Electrochemical cross-coupling reaction

Electrochemical cross-coupling reaction from functionalized organic halides requires generally catalytic amounts of a transition metal. This transition metal is most often nickel. The success of these reactions is due to the formation and the reactivity of an organometallic intermediate formed in situ by electrochemistry. To the best of our knowledge, these compounds have not been yet isolated and characterized. 

Much work has been directed towards the synthesis of thiophene oligomers and polymers. This is due to the current interest in research on conducting polymers and molecular electronics (92CRV711). Two main approaches have been used for making such polymers (i) chemical (e.g. FeCl3) or electrochemical oxidation of monomeric thiophenes and (ii) transition metal-catalyzed cross-coupling reactions. 

A, A -dimethylaniline group has been synthesized by a copper-free Sonogashira cross-coupling reaction using microwave irradiation as the source of energy <2006EJO2344>. The electrochemical and photophysical properties of the triad were systematically investigated by techniques such as time-resolved fluorescence and transient absorption spectroscopy. 

The first studies that intentionally used colloidal nanocatalysts were reported independently by Beller et al. [50] and Reetz et al. [51] using chemical reduction and electrochemical techniques, respectively, to synthesize colloidal palladium nanoparticles for the Heck reaction. Both Beller and Reetz concluded that the solution-phase catalysis occurred on the surface of the nanoparticle, without confirming that a homogeneous catalytic pathway was nonexistent. Le Bars et al. [52] demonstrated an inverse relationship between the size of Pd nanoparticles and the TOF (normalized to the total number of surface atoms) for the Heck reaction (Fig. 18.4a). After normalizing the rate to the density of defect sites (for each nanoparticle size) (Fig. 18.4b), the TOF for all particle sizes was identical. Colloidal PVP-capped palladium nanoparticles synthesized by ethanol reduction are effective catalysts for Suzuki cross-coupling reactions in aqueous solution [53]. The El-Sayed group reported that the initial rate of reaction increased linearly with the concentration of Pd nanoparticles [53] and the catalytic activity was inversely proportional to the... 

Cross-coupling reactions of two carboxylates with different alkyl groups by anodic decarboxylation (mixed Kolbe electrolysis) is an electrochemical method that allows the synthesis of unsymmetrical compounds (Scheme 7). 

The electrochemical reductive coupling reaction of various aikenes with benzyl bromides can also been achieved in the absence of supporting electrolyte using the microflow cell (Figure 7.20) [lOlj. When the inter-electrode gap is 160 pm, the desired cross coupling product is obtained effectively, whereas a significant amount of homocoupling product is obtained when the gap is 320 pm. 

On these grounds, a synthetic methodology for the preparation of oligothiophene-5, -dioxides was developed based on the Stille cross-coupling reaction of thienylstannanes with bromo-substituted thienyl-5, 5 -dioxide units in the presence of Pd(0) catalysts [13], Subsequently, copolymers of thiophene-5, S -dioxide with thiophene were prepared both electrochemically and by action of ferric chloride [14, 15]. 

Synthesis of Polyynes. Conjugated diyne and polyyne units, which exhibit unusual electrochemical, optical, and structural properties, can be efficiently constructed from 1-haloalkynes via homologation by one acetylene unit with TIPS-acetylene through a transition metal-catalyzed cross-coupling reaction, 2 which allows an access to a new class of polyyne framework. 

Then, the use of nickel(II) or cobalt(II) complexes as catalyst associated to the sacrificial anode process allows synthesis of functionalized mono- or diorganozinc species in a simple and efficient manner. Alternating Jt-conjugated copolymers, based on this electrochemical preparation of intermediate aryldizinc species and their subsequent palladium-catalyzed coupling with unsaturated dihalogenated compounds, can be synthesized. Furthermore, aromatic ketones are synthesized efficiently via cobalt-catalyzed cross-coupling reaction between arylzinc bromides and acid chlorides. 

Reductive Cross-Coupling of Nitrones Recently, reductive coupling of nitrones with various cyclic and acyclic ketones has been carried out electrochem-ically with a tin electrode in 2-propanol (527-529). The reaction mechanism is supposed to include the initial formation of a ketyl radical anion (294), resulting from a single electron transfer (SET) process, with its successive addition to the C=N nitrone bond (Scheme 2.112) (Table 2.9). 

Cross coupling between an aryl halide and an activated alkyl halide, catalysed by the nickel system, is achieved by controlling the rate of addition of the alkyl halide to the reaction mixture. When the aryl halide is present in excess, it reacts preferentially with the Ni(o) intermediate whereas the Ni(l) intermediate reacts more rapidly with an activated alkyl halide. Thus continuous slow addition of the alkyl halide to the electrochemical cell already charged with the aryl halide ensures that the alkyl-aryl coupled compound becomes the major product. Activated alkyl halides include benzyl chloride, a-chloroketones, a-chloroesters and amides, a-chloro-nitriles and vinyl chlorides [202, 203, 204], Asymmetric induction during the coupling step occurs with over 90 % distereomeric excess from reactions with amides such as 62, derived from enantiomerically pure (-)-ephedrine, even when 62 is a mixture of diastereoisomcrs prepared from a racemic a-chloroacid. Metiha-nolysis of the amide product affords the chiral ester 63 and chiral ephedrine is recoverable [205]. 

Becking and Schafer have shown that mixed Kolbe coupling reactions can provide useful yields (40-60%) of cyclic products.142 In the example provided in equation (4), 1 equiv. of acid (51) and 4 equiv. of acid (52) are electrochemically cooxidized, and the cyclic cross adduct (53) is formed in 53% yield. Because the rates of oxidation of (51) and (52) are similar, the concentration of radicals derived from (52) is higher. Thus, radicals derived from (51) are more likely to cross couple than to self couple. The strength of the mixed Kolbe method is that two carbon-carbon bonds are formed rather than one because the cyclic radical is removed by radical/radical coupling. 

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