Stereoselectivity

Source: Wikipedia, the free encyclopedia.

In

electronic effects in the mechanistic pathways leading to the different products. Stereoselectivity can vary in degree but it can never be total since the activation energy
difference between the two pathways is finite: both products are at least possible and merely differ in amount. However, in favorable cases, the minor stereoisomer may not be detectable by the analytic methods used.

An enantioselective reaction is one in which one enantiomer is formed in preference to the other, in a reaction that creates an optically active product from an achiral starting material, using either a chiral catalyst, an enzyme or a chiral reagent. The degree of selectivity is measured by the enantiomeric excess. An important variant is kinetic resolution, in which a pre-existing chiral center undergoes reaction with a chiral catalyst, an enzyme or a chiral reagent such that one enantiomer reacts faster than the other and leaves behind the less reactive enantiomer, or in which a pre-existing chiral center influences the reactivity of a reaction center elsewhere in the same molecule.

A diastereoselective reaction is one in which one

diastereomeric excess
.

Stereoconvergence can be considered an opposite of stereospecificity, when the reaction of two different stereoisomers yield a single product stereoisomer.

The quality of stereoselectivity is concerned solely with the products, and their stereochemistry. Of a number of possible stereoisomeric products, the reaction selects one or two to be formed.

Stereomutation is a general term for the conversion of one stereoisomer into another. For example, racemization (as in SN1 reactions), epimerization (as in interconversion of D-glucose and D-mannose in Lobry de Bruyn–Van Ekenstein transformation), or asymmetric transformation (conversion of a racemate into a pure enantiomer or into a mixture in which one enantiomer is present in excess, or of a diastereoisomeric mixture into a single diastereoisomer or into a mixture in which one diastereoisomer predominates).[4]

Examples

An example of modest stereoselectivity is the

geometric isomers
are also classified as diastereomers, this reaction would also be called diastereoselective.

Stereoselective dehalogenation

Cram's rule predicts the major diastereomer resulting from the diastereoselective nucleophilic addition to a carbonyl group next to a chiral center. The chiral center need not be optically pure, as the relative stereochemistry will be the same for both enantiomers. In the example below the (S)-aldehyde reacts with a thiazole to form the (S,S) diastereomer but only a small amount of the (S,R) diastereomer:[6]

Stereoselective addition of a thiazole to an aldehyde

The

allylic alcohol substrate is transformed into an optically active epoxyalcohol. In the case of chiral allylic alcohols, kinetic resolution results. Another example is Sharpless asymmetric dihydroxylation. In the example below the achiral alkene yields only one of the possible 4 stereoisomers.[7]

Stereoselective Sharpless oxidation

With a

t-butyl
group resulting in high facial diastereoselectivity:

Stereoselective reaction with carbocation Bach 2005

Stereoselective biosynthesis

Forsythia intermedia. This protein has been found to direct the stereoselective biosynthesis of (+)-pinoresinol from coniferyl alcohol monomers.[11] Recently, a second, enantiocomplementary dirigent protein was identified in Arabidopsis thaliana, which directs enantioselective synthesis of (−)-pinoresinol.[12]

Reaction of monolignol radicals in the presence of dirigent protein to form (+)-pinoresinol

See also

Notes and references

  1. ^ (a)"Overlap Control of Carbanionoid Reactions. I. Stereoselectivity in Alkaline Epoxidation," Zimmerman, H. E.; Singer, L.; Thyagarajan, B. S. J. Am. Chem. Soc., 1959, 81, 108-116. (b)Eliel, E., "Stereochemistry of Carbon Compound", McGraw-Hill, 1962 pp 434-436.
  2. ^ For instance, the SN1 reaction destroys a pre-existing stereocenter, and then creates a new one.
  3. ^ Or fewer than all possible relative stereochemistries are obtained.
  4. ^ Eliel, E.L. & Willen S.H., "Stereochemistry of Organic Compounds", John Wiley & Sons, 2008 pp 1209.
  5. ^ Effects of base strength and size upon in base-promoted elimination reactions. Richard A. Bartsch, Gerald M. Pruss, Bruce A. Bushaw, Karl E. Wiegers
    doi:10.1021/ja00791a067
  6. ^ Organic Syntheses, Coll. Vol. 10, p.140 (2004); Vol. 77, p.78 (2000). Link
  7. ^ Organic Syntheses, Coll. Vol. 10, p.603 (2004); Vol. 79, p.93 (2002).Link
  8. doi:10.1055/s-2005-918482
  9. doi:10.1021/ja050626v
  10. doi:10.1002/anie.200905235
  11. S2CID 41957412
    .
  12. S2CID 195313754
    .
Retrieved from "https://en.wikipedia.org/w/index.php?title=Stereoselectivity&oldid=1197236208"