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In chemistry, stereospecificity is the property of a reaction mechanism that leads to different stereoisomeric reaction products from different stereoisomeric reactants, or which operates on only one (or a subset) of the stereoisomers.^[1]^[2]^[3]

In contrast, stereoselectivity^[1]^[2] is the property of a reactant mixture where a non-stereospecific mechanism allows for the formation of multiple products, but where one (or a subset) of the products is favored by factors, such as steric access, that are independent of the mechanism.

A stereospecific mechanism specifies the stereochemical outcome of a given reactant, whereas a stereoselective reaction selects products from those made available by the same, non-specific mechanism acting on a given reactant. Given a single, stereoisomerically pure starting material, a stereospecific mechanism will give 100% of a particular stereoisomer (or no reaction), although loss of stereochemical integrity can easily occur through competing mechanisms with different stereochemical outcomes. A stereoselective process will normally give multiple products even if only one mechanism is operating on an isomerically pure starting material.

The term stereospecific reaction is ambiguous, since the term reaction itself can mean a single-mechanism transformation (such as the Diels-Alder reaction), which could be stereospecific, or the outcome of a reactant mixture that may proceed through multiple competing mechanisms, specific and non-specific. In the latter sense, the term stereospecific reaction is commonly misused to mean 'highly stereoselective reaction'.

Chiral synthesis is built on a combination of stereospecific transformations (for the interconversion of existing stereocenters) and stereoselective ones (for the creation of new stereocenters), where also the optical activity of a chemical compound is preserved.

The quality of stereospecificity is focused on the reactants and their stereochemistry; it is concerned with the products too, but only as they provide evidence of a difference in behavior between reactants. Of stereoisomeric reactants, each behaves in its own specific way.

Examples

Nucleophilic substitution at sp³ centres can proceed by the stereospecific S_N2 mechanism, causing only inversion, or by the non-specific S_N1 mechanism, the outcome of which can show a modest selectivity for inversion, depending on the reactants and the reaction conditions to which the mechanism does not refer. The choice of mechanism adopted by a particular reactant combination depends on other factors (steric access to the reaction centre in the substrate, nucleophile, solvent, temperature).

Stereospecificity in substitution reactions

S_N1 mechanism non-stereospecific	S_N2 mechanism stereospecific

For example, tertiary centres react almost exclusively by the S_N1 mechanism whereas primary centres (except neopentyl centres) react almost exclusively by the S_N2 mechanism. When a nucleophilic substitution results in incomplete inversion, it is because of a competition between the two mechanisms, as often occurs at secondary centres, or because of double inversion (as when iodide is the nucleophile).

The addition of singlet carbenes to alkenes is stereospecific in that the geometry of the alkene is preserved in the product. For example, dibromocarbene and cis-2-butene yield cis-2,3-dimethyl-1,1-dibromocyclopropane, whereas the trans isomer exclusively yields the trans cyclopropane.^[4]

This addition remains stereospecific even if the starting alkene is not isomerically pure, as the products' stereochemistry will match the reactants'.

The disrotatory ring closing reaction of conjugated trienes is stereospecific in that isomeric reactants will give isomeric products. For example, trans,cis,trans-2,4,6-octatriene gives cis-dimethylcyclohexadiene, whereas the trans,cis,cis reactant isomer gives the trans product and the trans,trans,trans reactant isomer does not react in this manner.

References

^ ^a ^b "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.
^ ^a ^b Eliel, E., "Stereochemistry of Carbon Compound", McGraw-Hill, 1962 pp 434-436
^ March, Jerry (1985), Advanced Organic Chemistry: Reactions, Mechanisms, and Structure (3rd ed.), New York: Wiley, ISBN 0-471-85472-7
^ Skell, P.S. and Garner, A.Y. (1956). "The Stereochemistry of Carbene-Olefin Reactions. Reactions of Dibromocarbene with the cis- and trans-2-Butenes". Journal of the American Chemical Society 78 (14): 3409–3411. doi:10.1021/ja01595a040.

English Journal

Regio- and Stereoselective Cyanotriflation of Alkynes Using Aryl(cyano)iodonium Triflates.

Wang X1, Studer A1.
Journal of the American Chemical Society.J Am Chem Soc.2016 Mar 9;138(9):2977-80. doi: 10.1021/jacs.6b00869. Epub 2016 Feb 25.
A novel, mild, and versatile approach for regioselective syn-addition of both the CN and OTf groups of aryl(cyano)iodonium triflates to alkynes is described. The reaction uses Fe-catalysis and can be conducted in gram scale. Products of the vicinal cyanotriflation can be stereospecifically readily f
PMID 26891077

Improved conversion of cinnamaldehyde derivatives to diol compounds via a pyruvate decarboxylase-dependent mechanism in budding yeast.

Miyakoshi S1, Negishi Y2, Sekiya Y2, Nakajima S2.
Journal of bioscience and bioengineering.J Biosci Bioeng.2016 Mar;121(3):265-7. doi: 10.1016/j.jbiosc.2015.06.013. Epub 2015 Jul 27.
Cinnamaldehyde is stereospecifically converted to (2S,3R) 5-phenylpent-4-ene-2,3-diol, an important starting material for the synthesis of biologically active compounds, by the budding yeast Saccharomyces cerevisiae. Immobilization of the yeast in calcium alginate capsules suppressed the formation o
PMID 26228910

A pyridoxal phosphate-dependent enzyme that oxidizes an unactivated carbon-carbon bond.

Du YL1, Singh R2, Alkhalaf LM1, Kuatsjah E3, He HY1, Eltis LD2, Ryan KS1.
Nature chemical biology.Nat Chem Biol.2016 Mar;12(3):194-9. doi: 10.1038/nchembio.2009. Epub 2016 Jan 25.
Pyridoxal 5'-phosphate (PLP)-dependent enzymes have wide catalytic versatility but are rarely known for their ability to react with oxygen to catalyze challenging reactions. Here, using in vitro reconstitution and kinetic analysis, we report that the indolmycin biosynthetic enzyme Ind4, from Strepto
PMID 26807714

Japanese Journal

Stereospecific Aerobic Oxidative Dehydrocoupling of P(O)–H Bonds with Amines Catalyzed by Copper

, , , , ,
Bulletin of the Chemical Society of Japan 87(3), 400-402, 2014
… Mechanistic studies showed that this dehydrocoupling reaction proceeded stereospecifically with inversion of stereochemistry at phosphorus. …
NAID 130004153285

好塩性アーキアの膜脂質イソプレノイド生成機構-ロイシン-メバロン酸経路と関連酵素の存在

山内敬明,田上諒
日本地球化学会年会要旨集 60(0), 139, 2013
好塩性古細菌の特徴的なイソプレノイド脂質コア部分の生合成機構の詳細について，ロイシンからメバロン酸への短絡経路と関連酵素の存在を指摘するため，立体特異的重水素化標識ロイシンを用いて検討した。(3S)- および(3R)-[3-d]ロイシンを調製し，その取り込み実験より，プロキラル水素が立体特異的に変換されていることが示され，これは相当するイソバレリル-CoA 脱水素酵素が基質を立体特異的に認識してい …
NAID 130004593890

Differential Effect of Schisandrin B Stereoisomers on ATR-Mediated DNA Damage Checkpoint Signaling

, , [他], , , ,
Journal of Pharmacological Sciences 122(2), 138-148, 2013
… These results indicate that GN is the active component of SchB and suggest that GN inhibits the DNA damage checkpoint signaling by stereospecifically interacting with ATR. …
NAID 130003362714