Electrode chiral

In 1975, the fabrication of a chiral electrode by permanent attachment of amino acid residues to pendant groups on a graphite surface was reported At the same time, stimulated by the development of bonded phases on silica and aluminia surfaces the first example of derivatized metal surfaces for use as chemically modified electrodes was presented. A silanization technique was used for covalently binding redox species to hydroxy groups of SnOj or Pt surfaces. Before that time, some successful attemps to create electrode surfaces with deliberate chemical properties made use of specific adsorption techniques... 

A qualitatively new approach to the surface pretreatment of solid electrodes is their chemical modification, which means a controlled attachment of suitable redox-active molecules to the electrode surface. The anchored surface molecules act as charge mediators between the elctrode and a substance in the electrolyte. A great effort in this respect was triggered in 1975 when Miller et al. attached the optically active methylester of phenylalanine by covalent bonding to a carbon electrode via the surface oxygen functionalities (cf. Fig. 5.27). Thus prepared, so-called chiral electrode showed stereospecific reduction of 4-acetylpyridine and ethylph-enylglyoxylate (but the product actually contained only a slight excess of one enantiomer). 

Sulfoxides yielded have the ( -configuration. A detailed mechanism of the origin of asymmetric induction is unknown. These chiral electrodes are reusable without loss of enantioselectivity. This approach is synthetically promising, but preparation of coated electrodes is a delicate undertaking, making the method difficult in practice unless robust chiral electrodes become commercially available. 

Chiral Electrode Formation. This was described in Section 11.3. 

Very significantly, recent studies by Pleus and coworkers108 have revealed that the conducting films 5 and 6 may be employed as chiral electrodes for the enantioselec-tive electroreduction of acetophenone and 4-methylbenzophenone in DMF to their corresponding alcohols, using DMF/LiBr as electrolyte and phenol as the proton donor. The nature of the electrolyte used for electrodeposition of the polymers influenced the optical purity of the chiral alcohol products obtained with enantioselec-tivities as high as 58% being achieved. 

Leon Kane-Maguire, Ph.D., is currently a project leader in the ARC Centre of Excellence for Electromaterials Science at the University of Wollongong. His research interests are the synthesis of conducting organic polymers and characterizing their stereochemical and photophysical properties. A strong focus of his recent work has been the synthesis of chiral conducting polymers (especially polyanilines) and their use as novel chiral electrodes for electrochemical asymmetric synthesis. 

Enantioselective hydrogenation of prochiral aromatic ketones has been successfully achieved employing a chiral electrode covered with a polymer obtained by oxidative electropolymerization of a complex based on the 2,2 -bipyridyl 58 (composition [Rh(133)2Cl2] ) [186]. 

The catalytic asymmetric oxidation of prochiral sulfides by chemical means is a difficult task. While a number of workers have been active in this area during the past few years, few systems simultaneously show good induction of chirality and good catalytic activity. The most common catalysts involve transition-metal complexes (homogeneous or supported) as well as chiral electrodes. These approaches are described successively below. 

Papillon, J., E. Schulz, S. Gelinas, J. Lessars, and M. Lemaire. 1998. Towards the preparation of modified chiral electrodes for heterogeneous asymmetric catalysis Synthesis and electrochemical properties of (S,S)-5,5 -his-[3-(3-methyl-pentyl)-thiophen-2-yl]-[2,2 ]-hipyridine. Synth Met 96 155-160. 

SCHEME 1 Asymmetric electro-oxidation of a sulfide to a chiral sulfoxide using a chiral electrode. 

The incorporation of a transition metal within a chiral ECP film should make it possible to expand the application of chiral electrodes to enantioselective electrocatalytic reactions. Such an approach has been validated by Moutet and co-workers, who have electrosynthesized a functionalized PPy film from a rhodium(III) complex with a pendant chiral bipyridyl ligand bound to a pyrrole unit [219]. The efficiency of this material was demonstrated through the electrocatalytic hydrogenation of prochiral ketones. The 5-configurated corresponding alcohols were produced with current efficiencies of around 60% and with optical yields between 5% and 12%. However, enantioselectivity was found to be strongly dependent on the film thickness and the electropolymerization... 

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