Stereospecificity reagents

Rodriguez, S., Kayser, M.M. and Stewart, J.D. (2001) Highly stereospecific reagents for /3-keto ester reductions by genetic engineering of baker s yeast. Journal of the American Chemical Society, 123, 1547-1555. 

Simmons-Smith reagent Named after the duPont chemists who discovered that diiodo-mechane would react with an active zinc-copper couple in ether to give a reagent with molecular formula ICHiZnl. The reagent adds stereospecifically cis- to alkenes to give cyclopropanes in high yields. 

Electron deficient carbon-carbon double bonds are resistant to attack by the electrophilic reagents of Section 5.05.4.2.2(t), and are usually converted to oxiranes by nucleophilic oxidants. The most widely used of these is the hydroperoxide ion (Scheme 79). Since epoxidation by hydroperoxide ion proceeds through an intermediate ct-carbonyl anion, the reaction of acyclic alkenes is not necessarily stereospecific (Scheme 80) (unlike the case of epoxidation with electrophilic agents (Section 5.05.4.2.2(f)) the stereochemical aspects of this and other epoxidations are reviewed at length in (B-73MI50500)). 

In the past few years metal deuterides have become commercially available at reasonable prices. This has encouraged the use of these reagents for reactions involving deuteride displacements of suitable leaving groups. The attractive feature of these reactions is the stereospecificity of the deuterium insertion. 

In a further extension of this reaction Winstein and Dauben showed that the action of the methylene-transfer reagent (1) on A -cycloal-kenols, e.g., (2), proceeds by stereospecific cis addition to give the cw-cyclo-propyl carbinol (5). It was also observed that both the rate and yield of the hydroxyl-assisted reaction are increased substantially. It has been suggested that the high stereoselectivity observed in these instances is best explained by complex formation or reaction of the reagent (1) with the hydroxyl group of (2) followed by intromolecular transfer of methylene. 

The first substance examined in the steroid field was 3j6-hydroxycholest-4-ene (1) and the epimeric 3a-alcohol (3). These compounds react stereospecifically in dry ether with the Simmons-Smith reagent to yield the isomeric cyclopropyl carbinols (2) and (4) in 90 % and 67 % yields, respectively. The rate of this reaction is about one fifth of that observed with simple cyclic car-binols. ... 

Hexafluoropropyldiethylamine is a particularly useful reagent for conversion of secondary benzylic hydroxy esters into the corresponding secondary benzyl fluorides The reactions proceed with inversion of configuration and a high degree of stereospecificity [86, 87] (equation 53)... 

Perfluorovinyl iodides readily undergo stereospecific coupling with the tn-fluoroinethylcopper solution [224] (equation 151) prepared from dibromodifluo-romethane [210] With longer-chain perfluoroalkylcopper reagents, the coupling reaction is accompanied by some exchange processes [225] (equation 152)... 

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