Electrocarboxylation of Organic Halides

The electrocarboxylation of organic halides ideally involves the following reaction scheme [21]  

Presumably, this is also true for the coordination of R , so that this strong base is kinetically stabilized and can wait to react almost exclusively with C02 instead of being partially protonated in the medium. 

As noted above, the electrocarboxylation process has been widely investigated for the production of some NSAIDs, starting from parent arylethyl halides [6, 7]. On the other hand, the direct process at conventional cathodes requires quite negative potentials, and gives rise in some cases to moderate faradic efficiencies. Furthermore, attempts to scale-up the process in the case of the reduction of l-(3-phenoxyphenyl)-l-chloroethane [18] and l-(4-isobutylphenyl)-l-chloroethane [16] (which are the precursors of fenopren and ibuprofen, respectively) gave very different results with respect to those obtained in syntheses performed in bench-scale systems. In particular, passivation of the cathode surface was observed, and this resulted in lower yields and selectivities. Similar results were also observed during the electrocarboxylation of chloroacetonitrile [19]. 

In the first case, for example, with NiCl2(dppe) as the catalyst, C02 oxidatively adds to the organometal(I) center to yield R-MeIII(C02), which gives rise to the formation of a carboxylate anion and of a metal(I) species that is reduced back to Me0 so that a new cycle can start (Equations 12.14-12.16)  

In the second case, for example, with PdCl2(PPh3)2 as the catalyst, RMe° gives rise to Me11 which is reduced to the zerovalent metal species (Equation 12.10) and to the anion R (Equation 12.17) which reacts with C02 (Equation 12.4) [21], 

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The catalyst is not necessary either for the electrocarboxylation of aryl halides or various benzylic compounds when conducted in undivided cells and in the presence of a sacrificial anode of aluminum [105] or magnesium [8,106], Nevertheless both methods, i.e., catalysis and sacrificial anode, can be eventually associated in order to perform the electrocarboxylation of organic halides having functional groups which are not compatible with a direct electroreductive process. 

Our synthetic method for CO2 fixation was based on the use of transition-metal catalysis combined with electrochemical techniques [10]. Within this methodology, the electrochemical CO2 fixation into some alkenes has been reported to afford carbo lic acids in a reductive hydrocarboxylation-tjqje reaction catalyzed by nickel complexes, under mild conditions [11]. The electrocarboxylation of organic halides to the corresponding carboxylic acids has also been reported [12], yields and efficiency of the reaction being strongly dependent on the reaction conditions. 

Electroassisted Barbier Reactions 1985 and 1986. More recently a new and efficient method was reported for the electrocarboxylation of organic halides, using an undivided electrolytic cell and a sacrificial magnesium electrode [101]. 

Co(TPP) has been demonstrated to act as a catalyst for the electrocarboxylation of benzyl chloride and butyl bromide with CO - to give PhCHiCfOiOCH Ph and Bu0C(0)C(0)0Bu, respectively. The propo.sed mechanism involved Co(TPP)R and [Co(TPP-N-R) as intermediates (the latter detected by spectroscopy) in the catalytic production of free R or R-, which then reacted directly with Co(TPP) precipitated on graphite foil has been successfully used for the determination of organic halides, including DDT and 1,2,3,4,5,6-hexachlorocyclohexane (lindane), to sub-ppm level in aqueous solution. Deoxygenation of the solutions is not required, and the technique is moderately insensitive to the ionic composition of the solution. ... 

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