M.I. Montenegro

Studies of electrode reactions in low ionic strength media using microelectrodes: Part 1. The reduction of nitrobenzenes in aprotic media

Abstract The influence of ionic strength on the current-potential responses for the electrochemical reduction of a series of nitrobenzenes (nitrobenzene, nitrotoluene, bromonitrobenzenes) in aprotic solvents has been investigated using microelectrodes. It is confirmed that it is possible to obtain reproducible data for solutions with a low ionic strength including media without the deliberate addition of electrolyte. However, their interpretation requires a consideration of the influences of ionic strength on the stability and chemistry of intermediates in addition to mass transport and ohmic drop. The results imply that the interaction between the anion radicals and the tetraalkylammonium cations is always weak but the cations interact strongly with more reduced intermediates.

Deprotection by electrolysis: Part I. The application of homogeneous redox catalysis to the study of the reduction of tosyl esters and amides

Abstract The mechanism of the cathodic cleavage of tosylate protecting groups from alcohols, amines and phenols in dimethylformamide has been probed using the technique of homogeneous redox catalysis. Some nine tosyl esters and six tosyl amides have been investigated and it is confirmed that these deprotection reactions occur by cleavage of the anion radicals. The formal electrode potentials for the couples, neutral molecule/anion radical, are reported and it is shown that the rate constants for the cleavage of the anion radicals lie in the range 104 s−1 to >108 s−1. Indeed for aromatic amines and phenols, the homogeneous charge transfer between the catalyst and the substrate becomes the rate determining step.

The Use of Cyclic Voltammetry to Detect Biofilms formed by Pseudomonas fluorescens on Platinum Electrodes

The development of an electrochemical detector to monitor the in situ formation of biofilms is described. The detector consisted of an electrochemical cell containing three electrodes, whose response to the application of a potential profile to the working electrode was sensitive to the amount of biofilm present on the surface. The electrochemical technique used was repetitive cyclic voltammetry. Differences between the response of the uncolonised electrode and after Pseudomonas fluorescens biofilms of different ages were grown on its surface were determined. The results show that cyclic voltammetry applied to platinum electrodes can be used to detect young biofilms. The development of the shape of the voltammogram as the potential is cycled may constitute a means of providing information on the coverage of the surface. Observation of the platinum electrodes before and after the electrochemical measurements showed that even after 30 min of recycling, most of the cells were still adhered to the surface, although some may have lost viability.

Further studies of the reduction of styrene in toluene/N,N-dimethylformamide mixtures

Abstract We report further studies in order to interpret the shapes of the steady state, reduction wave for styrene in toluene/DMF mixtures at a 6-μm radius gold microdisc electrode. The viscosities and conductivities of the solutions of Bu4NBF4 in the solvent mixtures have been determined. The voltammograms are corrected for IR drop and simulations have been carried out in order to confirm that the drawn out wave could result from slow electron transfer kinetics in an ece mechanism. In addition, some experiments have been carried out with 1,1-diphenylethylene and the results for the two olefins are compared.

Effect of the medium composition on the current of steady state voltammograms of neutral and charged species in dimethylformamide/toluene mixtures

Abstract Steady state voltammetric currents of both neutral and charged species obtained at microelectrodes are affected by the medium composition. Experiments carried out in N , N -dimethylformamide (DMF) and toluene/DMF mixtures for the reduction of dicyano(fluoren-9-ylidene)methane (DCN) and the methylviologen cation (MV 2+ ) and for the oxidation of ferrocene (Fc), in the presence of different electrolyte concentrations indicate that both viscosity of the solution and mass transport by migration can have a strong effect on the measured limiting currents. These observations are particularly important in electroanalysis since the appropriate choice of the medium may substantially improve both sensitivity and detection limit of the electrochemical method.

The application of microelectrodes to the study of the methoxylation of furan

Abstract Steady state microelectrode techniques are used to investigate the bromide mediated, anodic oxidation of furan in methanol, a reaction of considerable synthetic interest. It is confirmed that the reaction occurs via a rate determining step between bromine and the furan. The dependence of the steady state limiting current for the Br − /Br 2 reaction on the radius of the microdisc electrode is analysed to obtain a rate constant of 68 dm 3 mol −1 s − for the reaction of bromine with furan.

The cathodic cleavage of the benzyloxycarbonyl group from aliphatic amines in dimethylformamide

Abstract It is shown that the reduction of the benzyloxycarbonyl derivatives of n-butylamine, cyclohexylamine, piperidine and morpholine at vitreous carbon cathodes in DMF only occurs at very negative potentials, approx. −2.8 V vs. SCE, but the reaction leads to cleavage and hence to toluene and free amines in good yields ( > 80%). Cyclic voltammetry and controlled potential electrolysis have been used to probe the mechanism of these reductions both in the aprotic medium and in the presence of low concentrations of proton donor. The behaviour of protected primary and secondary amines is shown to differ slightly and the conclusions of the study are used to comment on the mechanism of deprotection of amino acids and peptides.

Electrochemical cleavage of some protecting groups from the sulphydryl function in aprotic solvents

Abstract It has been confirmed that both the benzyl and 4-picolyl groups may be cleaved from thioethers at a vitreous carbon cathode in N , N -dimethylformamide. The yield of thiol is good (> 80%). In contrast, the reduction of 4-nitrobenzyl thioethers does not lead to simple cleavage of the C-S bond.