Romano Andruzzi

Dimethylpyrrolidinium, a catalyst for organic electroreductions

The electroreduction of dimethylpyrrolidinium (DMP+) in diglyme on mercury cathodes was investigated. It was found to proceed via a fast one-electron transfer resulting in an insoluble “amalgam”. The DMP+ amalgam was reactive and transferred electrons to suitable substrate with regeneration of DMP+. Thus DMP+ could be used as a catalyst for electroreduction and this possibility was studied. It was found to be a suitable catalyst for the reduction of several difficult-to-reduce substrates (E1/2 < −2.8 V vs. SCE). Among others it effected the reduction of fluorobenzene, an aliphatic chloride, and simple conjugated dienes. A possible mechanism and the utility of the reaction are discussed.

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.

A solid electrode with periodical renewal of the diffusion layer: The pulsed platinum electrode

Abstract The voltammetric behaviour of a new type of solid electrode with periodical renewal of the diffusion layer is described. The renewal is achieved simply by means of a movement in an up and down sense of the electrode (pulsed electrode) with respect to the quiet solution.

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.

Chemical and electrochemical synthesis of quinoneimine n-oxides from indolinone-3-arylimino nitroxide radicals

Abstract 2,2-Diphenyl-3-arylimino-indoline-1-oxyls are efficiently converted to the corresponding isomeric 5- and 7-quinoneimine N-oxides either by chemical (cerium ammonium nitrate) or electrochemical oxidation in aqueous acetonitrile. The electrochemical studies indicate that mono-electronic oxidation of the starting compounds leads to the corresponding oxoammonium ions. These are stable in anhydrous acetonitrile, as indicated by cyclic voltammetry, but in non-anhydrous conditions undergo rapid nucleophilic attack by water molecule followed by further oxidation to the final products. The relative economic and practical advantages of the electrochemical unvs the chemical process are discussed. The ESR parameters for the new nitroxide radicals involved in the present investigation are also reported.

Anodic oxidation mechanism of 3-arylaminoindoles at a rotating and at a (new type of) pulsing platinum electrode in dipolar aprotic solvents

Abstract The oxidation of 2-phenyl-3-arylaminoindoles (1a–e) has been studied in acetonitrile, dimethylformamide and propylene carbonate at a new type of platinum electrode with periodical renewal of the diffusion layer and at a rotating platinum electrode by cyclic voltammetry, d.c. and a.c. voltammetry, controlled potential coulometry and spectrometry (e.s.r. and u.v.). In the examined aprotic solvents and in the presence of a base (like water or diphenylguanidine) amines 1a–e undergo a two-electron oxidation to the corresponding imines. In a protophobic medium (acetonitrile or propylene carbonate) 1a–e are oxidized in two one-electron steps, the first of which leads to the formation of a cation radical (identified by e.s.r. spectrum), oxidizable at more positive potentials (second step). The second electron transfer, however, must be simultaneous with, or be immediately followed by, a very fast deprotonation reaction. The same e.e.c. sequence explains the observed bielectronic step of 1a–e in a protophilic aprotic solvent, like dimethylformamide.

ELECTROCHEMICAL AND ESR-STUDIES ON THE REDUCTION OF 3-ARYLIMINO-3H-INDOLE-N,N′-DIOXIDES

Abstract 3-Arylimino-3H-indole-N,N′-dioxides have been reduced in DMF to the corresponding radical anions, and studied using polarographic and controlled potential electrolysis, as well as UV and ESR spectrometry. A multistep scheme has been suggested in order to explain the various experimental results. ESR spectra were explained in terms of HFSC evaluation and theoretically reconstructed in order to check the validity of the attributions. The same radicals were obtained with DMSO/t-BuOK as reducing agent.

Characterization and utilization of a long-lasting sessile-drop mercury electrode in differential pulse anodic-stripping voltammetric subtrace metal analysis of natural waters.

Construction details and voltammetric characterization of a long-lasting sessile-drop mercury electrode are reported. The results obtained in the fully automated differential pulse anodicstripping voltammetric determination of cadmium in a simple purified electrolyte as well as of zinc, cadmium and lead in sea-water samples suggest that this new kind of semi-stationary mercury electrode may be employed in voltammetric trace and ultratrace analysis of natural waters for metal ions.