Electroreduction asymmetric

Electroreductive asymmetric alkylation [480,481] and electrooxidative asymmetric fluorination [482,483] of chiral substrates have been reported. 

Asymmetric electroreduction of ketones to the corresponding chiral alcohols has recently been reported. Typical examples are the reduction of ketones bearing chiral auxiliaries [68, 69], and the indirect reduction of ketones with alcohol dehydrogenase (ADH), as a mediator (Scheme 32) [70]. 

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],... 

Asymmetric reduction of activated olefins has been performed using prochiral cumarin derivatives [414—416]. A maximum asymmetric yield reported was 17%, as in Eq. (62) [415], and an asymmetry induction mechanism has also been discussed [416]. Later, Schafer and coworkers carried out similar enantioselective electroreduction of various 4-substituted cumarins by systematic variation of the electrolysis conditions, and they obtained optical yields as high as 67% [Eq. (62)] [417-419]. Computational chemistry using semiempirical AMI demonstrates that ri-protonation of the intermediate anion by protonated yohimbine to form the (i )-isomer is energetically favored [418,419]. 

A highly asymmetric induction in the nickel-catalyzed electroreductive couplings of aryl halides with a-chloropropionic acid derivatives was reported by Nedelec and coworkers [Eq, (65)] [495]. 

Recently, asymmetric electroreduction methods based on new principles have been developed. 

Regarding the chemical behavior of (-)-emetine (1), the stereochemistry of asymmetric electroreductions of bromocyclopropanes at a mercury cathode in the presence of adsorbed 1 has been studied 95,96). Bertz et al. (97) have reported asymmetrie induction (with 70 30 R/S ratio, but in only 6% chemical yield) that occurred in the conjugate addition of an (-)-emetine-incorporating phenylamidocuprate to 2-cyclohexenone. Fraser-Reid s group 98,99) has reported that the NH group of 1 is readily protected as the A-pent-4-enoyl derivative, which can be rapidly... 

---