Editing Stereoselectivity

{{Short description|Ability of a chemical reaction to produce an unequal mixture of stereoisomers}}
In [[chemistry]], '''stereoselectivity'''<ref>(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.</ref> is the property of a [[chemical reaction]] in which a single [[reactant]] forms an unequal mixture of [[Stereoisomerism|stereoisomers]] during a non-[[stereospecificity|stereospecific]] creation of a new [[stereocenter]] or during a non-stereospecific transformation of a pre-existing one.<ref>For instance, the S<sub>N</sub>1 reaction destroys a pre-existing stereocenter, and then creates a new one.</ref> The selectivity arises from differences in [[steric effects|steric]] and [[electronic effects]] in the [[Reaction mechanism|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 [[diastereomer]] is formed in preference to another (or in which a subset of all possible diastereomers dominates the product mixture), establishing a preferred relative stereochemistry. In this case, either two or more chiral centers are formed at once such that one relative stereochemistry is favored,<ref>Or fewer than all possible relative stereochemistries are obtained.</ref> or a pre-existing chiral center (which needs not be optically pure) biases the stereochemical outcome during the creation of another. The degree of relative selectivity is measured by the [[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 S<sub>N</sub>1 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).<ref>Eliel, E.L. & Willen S.H., "Stereochemistry of Organic Compounds", John Wiley & Sons, 2008 pp 1209.</ref>

==Examples==
An example of modest stereoselectivity is the [[dehydrohalogenation]] of 2-iodobutane which yields 60% ''trans''-2-butene and 20% ''cis''-2-butene.<ref>'' Effects of base strength and size upon in base-promoted elimination reactions.'' Richard A. Bartsch, Gerald M. Pruss, Bruce A. Bushaw, Karl E. Wiegers 
[[J. Am. Chem. Soc.]]; '''1973'''; 95(10); 3405-3407. {{doi|10.1021/ja00791a067}}</ref> Since alkene [[geometric isomer]]s are also classified as diastereomers, this reaction would also be called diastereoselective.

:[[File:Stereoselectivedehalogenation.svg|411px|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:<ref>Organic Syntheses, Coll. Vol. 10, p.140 (2004); Vol. 77, p.78 (2000). [http://orgsynth.org/orgsyn/pdfs/V77P0078.pdf Link]</ref>

:[[File:Stereoselectiveadditiontoathiazole.svg|370px|Stereoselective addition of a thiazole to an aldehyde]]
The [[Sharpless epoxidation]] is an example of an enantioselective process, in which an [[Chirality (chemistry)|achiral]] [[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.<ref>Organic Syntheses, Coll. Vol. 10, p.603 (2004); Vol. 79, p.93 (2002).[http://orgsynth.org/orgsyn/pdfs/V77P0078.pdf Link]</ref>

:[[File:StereoselectiveSharplessOxidation.svg|309px|Stereoselective Sharpless oxidation]]

With a [[stereogenic]] center next to the carbocation the substitution can be stereoselective in inter- <ref>'' Diastereoselective Friedel-Crafts Cyclization Reactions to 2-Substituted 1-Phenyl-1,2,3,4-tetrahydronaphthalenes'' Friedrich Mühlthau, Thorsten Bach Synthesis '''2005''': 3428-3436 {{doi|10.1055/s-2005-918482}}</ref> and intramolecular <ref>'' High Facial Diastereoselectivity in Intra- and Intermolecular Reactions of Chiral Benzylic Cations'' Friedrich Mühlthau, Oliver Schuster, and Thorsten Bach [[J. Am. Chem. Soc.]], '''2005''', 127 (26), pp 9348–9349 {{doi|10.1021/ja050626v}}</ref><ref>''Stereoselective Reactions with Stabilized Carbocations'' Pier Giorgio Cozzi and Fides Benfatti [[Angew. Chem. Int. Ed.]] '''2009''', 48 {{doi|10.1002/anie.200905235}}</ref> reactions. In the reaction depicted below the nucleophile (furan) can approach the carbocation formed from the least shielded side away from the bulky [[t-butyl]] group resulting in high facial diastereoselectivity:

:[[File:Stereoselectivereactionwithcarbocations.svg|Stereoselective reaction with carbocation Bach 2005]]

== Stereoselective biosynthesis ==
[[Pinoresinol]] biosynthesis involved a protein called a [[dirigent protein]]. The first dirigent protein was discovered in ''[[Forsythia intermedia]]''. This protein has been found to direct the stereoselective biosynthesis of (+)-[[pinoresinol]] from [[coniferyl alcohol]] monomers.<ref>{{cite journal  |vauthors=Davin LB, Wang HB, Crowell AL, et.al. |title=Stereoselective bimolecular phenoxy radical coupling by an auxiliary (dirigent) protein without an active center |journal=Science |volume=275 |issue=5298 |pages=362–6 |year=1997 |pmid=8994027 |doi=10.1126/science.275.5298.362|s2cid=41957412 }}</ref> Recently, a second, enantiocomplementary [[dirigent protein]] was identified in ''[[Arabidopsis thaliana]]'', which directs enantioselective synthesis of (−)-pinoresinol.<ref>{{cite journal|vauthors=Pickel B, Constantin MA, Pfannsteil J, Conrad J, Beifuss U, Schaffer A |title=An Enantiocomplementary Dirigent Protein for the Enantioselective Laccase-Catalyzed Oxidative Coupling of Phenols|journal=Angewandte Chemie|volume=53|issue=4|pages=273–284|date=March 2007|doi=10.1007/s10086-007-0892-x|s2cid=195313754 |doi-access=free}}</ref>

[[File:(+)-Pinoresinol Biosynthesis.svg|400px|Reaction of monolignol radicals in the presence of dirigent protein to form (+)-pinoresinol]]

== See also ==
* [[Dynamic stereochemistry]]
* [[Torquoselectivity]]
* [[Regioselectivity]]
* [[Chemoselectivity]]

==Notes and references==
{{Reflist}}

{{Organic reactions}}
{{Authority control}}

[[Category:Stereochemistry]]