Electrochemical reoxidation

For enzymatic reductions with NAD(P)H-dependent enzymes, the electrochemical regeneration of NAD(P)H always has to be performed by indirect electrochemical methods. Direct electrochemical reduction, which requires high overpotentials, in all cases leads to varying amounts of enzymatically inactive NAD-dimers generated due to the one-electron transfer reaction. One rather complex attempt to circumvent this problem is the combination of the NAD+ reduction by electrogenerated and regenerated potassium amalgam with the electrochemical reoxidation of the enzymatically inactive species, mainly NAD dimers, back to NAD+ [51]. If one-electron... 

Pd(II)-catalyzed dialkoxy- or dihydroxycarbonylation of alkynes (Eq. 22, R = alkyl or H, respectively) with formation of maleic and fumaric esters or acids (and, in the case of acetylene, of muconic esters too), has been reported to occur in the presence of CuCl2 and/or 02 as oxidant [73-79] electrochemical reoxidation of Pd(0) has also been described [80]. 

An important drawback of the original Wacker process is the highly corrosive nature of the aqueous acidic PdCl2—CuCl2 system. Attempts were made to apply electrochemical reoxidation of palladium,513,514 and to use other oxidants,495 such as Fe(III) salts, Mn02, quinones,514-516 peroxides,517,518 and more recently, heteropoly acids.516,519"522... 

For the enantioselective dehydrogenation of the ( -enantiomer from a racemic mixture of 3-hydroxybutyrate three different methods were used to regenerate NADP (Table 27) (83). The first method worked with catalytic concentrations of NADP and anthraquinone-2.6-disulphonate. NADPH was reoxidized by oxidized anthraquinone-2.6-disulphonate catalysed by AMAPOR (Reaction [31a]). The reduced anthraquinone-2.6-disulphonate was electrochemically reoxidized. In experiment 2 a method described by the Whitesides group (106) was applied for NADP regeneration. Oxidized anthra-quinone-2.6-disulphonate could also be used in stoichiometric concentrations (exp. 3). In such a case its electrochemical reoxidation was not necessary. Obviously in experiment 1 the product isolation is especially simple. 

Extraction at +0.30 V turns the electrode transparent. As seen in Fig. 2, the electrode returns to the original state as expected from a reversible insertion reaction. The cycled electrode seems to be more oxidized since the low binding energy states in the core level spectrum and the bandgap states are now almost absent. The width of the W 4f core levels of an electrochemically reoxidized electrode is now 0.95 eV, considerably less than for an ion-inserted electrode. 

The oxidation of naphthalene was carried out with chromic acid and chromates, regenerated by electrochemical reoxidation. This method was first used by the Farbwerke Hoechst. In 1891, it was accidentally discovered at BASF haX naphthalene could be oxidized by concentrated sulfuric add in the presence of mercury. In 1897, introduced the first synthetic indigo to the market, followed shortly thereafter by Farbwerke Hoechst, Synthetic indigo rapidly replaced the natural product in the market in spite of harsh competition of the producers of natural indigo, especially of the Provence/France. 

FIGURE 2.14 SEM images of platinum deposited on (lOO)MgO. (a) Initial platinum substrate. (b) After electrochemical modification in 0.1 mol.L" Nal in DMF, reduction with a charge density of 24 mC.cm and then contact with air. (c) After reduction followed by an electrochemical reoxidation. (Ghilane, J., M. Guilloux-Viry, C. Lagrost, P. Hapiot, and J. Simonet, 2005, J. Phys. Chem. B 109 14925. Used with permission.)... 

The arrest of the oxidation of QHpOq is located at less positive potentials in the sequence platinum, platinum-ruthenium (36 % Ru) and platinum-rhenium (8 % Re). Curves a and d were taken with 2mA/cm, the other curves with 5mA/cm. The upper curves were obtained in the absence of CgHpOq. The appearance of the oxidation region on platinum-rhenium at less positive potentials was attributed [72] to the oxidation of the chemisorbed species by a rhenium oxide and the electrochemical reoxidation of rhenium. 

From these investigations, during the electrochemical studies, was been possible to value the applicability of the palladium electrochemical reoxidation to develop a system, that not use conventional chemistry to report palladium zero in oxidation form. A selective and enviroiunent friendly methodology, based on the use of electrochemistry and palladium catalysts, for fine chemical preparation is here reported. 

The electrochemical reoxidation of Pd(0) to Pd(II) was applied to the efficient synthesis of N,N -disubstituted meas, starting from amines and carbon monoxide. The advantage the anodic recycling at a graphite electrode of Pd(II) is that it proceeds efficiently under atmospheric pressure of carbon monoxide, avoids the use of copper or halide ions and high pressure of O2 gas which also implies the formation of water and causes undesired side reactions this way, the performances of the catalysts should be enhanced. 

---