This article Stereospecific Reactions mentions properties and their types based on mechanism and starting material stereochemistry.
What are the properties of Stereospecific reactions?
- Reactions in which the stereochemistry of the product is determined by the stereochemistry of the starting material are called stereospecific
- Stereospecific reactions lead to the production of a single isomer.
- It depends upon the mechanism of the reaction and the stereochemistry of the starting material.
- There is no choice in the reaction pathway.
- The reaction gives a different diastereoisomer of the product from each stereoisomer of the starting material
Different types of Stereospecific reactions?
E2 eliminations from cyclohexanes
EXAMPLE 1
- Reactant stereochemistry: equatorial substituted cyclohexane
- Mechanism: anti-periplanar transition state
- Product stereochemistry: No product
- Reason: equatorial C-X anti-periplanar to two equatorial C-C bonds hence can not E2 eliminated
EXAMPLE 2
- Reactant stereochemistry: axial substituted cyclohexane
- Mechanism: anti-periplanar transition state
- Product stereochemistry: Cyclohexene
- Reason: though axial substituted cyclohexane unstable, ring flipping possible E2 elimination easily, as axial C-X anti-periplanar to two axial C-H
Epoxidation of alkene’s
EXAMPLE 1
- Reactant stereochemistry:cis-alkene
- Mechanism: new C–O bonds are formed on the same face of the alkene’s π bond,
- Product stereochemistry: cis-epoxide
- Reason: the geometry of the alkene is reflected in the stereochemistry of the epoxide
EXAMPLE 2
- Reactant stereochemistry: trans-alkene.
- Mechanism: SN2 reaction has no choice but to go with inversion,
- Product stereochemistry: trans epoxide.
- Reason: the geometry of the alkene is reflected in the stereochemistry of the epoxide
Epoxide ring opening
EXAMPLE 1
- Reactant stereochemistry EPOXIDE top face of the ring
- Mechanism: SN2 reaction has no choice but to go with an inversion
- Product stereochemistry: anti diastereoisomer, but each gives an opposite enantiomers
- Reason: Me2NH will attack the two identical ends of the epoxide with precisely equal probability
