Electrochemical enantioselective

Scheme 55 Electrochemical enantioselective Sharpless dihydroxylation of alkenes.

There has been a trend toward electrochemical reactions in lab-on-a-chip devices in the last few years.51,52 This is mainly because miniaturized electrodes can be fabricated using microfabrication methods and solutions can be transferred by microfluidics approaches.5354 Flow injection analysis and sequential injection analysis techniques were also employed for electrochemical enantioselective high-throughput screening of drugs.55... 

There are several methods for accomplishing electrochemical enantioselective reactions with the use of chiral inductors. They are the use of... 

Prasad, B., Kumar, D., Madhuri, R., and Tiwari, M. P. (2011). Metal ion mediated imprinting for electrochemical enantioselective sensing of 1-histidine at trace level. Biosens. Bioelectron. 28,117-126. 

Any effective chiral solvent for electrochemical enantioselective reaction have not been found until now. 

Enantioselective electron transfer reactions are not possible in principle because the electron cannot possess chirality. Whenever the choice of enantiodifferentiation becomes apparent, it will occur in chemical steps subsequent (or prior) to electron transfer. Thus, enantioselectivities require a chiral environment in the reaction layer of electrochemical intermediates although asymmetric induction was report-... 

Several alkyl aryl sulfides were electrochemically oxidized into the corresponding chiral sulfoxides using poly(amino acid)-coated electrodes . Although the levels of enan-tioselection were quite variable, the best result involved t-butyl phenyl sulfoxide which was formed in 93% e.e. on a platinum electrode doubly coated with polypyrrole and poly(L-valine). Cyclodextrin-mediated m-chloroperbenzoic acid oxidation of sulfides proceeds with modest enantioselectivity . 

There is huge potential in the combination of biocatalysis and electrochemistry through reaction engineering as the linker. An example is a continuous electrochemical enzyme membrane reactor that showed a total turnover number of 260 000 for the enantioselective peroxidase catalyzed oxidation of a thioether into its sulfone by in situ cathodic generated hydrogen peroxide - much higher than achieved by conventional methods [52],... 

The electrochemical reduction of phenylglyoxylate oxime in the presence of strychnine afforded the corresponding optically active amines (equation 4). Also, the electrochemical reduction of oximes by utihzing poly-L-vahne-coated graphite electrode afforded optically active amine. However, in both cases the enantioselectivities were very low. 

Moderate enantioselectivity factors have also been found for electron transfer reactions between HRP or GO and resolved octahedral ruthenium or osmium complexes, respectively. In particular, the rate constants for the oxidation of GO(red) by electrochemically generated and enantiomers of [Os(4,4 - 2 ) ]3 + equal 1.68 x 106 and 2.34 x 106 M-1 s-1, respectively (25 °C, pH 7) (41). The spectral kinetic study of the HRP-catalyzed oxidation of and A isomers of the cyclo-ruthenated complex [Ru(phpy)(phen)2]PF6 (Pig. 21) by hydrogen peroxide has revealed similarities with the oxidation of planar chiral 2-methylferrocene carboxlic acid (211). In both cases the stereoseleci-vity factor is pH dependent and the highest factors are not observed at the highest rates. The kA/kA ratio for [Ru(phpy)(phen)2]PF6 is close to 1 at pH 5-6.5 but increases to 2.5 at pH around 8 (211). 

R.I. Stefan, H.Y. Aboul-Enein and J.F. van Staden. In H. Baltes, G.K. Fedder and G. Korvink (Eds.), Enantioselective Electrochemical Sensors in Sensors Update, Vol. 10, Wiley-VCH, Weinheim, Germany, 2001. 

The mechanism of the enantioselective 1,4-addition of Grignard reagents to a,j3-unsaturated carbonyl compounds promoted by copper complexes of chiral ferrocenyl diphosphines has been explored through kinetic, spectroscopic, and electrochemical analysis.86 On the basis of these studies, a structure of the active catalyst is proposed. The roles of the solvent, copper halide, and the Grignard reagent have been examined. 

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