Enantioselective cathodic reduction

Yadav AK, Manju M, Chhinpa PR (2003) Enantioselective cathodic reduction of some prochiral ketones inthe presence of (—)-N, N-dimethylquininium tetrafluoroborate atmercury cathode. Tetrahedron Asymmetry 14 1079-1081... 

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). 

The HOPG (highly oriented pyrolytic graphite) carbon electrode chemically modified with (5[-phenylalanine at the basal surface led to 2% ee in the reduction of 4-acetylpyridine [377]. A cathode modified with a chiral poly(pyrrole) reduced 4-methylbenzophenone or acetophenone in DMF/LiBr and phenol as proton donor to 1-phenylethanol with up to 17% ee [382]. Alkyl aryl ketones have been reduced to the corresponding alcohols at a Hg cathode in DMF/water in the presence of (1R,2S)-A,A-dimethylephedrinium tetrafluorobo-rate (DET), producing (5 )-l-phenylethanol with 55% ee from acetophenone. Cyclovoltammetry supports an enantioselective protonation of the intermediate (PhCOH(CH3)) [383]. 

The asymmetric reduction of C=N double bonds in prochiral oximes afforded a maximum of 18% ee [380, 384, 385]. Prochiral azomethines were reduced to the corresponding 1,2-diamines and secondary amines using 36 optically active supporting electrolytes. The dimers were optically inactive, while the monomers showed low optical inductions (<11% ee). The effect of electrolyte, substrate concentration, temperature, pH, and cathode potential on the induction was studied. It was proposed that the enantioselectivity... 

Cleavage Gem-dihalides and monohalides have been dehalogenated to chiral monohalides in the presence of alkaloids [397, 398]. l,l-Diphenyl-2-bromo-2-carboxyl (bromo or methyl carboxylate) cyclopropanes are cathodically debromi-nated in the presence of alkaloid cations with enantioselectivities up to 45% ee. A mechanism is proposed whereby the alkaloid is adsorbed at the Hg cathode, which protonates face selectively the carbanion generated by 2e reduction from the bromide [399]. 

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