Electroreductive coupling, metal complex

The complex [Ni(bpy)2]2+ catalyzes the electroreductive coupling of organic halides and carbon monoxide into ketones under a CO atmosphere,226 or in the presence of a metal carbonyl,227 especially iron pentacarbonyl. Unsymmetrical ketones have been obtained from mixtures of two different organic halides.228 CO is very reactive towards reduced Ni° species to form the stable [Ni°(bpy)(CO)2]° complex, which probably evolves to a transient arylnickel [Nin(bpy)(R)(CO)X]° complex in the presence of both ArX and [Ni°(bpy)]° species.229,230... 

Organic electroreductive coupling reactions using transition-metal complexes as catalysts have been widely investigated. Reviews on the subject have been published [89, 90]. The method involving the most common transition-metal complexes (nickel, cobalt, palladium) appears to be a useful tool to synthetize heterocycles from organic halides via radical intermediates. Nickel catalyst precursors are nickel(II) salts that are cathodically reduced either to nickel(I) or to nickel(O) and cobalt catalyst... 

Organic Electroreductive Coupling Reactions Using Transition Metal Complexes as Catalysts... 

Organic Electroreductive Coupling Reactions using Transition Metal Complexes as Catalysts Table 2. Reductive electropolymerisation of aryl dihalides using nickel catalysts... 

Alternatively, CO2 can be used as source of CO. Indeed, it is well known that low-valent transition metal complexes can catalyze the chemical or electrochemical reduction of CO2 into CO. This approach was used to generate the mixed nickel complex Ni°bpy(CO)2 by the electrochemical reduction of Nibpy in NMP or DMF in the presence of CO2. The reduced complex can react with alkyl, benzyl, and allylhalides to give the symmetrical ketone along with the regeneration of Nibpy ". A two-step method alternating electroreduction and chemical coupling leading to the ketone has thus been set up (Scheme 9) [126,127]. 

Nedelec, J.-Y., J. Perichon, and Troupel, M. Organic Electroreductive Coupling Reactions Using Transition Metal Complexes as Catalysts. 185, 141-174 (1997). 

N d lec J-Y,P richon J,Troupel M (1997) Organic Electroreductive Coupling Reactions Using Transition Metal Complexes as Catalysts. 185 141-174 Nicotra F (1997) Synthesis of C-Glycosides of Biological Interest. 187 55-83 Nishimura J,see Inokuma S (1994) 172 87-118 Nolte RJM,see Sijbesma RP (1995) 175 25-56 Nordahl A,see Carlson R (1993) 166 1-64... 

ELECTROREDUCTIVE COUPLING USING METAL COMPLEX CATALYSTS... 

Two aspects of porphyrin electrosynthesis will be discussed in this paper. The first is the use of controlled potential electroreduction to produce metal-carbon a-bonded porphyrins of rhodium and cobalt. This electrosynthetic method is more selective than conventional chemical synthetic methods for rhodium and cobalt metal-carbon complexes and, when coupled with cyclic voltammetry, can be used to determine the various reaction pathways involved in the synthesis. The electrosynthetic method can also lead to a simultaneous or stepwise formation of different products and several examples of this will be presented. 

Equation (1) describes the chemisorption of O2 on a surface site A of a metal (Ma) in an acid medium, where coupled to a proton and an electron transfer leads to the formation of an adsorbed end-on complex HOO-Ma. The unstable intermediate subsequently dissociates into two adsorbed species, one adsorbing on A sites, 0-Ma and the OH species adsorbing on B sites, HO-Mb (Eq. 2). In the rest of the electroreduction steps, represented by Eq. (3), adsorbed O and OH are reduced to H2O and the water molecules are eventually desorbed from the metal surface. Actually, Eqs. (1)-(3) can also be used to interpret the ORR activity for Pt-skin surfaces. The electronic sfructures of surface Pt atoms are not identical due to the existence of 3d metal in the sublayers. Ma and Mb can be looked as two Pt surface sites with different activities for reactions (l)-(3). Ma site possess better performance for the formation of the OOH complex, and Mb site may enhance the dissociation of OOH. The overall ORR is thus facilitated by the skin sfructures. 

Additional alterations in the work terms with the electrode material for outer-sphere reactions may arise from discreteness-of-charge effects or from differences in the nature of the reactant-solvent interactions in the bulk solution and at the reaction plane. Thus metals that strongly chemisorb inner-layer solvent (e.g., HjO at Pt) also may alter the solvent structure in the vicinity of the outer plane, thereby influencing k bs variations in the stability of the outer-sphere precursor (and successor) states. Such an effect has been invoked to explain the substantial decreases (up to ca. 10 -fold) in the rate constants for some transition-metal aquo couples seen when changing the electrode materiaf from Hg to more hydrophilic metals such as Pt. Much milder substrate effects are observed for the electroreduction of more weakly solvated ammine complexes . 

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