloroacetic or salicylic acid, the two normal waves found with the purely aprotic medium are gradually replaced by a single two-electron wave (A) at less negative potentials and a new wave (D) appears at more negative potentials. The electrochemical reduction product is 1-hydroxy-2-phenyl-3-arylaminoindole corresponding to the first wave (A) and 2-phenyl-3-arylaminoindole corresponding to the second wave (D). A multi-step scheme for the electrochemical reduction of (I a-f ) is suggested to explain most of the experimental results obtained.

Electrochemical reduction of bis(2-phenyl-3-indolinone)-azine in dimethylformamide

Summary The electrochemical behaviour of bis(2-phenyl-3-indolinone)azine (I) in dimethylformamide solution was studied by polarography, cyclic voltammetry, controlled-potential coulometry and ultraviolet spectroscopy. In aprotic medium (dimethylformamide with tetraethylammonium perchlorate as background electrolyte) compound (I) is reduced at a mercury electrode in two reversible one-electron steps: first to an anion radical and then to the di-anion. In the presence of 3,4-dimethylphenol the second wave of compound (I) shifts to more positive potentials, while the first wave is unchanged. With a sufficient amount of the proton donor the two waves merge and give a single two-electron wave. When benzoic acid is used as proton donor a new wave appears, less negative than the two normal waves, but the total height of this polarogram is equal to the total height of the two waves of the aprotic medium. If the concentration of protonating agent is increased the parallel increment in the first wave is accompanied by a decrease in the third wave. In the presence of a sufficient amount of benzoic acid only one wave of two-electron height is obtained. A multi-step scheme suggested for the electrochemical reduction of (I) explains the various experimental results obtained.

On the electrochemical behaviour of azomethine derivatives of α-dicarbonylic compounds in aqueous media: II. Reduction of phenylglyoxalaldoxime and phenylglyoxalhydrazone

Summary The electrochemical behaviour of phenylglyoxalaldoxime (I) and phenylglyoxalhydrazone (II) in aqueous buffered media has been studied by polarographic, coulometric, and spectrophotometric means. The main feature of the reduction polarograms of compounds (I) and (II) is the presence of two waves, A and B, caused by a stepwise reduction to phenacylamine (VI) and acetophenone (V). In acidic media (pH → − NH 3 phenylglyoxalaldimine (VII), are in agreement with a hypothesis, previous formulated by the authors, that the electrochemical reduction of α-ketooximes and α-ketohydrazone takes place via an e-c-e mechanism.

Electrochemical reduction of 3-iminoindolenine derivatives in dimethylformamide

Summary The electrochemical behaviour of 3-iminoindolenine derivatives (Ia−g) in DMF solution was studied by polarography, cyclic voltammetry, controlled-potential coulometry and ultraviolet spectroscopy. In aprotic medium (DMF with KClO4 as background electrolyte) compounds (Ia−g) are reduced at a mercury dropping electrode in two-electron steps. The potentials of the first reversible step have been correlated with the Hammett constants. In the presence of 3,4-dimethylphenol the first wave grows in height at the expense of the second until with sufficient protonating agent only one wave of two-electron height is observed. On addition of benzoic or monochloroacetic acid as protonating agent, the two normal waves of aprotic medium are gradually replaced by a single two-electron wave at less negative potentials. The electrochemical reduction products of compounds (Ia−g) are 3-aminoindole derivatives (IIa−g). A multi-step scheme for the electrochemical reduction of (Ia−g) is suggested to explain the various experimental results obtained.

On the electrochemical behaviour of azomethine derivatives of α-diketones in aqueous medium: I. Reduction of 2-oxyimino-1,2-dihydropyrrolizin-1-one

Summary The electrochemical behaviour of 2-oxyimino-1,2-dihydropyrrolizin-1-one (I) in aqueous buffered medium (2 The main feature of the polarogram is the presence of two waves (A and B), caused by the stepwise reduction of (I) to the corresponding α-aminoketone and ketone. In an alkaline medium (pH>9.5), where (I) is present almost completely in its anionic form (p K =8.6), the slowing-down of the proton recombination step of the anion of (I) causes a gradual decrease of wave A and the appearance of a new wave A′, caused by the direct reduction of the same anion. A mechanism is suggested for the reduction as a whole and an analysis given for the role played by the carbonyl group in the reduction.

Electrochemical reduction of 2-cyano-1-methylpyridinium ion

Abstract 2-Cyano-1-methylpyridinium ion, I, exhibits three well-defined polarographic waves in Britton-Robinson buffer at pH 5 to 8. Electrolysis at the plateau potential of the most positive wave, involving a one-electron uptake, leads to the formation of a pyridinyl radical which probably dimerizes to a 2,4′-linked structure, VIII. Electrolysis at the plateau potential of the second wave, involving a number of electrons variable between nearly 4 (at pH≈5) and ≈3 (at pH≈8), leads to the formation of a mixture of 2-aminomethyl-1-methylpyridinium ion and 1-methylpyridinium ion. At the third wave potentials the second wave products are further reduced to give basic species with catalytic activity.

Electrochemical reduction of 3-diazocamphor

Summary The electrochemical reduction of 3-diazocamphor (I) has been studied by polarographic coulometric and spectrophotometric means. The experiments were carried out in aqueous solutions between pH≈-6 and pH≈-12; in this pH range (I) does not undergo appreciable hydrolysis. On the reduction of (I) at pH

Electrochemical reduction of azomethine derivatives of α-diketones: III. 1,2-dihydropyrrolizine-1,2-dione dioxime

Summary The electrochemical behaviour of 1,2-dihydropyrrolizine-1,2-dione dioxime (I) in aqueous buffered media has been studied by polarographic, coulometric and spectrophotometric means. The main feature of the polarogram in acidic media is the presence of three waves (A, B and C). Wave A is caused by the reduction of (I) to the corresponding ene-diamine (II), which is then reduced to ene-amine (V) (wave B) and to amine (VI) (wave C). In a controlled potential electrolysis, performed at potentials of the plateau of wave A, the reduction yields 1,2-dihydro-2-aminopyrrolizin-1-one (III), formed via hydrolysis of (II); a further reduction of (III) yields ketone (IV). When pH is increased, waves B and C are progressively lowered, and finally vanish at pH∼-8, because of a slowing down of the pre-protonation steps. At pH>8, wave A is split into three waves (A, A′ and A″), the first of which increases, and the second and third decrease, as pH attains higher and higher values; this behaviour can be ascribed to the reduction of the protonated, neutral, and anionic form of (I).