Radical vs Anionic Pathway in Mediated Electrochemical Reduction of Benzyl Bromide in a Bicontinuous Microemulsion

Abstract

Products derived from benzyl radicals or anions were obtained by reduction of benzyl bromide mediated by vitamin B12, Co(salen), and cobalt phthalocyaninetetrasulfonate in a bicontinuous microemulsion and homogeneous solvent. The reactions begin by electrochemical generation of a Co(I) complex (CoIL), followed by oxidative addition of benzyl bromide to give benzyl−CoIIIL. Reductive cleavage of the Co−C bond yields benzyl radicals or anions depending on the potential of reduction of benzyl−CoIIIL. Rate constants of Co−C bond formation (k 1) are correlated with the formal potential E°‘Co(II)/Co(I), indicative of control by the inherent activation free energy of the oxidative addition rather than by reactant distribution between phases in the microemulsion. Vitamin B12 gave a benzyl−CoIIIL intermediate reduced at −1.1 V vs SCE and yielded bibenzyl as the sole product of a radical pathway. The reduction potential of benzyl−CoIII(salen) is negative enough to reduce the benzyl radical to an anion, so reduction of benzyl bromide mediated by Co(salen) gave toluene in the microemulsion and a mixture of toluene and bibenzyl in DMF. Although Co(salen) reacts very rapidly with benzyl bromide, a slow rate of reductive cleavage of benzyl−CoIII(salen) creates a bottleneck in the catalytic pathway. The one-electron catalyst vitamin B12, with a smaller rate of oxidative addition, gives faster catalytic reduction for a given k 1 relative to the two-electron catalyst Co(salen). Thus, a radical or anionic pathway can be chosen by controlling the potential of the benzyl−CoIII reduction. The facile formation of bibenzyl in the microemulsion suggests that it should be applicable to electro-organic syntheses featuring radical-based formation of carbon−carbon bonds.

Green synthesis via electrolysis in microemulsions

“…3) was found, with the same line describing reactions in DDAB microemulsions and in organic solvents. A similar linear plot was found for the catalytic reductions of benzyl bromide and alkyl iodides in microemulsions and organic solvents [19,20]. These linear plots indicate that the bimolecular reactions in the microemulsions are controlled by the intrinsic activation free energy of the reaction as controlled by E°′ of the catalyst, rather than by partition of reactants between oil and water phases.…”

“…Also, coadsorption of reactants and catalysts into an adsorbed surfactant layer on an electrode in microemul- sions can lead to high concentrations in a restricted reaction volume that can enhance rates of bimolecular reactions. The carbon-carbon bond-forming catalysis shown above features two chemical steps following the electrochemical formation of the active form of the catalyst, Co I L. While Co(salen) reacts much faster with benzyl bromide than the cobalt corrin vitamin B 12 , the rate of formation of dimeric products was still larger for vitamin B 12 [20]. This reflects the weakness of the alkylcobalt bond of vitamin B 12 , and suggests that breaking this bond is the rate-determining step for carbon-carbon bond formation.…”

Assembly of Electroactive Ordered Multilayer Films of Cobalt Phthalocyanine Tetrasulfonate and Polycations

“…4a) with average midpoint potential of Ϫ0.28 V vs SCE at pH 5. These peaks appear at similar potentials to those of the Co(II)/Co(I) redox couple of Co II PcTS 4Ϫ dissolved in dimethylformamide (E 0 ϭ Ϫ0.32 V) and in a microemulsion (Ϫ0.25 V) (10,11). Thus, the CV results suggest that the catalytically active Co(I) form of the complex (10) can be produced reversibly in these films.…”

“…Association of metal phthalocyanine tetrasulfonates within the film was important for film stability. Cobalt phthalocyanine tetrasulfonate (Scheme 1) is a complex suitable for electrochemical catalysis of important carbon-bond forming reactions (10,11). We felt that association might also be a stabilizing factor in a layer-by-layer film growth technique we have been exploring for ultrathin electrode coatings.…”

Synthesis and spectral analysis of N-substituted pyrrole salicylaldehyde derivatives-electropolymerization of a new nickel(II)-Schiff base complex derived from 6-[3′-N-pyrrolpropoxy]-2-hydroxyacetophenone and 1,2-diaminoethane

“…On the other hand, the direct electrochemical reduction of halogenated benzene derivatives has been the subject of a modest number of publications, such as with bromobenzene [48], benzyl bromide [49], benzyl chloride [50], and for the reduction of benzotrichloride [51]. Thus, it seems that there has been no previous study of the catalytic reduction of alkyl halides using nickel (I)-salen species contained in polypyrrole matrices.…”