Electrochemical enantioselective reductions

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. 

The mechanism of the enantioselective 1,4-addition of Grignard reagents to a,j3-unsaturated carbonyl compounds (Scheme 5 R1 = alkyl R2 = alkyl, OR3), promoted by copper complexes of chiral ferrocenyl diphosphines (180), has been explored using kinetic, spectroscopic, and electrochemical analysis. The roles of the solvent, copper halide, and the Grignard reagent have been thoroughly examined. Kinetic studies support a reductive elimination as the rate-limiting step, in which the chiral catalyst,... 

The same system was employed in the reduction of 4-phenyl-2-butanone to (S)-4-phenyl-2-butanol using HLADH as well as 5-ADH from Rhodococcus sp. with high enantioselectivity [113]. With pentamethylcyclopentadienyl-4-ethoxy-methyl-2,2 -bipyridinechloro-rhodium(III) as mediator and HLADH as catalyst, after 5 h 70% of 4-phenyl-2-butanone was reduced to (S)-4-phenyl-2-butanol with 65% ee. Using 5-ADH. 76% of the ketone was converted to the (S)-alcohol after 5 h with 77% ee. Furthermore, this system has been applied in an electrochemical EMR with a polymer bound rhodium complex as mediator. 

Some special methods of enantioselective electrochemical reactions should be mentioned. D-Alanine was prepared with an ee close to 100% using the electrochemical reduction of pyruvic acid using an electrode on which amino acid oxidase and electron mediator were immobilized... 

Cheng has published a convenient and synthetically useful alternative method to the NHK reaction for the arylation of aromatic aldehydes in a mild and selective way with nickel(ll) bromide/zinc/dppe mediated protocol for the synthesis of diaryl carbinols. Durandetti reported an electrochemical coupling of aryl halides with aldehydes for the synthesis of diaryl carbinols which was catalytic in chromium and nickel salts. Comins utilized the NHK reaction to prepare 5-(l-hydroxyalkyl)-2,3-dihydro-4-pyridones, which were then explored in reductive, oxidative and substitutive reactions. " The first asymmetric catalytic synthesis of 5y -alk-l-ene-3,4-diols was developed the regio-, diastereo- and enantioselective addition of 3-chloropropenyl pivaloate to aldehydes was made possible by exploiting Salen r(II) species in a catalytic version of the NHK reaction. ... 

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