Supporting electrolytes chiral

Partially successful attempts towards chiral electrochemical synthesis have involved chiral supporting electrolytes chiral solvents and chiral adsorbates, mostly alkaloids With the latter method enantiometric excess values >40% have... 

Since the electroreduction of ketones shown in Scheme 29 has been well established [1-3, 12, 62-65], one more recent interest in the electroreduction of carbonyl compounds is focused on the stereo-selective reduction of ketones. For example, the diastereo-selective cathodic coupling of aromatic ketones has been reported. In the presence of a chiral-supporting electrolyte, a low degree of enantioselectivity has been found [66] (Scheme 30). 

Scheme 30 Diastereoselective cathodic coupling of aromatic ketones in the presence of chiral supporting electrolyte, yield 91%, 25% ee.

Amino-l-(3-hydroxyphenyl)ethanol has been prepared by an improved and possibly general method for this class of compounds.16 An interesting report on the partial asymmetric induction in the reduction of acetophenone-N-benzylimine at the mercury cathode with chiral supporting electrolytes may have potential for the chiral synthesis of alkaloids.17 For example, using ( -)-(R, S)-JV-methyI-ephedrine methiodide as the electrolyte resulted in the formation of (—)-(R)-N-benzyl-a-phenethylamine of 7.3% optical purity. Of interest for biosynthetic studies are the reports of the preparation of specifically labelled substituted -phenethylamines18 and of (+)-N-(o-chlorobenzyl)-a-methylphenethylamine hydrochloride 14C-labelled at the -carbon.19... 

The reduction of 2-oxoacids bound to different chiral auxiliaries gave the 2-hydroxyacid derivatives in a 64 to 76% yield and 42 to 86% de depending on solvent, proton donor, supporting electrolyte, temperature, and substituent R in the oxoacid. The results are in accordance with an ECE reduction of the 2-oxoamide to an enolate anion, which subsequently undergoes a face-selective protonation to the hydroxy acid [346, 347]. 

T Arai, M Ichinose, H Kuroda, N Nimura, T Kinoshita. Chiral separation by capillary affinity zone electrophoresis using an albumin-containing support electrolyte. Anal Biochem 217 7-11, 1994. 

Electroreduction [5b] (with chiral quat as the supporting electrolyte) has been compared with chemical reduction (NaBH4) in the presence of chiral quats for ketone (up to 28% op) and imine (up to 22% op) reductions [57,58], The reduction (NaBH4) of a chiral a,p-enone prostaglandin intermediate in the presence of ephedra-derived catalysts led to the formation of the enol with 70% de [59]. Other reductions with lower asymmetric inductions are noted for ketone [lli,24h,24i,47e,60], imine [5b,57], and hydrodehalogenation of a cyclic a,a-dichlo-roamide [61],... 

The electroreductive cyclization of chiral aromatic a-iminoesters 175, prepared from ( )-a-amino acids such as (6)-valine, (A)-leucine, and ( -phenylalanine, in the presence of chlorotrimethylsilane and triethylamine afforded mixed ketals of ar-2,4-disubstituted azetidin-3-ones 176 stereospecifically (>99% de and 85-99% ee) (Equation 46) <2003JA11591>. The best result was obtained using tetrabutylammonium chlorate as a supporting electrolyte and a platinum cathode. 

In simulation of the intramolecular asymmetric reduction using chiral but optically inactive (racemic) substrates, such as ketones [103,104] and gem-dihalopropanes [477], asymmeric yields higher than 80% could be obtained. It was also found that the intramolecular asymmetric reduction could be improved in asymmetric yield by using an optically active supporting electrolyte [445,478,479]. 

Komori and Nonaka [499] reported the first example of an electrochemical enantiomer-differentiating reaction When racemic 2,2-dimethyl-1-phenyl-1-propanol was oxidized at a poly(L-valine)-coated anode, 43% optically pure (S)-(—)-2,2-dimethy 1-1-phenyl-1-propanol was recovered as an unreacted part. Yamagishi and Aramata [500] also found electrooxidative optical resolution of a racemic Co(l,10-phenathroline)3 complex by a chiral clay-coated anode, and Yoshinaga and coworkers [501] electrore-ductively resolved racemic Co(acetylacetonato)3 by using optically active supporting electrolytes. 

The diastereoselective anodic fluorination of a-phenylsulfenyl esters via intramolecular asynunetric induction has been studied using various chiral auxiliaries. Of these chiral auxiliaries, die 8-phenyl-methyl group gives the best diastereoselectivities. Diastereoselectivity is also affected by the supporting electrolyte and it is found that Et4NF 3HF leads to better selectivity relative to Et3N 3HF or... 

Fig. 9 Voltammograms for glucose oxidation on the two chiral Pt(643) electrodes in 0.05 M H2SO4 supporting electrolyte, (a) Pt(643) electrode in 5 mM o-glucose (b) Pt(643) electrode in 5 mM i-glucose (c) Pt(643) electrode in 5 mM D-glucose and (d) Pt(643) electrode in 5 mM L-glucose. Scan rate 50 mV. (Reproduced with permission from Ref. [47].)...

Such high optical rotations have recently been confirmed on solutions of chemically synthesized 36 [104]. The electrochemical behavior of the two antipodic forms of 37 has been analyzed using d- and /-camphorsulfo-nates as supporting electrolyte, and the differences observed in the resulting electrochemical responses provided first evidence of enantioselective molecular recognition on chiral conducting polymers 1102]. 

To optimize such separation systems, on the one hand the nature and concentration of the chiral selector can be varied, while on the other hand the pH and organic additives (e.g., lower alcohols) make it possible to modify retention times and resolution. For electrophoretic applications, the electric field strength and the composition of the buffer ( background electrolyte , BGE) can also support enantiomer separations. 

The appearance of capillary electrophoresis in the mid-1980s coincided with a period of important expansion of enantioseparation techniques," and soon it was applied to the analysis of enantiomeric mixtures of the most diverse character." As in HPLC, a chiral selector, with the ability to recognize stereoselectively the enantiomers, has to be incorporated into the separative system. This CS is either added to the background electrolyte as an additional solute, acts as a chiral surfactant forming chiral micelles, or incorporated as a constituent of the chromatographic support, which is filling the capillary. The difference in mobilities observed for enantiomers is the result of the differentiated association between each one of them and the CS and/or the difference in mobility of the transient diastereomeric adsorbates formed.""... 

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