Chiral redox reactions

One might anticipate that there would be a rate difference for the reaction of enantiomers with a chiral compound. The first demonstration of stereoselectivity in an outer-sphere electron transfer was as recent as 1980. Since then such asymmetric induction has been established with a number of examples, nearly all involving outer-sphere redox reactions. Thus, consider the two reactions... 

Chart 2. Examples of chiral complexes studied in redox reactions with GO, HRP, and cytochrome c peroxidase. 

We have shown how the band structure of photoexcited semiconductor particles makes them effective oxidation catalysts. Because of the heterogeneous nature of the photoactivation, selective chemistry can ensue from preferential adsorption, from directed reactivity between adsorbed reactive intermediates, and from the restriction of ECE processes to one electron routes. The extension of these experiments to catalyze chemical reductions and to address heterogeneous redox reactions of biologically important molecules should be straightforward. In fact, the use of surface-modified powders coated with chiral polymers has recently been reputed to cause asymmetric induction at prochiral redox centers. As more semiconductor powders become routinely available, the importance of these photocatalysts to organic chemistry is bound to increase. 

Oxidations of carbon-heteroatom species often results in the destruction of a stereogenic center, as in the oxidation of a secondary alcohol to a ketone. In some instances, this reaction can be coupled with another to provide a chiral product (see Chapter 21). One example is the enzymatic acetylation of one enantiomer of a secondary alcohol, where a redox reaction with a transition metal catalyst equilibrates the unreactive isomer of the alcohol (Scheme 9.1).10 12 The redox reaction can also be performed by an enzyme.13... 

Although several research groups have been interested in transition metal enolates to use the metal centre as a potential site of asymmetry in the design of chiral catalysts, examples of well defined redox reaction involving middle to late transition elements and lanthanides are scarce in the literatnre. Based on Pearson s theory of hard and soft acids and bases", it has been proposed that combining a hard ligand with a soft late transition metal centre may lead to weak metal-heteroatom links, resnlting in reactive late metal-heteroatom bonds. 

The ability of clathrochelates to form ion pairs and covalently attached complexes is utilized in biochemistry [315-321], The stereoselectivity of the redox reactions of plastocyanine and horse heart cytochrome C with several cage complexes was reported in Ref 319. Studies on stereoselective electron transfer in different systems provide information on the importance of close ion pair association of a cage complex with protein in chiral discrimination. 

Chiral Synthons by Selective Redox Reactions Catalysed by Hitherto Unknown Enzymes Present in Resting Microbial Cells... 

Many stereo- and regioselective redox reactions have been carried out on a preparative scale by the catalysis of enzymes or microorganisms. The majority of these reactions involves the reduction of keto groups to chiral secondary alcohols or derivatives thereof Not only the reductions of CX-double bonds, but also selective dehydrogenations are of interest. A well-known example of a regioselective biocatalytic dehydrogenation is the formation of L-sorbose from D-sorbitol which is actually the key step in the synthesis of vitamin C. 

The boronic acid 20 reported by Shinkai is a rare example of an enantiopme chiral redox-active receptor." It was found to bind linear and cyclic sugars in aqueous solution at neutral pH due to the reversible reaction between the boronic acid moiety of the protonated ferrocene receptor and the sugar to form a boronate ester. Electrochemical studies revealed that complexation imparted cathodic (ca. - 50 mV) shifts in the Fc /Fc redox couple. Interestingly, moderate enantioselectivity was observed in that (+ ) — 1 bound the linear saccharide D-sorbitol more strongly than L-sorbitol (ATd/AIl = 1-4). However, differences in the electrochemical response to the binding of these enantiomers by 20 were not apparent. 

Chiral /3-phosphinoethyl p-tolyl sulfoxide (38) undergoes a rapid internal redox reaction between the sulfinyl and the phosphino group. On the other hand, an aromatic phosphino sulfoxide is much more stable a chiral sulfoxide 39a is recovered without any racemization even though at a much higher temperature (130-140 °C). 

In contrast to the resolution of secondary alcohols, where the more simple Upase technology is recommended instead of redox reactions, desymmetrization of primary diols of prochiral or meso-structure has been shown to be a valuable method for the synthesis of chiral lactones (Scheme 2.143) [1034]. 

Scheme 18 Chiral Mg-bisoxazolidine complex-catalyzed intramolecular redox reactions via [1,5]-HT/cyclization...

Scheme 29 Asymmetric sp C-H functionalization via a chiral Br0nsted acid-catalyzed redox reaction...

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