Reduction potentials electrosynthesis

The electrosynthesis of metalloporphyrins which contain a metal-carbon a-bond is reviewed in this paper. The electron transfer mechanisms of a-bonded rhodium, cobalt, germanium, and silicon porphyrin complexes were also determined on the basis of voltammetric measurements and controlled-potential electrooxidation/reduction. The four described electrochemical systems demonstrate the versatility and selectivity of electrochemical methods for the synthesis and characterization of metal-carbon o-bonded metalloporphyrins. The reactions between rhodium and cobalt metalloporphyrins and the commonly used CH2CI2 is also discussed. 

TPP)Rh(L)J+C1 in the presence of an alkyl halide leads to a given (P)Rh(R) or (P)Rh(RX) complex. The yield was nearly quantitative (>80X) in most cases based on the rhodium porphyrin starting species. However, it should be noted that excess alkyl halide was used in Equation 3 in order to suppress the competing dimerization reaction shown in Equation 1. The ultimate (P)Rh(R) products generated by electrosynthesis were also characterized by H l MR, which demonstrated the formation of only one porphyrin product(lA). No reaction is observed between (P)Rh and aryl halides but this is expected from chemical reactivity studles(10,15). Table I also presents electronic absorption spectra and the reduction and oxidation potentials of the electrogenerated (P)Rh(R) complexes. 

The model had been substantiated by measuring the potential dependent electrosorption isotherms of all species involved that show that the proto-nated alcaloids are those species that are by far most strongly adsorbed, whereas the acetyl pyridines are least strongly adsorbed, especially at lower pH relative to the electrosynthesis, which is performed at pH 4 to 4.5. The optically inductive reduction of 2- and 4-acetyl pyridine to the optically active carbinols demands the formation of a dense but not too densely packed surface layer of adsorbed protonated alcaloid, which still allows for insertion of the oxo-compound or the ketyl radical, respectively. Performing the reaction with too high alcaloid concentrations leads to compaction on the adsorbate layer, the ketyl radical is squeezed out, and optimal induction is no longer observed. 

Apart from a change in the course of the reduction, which will be discussed below in the section on stereospecific electrosynthesis, the epimers of a,a -dibromosuccinic acid show differences in half-wave potentials (134,135). The free acid of the erythro dibromosuccinic acid is reduced at more positive potentials than the threo-ioxm. For the anions of these acids, however, the reduction at higher pH values occurs at more positive potentials for the threo-epimer than for the erythro-anion. For the esters of dibromosuccinic acids, the difference in half-wave potentials of the two epimers is too small to be significant. 

It is recommended that organic electrosynthesis be carried out at a constant current at first, since the setup and operation are simple. Then the product selectivity and yield can be improved by changing current density and the amoimt of electricity passed [current (A) x time (i) = electricity (C)]. However, the electrode potential changes with the consumption of the starting substrate (more positive in case of oxidation or more negative in case of reduction). Therefore the product selectivity and current efficiency sometimes decrease, particularly at the late stage of electrolysis. 

Figure 6. A) Polarography of complex B in DMF a) under N2 b) under O2 and in the presence of CF3(X)0H B) a) Coulombic yield in H2O2 electrosynthesis during controlled potential reduction at 0.0 V for complex B b) theorectical yield.

The potential advantages associated with an electrosynthesis include high material utilization and significantly less energy requirement, ease of control of the reaction, less hazardous process, and the ability to perform wide range of oxidation and reduction reactions. Therefore, many electrosynthetic reactions have been reported so far [1]. However, only a few have been employed industrially. The conunercializa-tion of electrosynthetic processes has been restricted by the limited solubility of substrates and products in conventimial electrolytic solutions, the poor interphase mass transport characteristics associated with two phase system in which the reaction occurs at solid (electrode)-liquid (electrolyte) interfaces, the low selectivity for desired reaction products, and the complex processing schemes often used to recover products. 

Electrosynthesis is one of the most interesting and yet underutilized methods of preparing coordination compounds. Electrochemical reactions make use of the universal "chemical reagent" — the electron. 1 The electrosynthesis of metal complexes can, in principle, give rise to a high selectivity, because it is possible to control electrode potentials over a wide range. Electrons can be removed from, or added to, a system without the complications associated with the presence of chemical oxidants and reductants and the by-products associated with their use. However, a new set of complications in the form of electrodes and supporting electrolytes must be dealt with. 

Complex hydrides, particularly those of boron and aluminum, often are employed as reductants in coordination chemistry. The use of boron and aluminum hydrides in this area is the subject of Chapter 10. The reduction of complexes at electrodes is a promising direction in the synthesis of coordination compounds. Electrosynthesis is characterized by high reaction selectivity because the electrode potential is controllable. The use of this method in preparative coordination chemistry is the subject of Chapter 7. The present Chapter is devoted to a discussion of other reductants useful as reagents in synthetic coordination chemistry. 

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