Maria José Medeiros

Electroreductive cyclisation of unsaturated halides catalysed by nickel macrocyclic complexes

Abstract The indirect electroreduction of unsaturated aliphatic halides using Ni(II) complexes as electron-transfer mediators is achieved in N,N′ -dimethylformamide by constant current electrolysis between a cathode and a sacrificial anode in a diaphragmless cell. Cyclic compounds are obtained in low to high yields in conditions that avoid the usual preparation using organotin reagents.

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.

Intramolecular reductive cyclisations using electrochemistry: development of environmentally friendly synthetic methodologies

This review summarises electrochemical reductive intramolecular cyclisations, including transition-metal catalysed reactions. It presents some selected examples of organic halide electroreductions with further intramolecular coupling reactions, carbonyl group reductions with further coupling and intramolecular cyclisations involving electrogenerated bases.

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.

Electrochemical intramolecular cyclisation of propargyl bromoethers catalysed by nickel complexes

The electrochemical intramolecular cyclisation of propargyl 2-bromoethers 1–3 in N,N′-dimethylformamide at constant current in a diaphragmless cell has been developed using NiII complexes as electron-transfer mediators. During controlled-current electrolyses of solutions of NiII complexes in the presence of bromoethers, catalytic reduction of the latter proceeds via one-electron cleavage of the carbon–bromine bond to form a radical intermediate that undergoes cyclisation to afford the cyclic compounds in moderate to good yields.

Reductive cyclisation of propargyloxy and allyloxy α-bromoester derivatives using environmentally friendly electrochemical methodologies

The electrochemical intramolecular cyclisation of α-bromo propargyloxy (1 and 2) and allyloxy (3) esters was carried out in ethanol and in ethanol–water mixtures as environmentally friendly systems using Ni(II) complexes as the catalysts. The reduction of the substrates proceeds via one-electron cleavage of the carbon–bromine bond to form a radical-type intermediate that undergoes cyclisation to afford cyclic ethers in moderate to good yields.

Mechanism of the electrochemical oxidation of zero valent palladium complexes

Abstract Within long times scales, the electrochemical oxidation of Pd(0)(PPh3)4 proceeds through an ECE-like two-electron process to afford two cationic palladium(II) complexes in equilibrium: Pd(II)(PPh3)2(BF4)2 and Pd(II)(PPh3)3(BF4)2, whose relative concentrations depend on the triphenylphosphine concentration. At short time scales, one-electron oxidation affords a transient cationic palladium(I) complex involved in a fast follow-up endergonic equilibrium which is believed to consist in a geometrical rearrangement (tetrahedral to square planar), most probably induced by the solvent.

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.

Structural, morphological, ionic conductivity, and thermal properties of pectin-based polymer electrolytes

ABSTRACT New polymer electrolytes (PEs), potentially interesting for solid-state electrochemical devices applications, were synthesized by a solvent casting method using pectin and ionic liquid (IL) N,N,N-trimethyl-N-(2-hydroxyethyl)ammonium bis(trifluoromethylsulfonyl)imide ([N1 1 1 2(OH)] [NTf2]. The resulting electrolytes besides being moderately homogenous and thermally stable below 155°C, they also exhibited good mechanical properties. The SPE membranes were analyzed by differential scanning calorimetry (DSC), X-ray diffraction (XRD), scanning electron microscopy (SEM), atomic force microscopy (AFM), and complex impedance spectroscopy.