Electrocarboxylation

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. ... 

Complex (16), which has a similar structure to Co11 salen, catalyzes the electrocarboxylation of arylmethyl chlorides.274 The enhancement of the catalytic life of (16) as compared to Co-salen may be due to the absence of imino bond in its ligand. The catalytic reduction of halogenated compounds has also been attempted at poly[Mn(salen)]-coated electrodes (M = Ni,253 Co275), which might have potential use for determination of organohalide pollutants.275... 

Electrocarboxylation is carried out when C02 is used as electrophile offering an interesting alternative to organometallic synthesis. A prerequisite of this type of electroreduction in industrial scale is electrolytic cells especially adapted to use aprotic solvents. These cells must fullfil the following requirements [148,149] ... 

Electrochemistry offers new routes to the production of several commercially relevant a-arylpropionic acids, used as non-steroidal anti-inflammatory agents (NSAI) [178,182]. A preparative method based on sacrificial Al-electrodes has been set up for the electrocarboxylation of ketones [117,183-187] and successfully applied to the electrocarboxylation of aldehydes, which failed with conventional systems. The electrocarboxylation of 6-methoxy-acetonaphthone to 2-hydroxy-2-(6-methoxynaphthyl)propionic acid, followed by chemical hydrogenation to 2-(6-methoxynaphthyl)-2-propionic acid - one of the most active NSAI acids - has been developed up to the pilot stage [184,186],... 

The electrocarboxylation of aryl iodides or bromides can also be catalyzed by the Pd-PPhs system (Eq. 14) [101]. 

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. 

The direct carboxylation of unsaturated compounds has already been reported [107,108]. The process is however limited to reducible alkenes like aryl olefins or activated olefins. On the other hand, only a few examples of direct electrocarboxylation of alkynes have been reported [109],... 

Diynes have already been used for building polycylic compounds in the presence of CO2 and a stoichiometric amount of Ni(0) bicyclic pyrones were obtained [117]. With the electrocarboxylation method, linear or cyclic mono-carboxylic acids were obtained as main products from non-conjugated diynes depending on the ligand associated to Ni [118, 119]. Thus ring-fomation occurred with the Ni-bipyridine complex at normal pressure of CO2 on the other hand, in the presence of PMDTA as ligand and with a 5 atmosphere pressure of CO2, linear adducts were mainly formed as illustrated in Eq. (16) ... 

The electrocarboxylation of 1,3-diynes in the presence of Ni-PMDTA as catalyst led to ( )-2-vinylidene-3-yne carboxylic acids regio- and stereo-selec-tively [119,120]. Yields referred to a 70-100% conversion are good (Table 18). 

From a mechanistic perspective, the best understood system for C—CO2 bond formation is the catalytic electrocarboxylation of bromoarenes in the presence of a transition metal catalyst, reaction (11) [84-87]. This reaction is selective and occurs at room temperature and 1 atm of CO2. 

In general, electrocarboxylation reactions are carried out in aprotic solvents such as acetonitrile (ACN), N,N-dimethylformamide (DMF) or N-methyl-2-pyrrolidone (NMP) in a one-compartment cell by the use of sacrificial anodes (Al or Mg), as the use of these systems generally provide important advantages [7, 10-12] that include ... 

Recently, the role of metal cations has been studied in detail [13, 14], it having been observed that when A1 or Mg cations are added to the reaction medium, they have a strong influence on the reduction process of alkynes, ketones and halides, and in most cases this leads to a complete change of the mechanism. In contrast, when ions are supplied to the reaction medium by oxidation of the anode during electrocarboxylation, they show no effect on the cathodic process. Hence, it appears that metal cations produced by the consumable anode in undivided cells are intercepted and complexed by the carboxylated products before they can reach the cathode. Therefore, the cathodic process can occur in a medium without free metal ions. 

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

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]. 

Simple kinetic models for the direct and homogeneous electrocarboxylation of benzyl halides were recently developed based on competitions at two successive stages involving R and R, respectively, and on the competition between reduction processes involving C02 and RX. Interestingly, the experimental data obtained were in satisfactory agreement with theoretical predictions, both in the case of the direct process [33] (in spite of the fact that no adjustable parameters were used in the model) and of the process mediated by a HCTC [21]. 

Recently, the electrocarboxylations of benzyl and aryl halides and perfluoroalkyl-halides [39] in supercritical mixture or in supercritical carbon dioxide (scC02) and of aryl and benzyl halides in microemulsion [40], were also investigated in order to exploit the possible effect of the use of these solvents on the selectivity of the... 

The electrocarboxylation of aldehydes and ketones leads to the corresponding a-hydroxycarboxylic acids that can easily be converted into carboxylic acids via a hydrogenation reaction [7]. It has been reported that the electrocarboxylation of aromatic ketones occurs through the reaction of C02 onto the activated carbon atom of the carbonyl group of the ketyl radical anion generated upon electron transfer to the ketone [7]. Otherwise, the aforementioned intermediate is likely to be a resonance hybrid (see Equation 12.23), and its electrophilic reaction with C02 may take place both at the carbon or the oxygen atom [42, 43]. 

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