Reactions radicalaires dans des milieux protiques et aprotiques : comparaisons entre les cyclisations reductives electrochimiques catalysees par des complexes du nickel

The electrochemical reductive cyclisation of unsaturated organic halides in the presence of Ni(II) complexes as catalysts was examined in aprotic solvents such as DMF and in protic solvents such as ethanol, butanol or ethanolewater mixtures. The presence of the alcohol media enhanced the rate of recycling of the catalytic species.La cyclisation réductive électrochimique d’halogénures insaturés en présence de complexes de Ni(II) comme catalyseurs a été examinée dans des solvants comme le DMF et dans des solvants protiques comme l’éthanol, le butanol ou des mélanges éthanol-eau. La présence du milieu alcoolique augmente la vitesse du recyclage des espèces catalytiques.Fundação para a Ciência e a Tecnologia (FCT

Tetraphenylborate Anion as a Phenylating Agent: Chemical and Electrochemical Reactivity of BPh4--Rh Complexes toward Mono- and Dienes and Carbon Dioxide

A few aspects of the chemical and electrochemical reactivity of mono-, di-, and trinuclear η6-tertraphenylborate-Rh complexes [(C2H4)2Rh(η6-PhBPh3) (1), {[(C2H4)2Rh(η6-Ph)]2BPh2}O3SCF3 (2), and {[(C2H4)2Rh(η6-Ph)]3BPh}(O3SCF3)2 (3)] are described. The factors governing the phenyl transfer reaction from coordinated BPh4- anion to monoenes and dienes are highlighted. Phenyl transfer to ethylene affords, among other products, styrene and ethylbenzene; styrene is converted into trans-stilbene. Complexes 1 and 2 react with isoprene affording 1,1-dimethylindene and 1,2-dihydro-2-methyl- and 1,2-dihydro-3-methylnaphthalene, species formally involving phenyl transfer from B to coordinated isoprene and intramolecular cyclization of the resulting intermediate. The reactivity of 1-3 toward CO2 was studied and compared with that of (diphos)Rh(η6-PhBPh3). Using complexes 1-3, coordinated ethylene competes with CO2 toward coupling with phenyl. The electrochemical behavior of the dinuclear complex, 2, was studied. It undergoes two consecutive irreversible 1-electron reductions at -1.53 and -1.74 V vs. SCE, respectively Analogous experiments carried out in the presence of CO2 have clearly shown that the reduced species are both able to react catalytically with the heterocumulene under controlled-potential conditions (-1.5 and -1.8 V). CO2 was coupled with both ethylene and Ph affording propionic acid and HOBz. The former species was the only product under an ethylene/carbon dioxide atmosphere