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Stereoselective and stereospecific reactions
1. SAMRAT PRITHVIRAJ CHAUHAN
GOVERNMENT COLLEGE, AJMER
2020-2021
Department Of Chemistry
Stereoselective and Stereospecific Reactions
Submitted By:
Sakshi Arora
M.Sc.(Chemistry) Semester II
2. Index
1. Stereoselective Reaction
a. Addition in 4-methylcyclohexenone
b. Bromination of Cyclohexene
c. Stereoselectivity of Enzymes
2. Stereospecific Reaction
a. Bromination of (E)-2-Butene and (Z)-2-Butene
b. Epoxidation of (E)-2-Butene and (Z)-2-Butene
c. Dehydrohalogenation of 1-Bromo-1,2-diphenylpropane
3. Stereoselectivity vs Stereospecificity
4. Reference
3. Stereoselective Reaction
• Any reaction in which only one set of stereoisomers is formed
exclusively or predominantly is called stereoselective reaction.
• It yields predominantly one stereoisomer(or one pair of enantiomers) of
several possible diastereomers.
• Such a reaction is said to display stereoselectivity.
• The reaction selects one or two of possible stereoisomeric products to
be formed.
• The product stereoisomer is formed due to its reaction pathway being
more favorable than the other. The selectivity of the reaction pathway
depends on differences in steric effects and electronic effects.
4. • It is further subdivided into:
a) Enantioselectivity
It is formation of one of the two enantiomers predominantly
or exclusively.
b) Diastereoselectivity
It is formation of one of the two or more diastereomers
predominantly or exclusively.
• Stereoselective reactions proceed via the most favourable
pathway(kinetic control) or via the pathway which gives the most
stable stereoisomer as the major product(thermodynamic control).
5. 1. Addition in 4-methylcyclohexenone
• The conjugate addition of lithium dimethylcuprate to 4-
methylcyclohexenone is highly stereoselective(diastereoselective).
• The approach of the bulky cuprate reagent occurs predominantly on
the less sterically hindered face of the enone i.e. away from 4-methyl
group.
4-methylcyclohexenone
6. 2. Bromination of Cyclohexene
• When cyclohexene is brominated, the product is one stereoisomeric
product trans-1,2-dibromocyclohexane. No cis-isomer is formed.
• Anti addition of bromine occurs and the two bromine atoms in the
product are on opposite faces of the ring.
7. 3. Stereoselectivity of Enzymes
• The enzymes when in contact with prochiral molecules, react only with one of
the enantiotopic ligands or faces, a property called stereoselectivity.
• Glycerol undergoes phosphorylation exclusively at the pro-R hydroxymethylene
group with adenosine triphosphate(ATP) in presence of an enzyme glycerol
kinase.
Glycerol (R)-Glycerol-1-phosphate
8. Stereospecific Reaction
• A reaction in which stereochemically different molecules react
differently is called a stereospecific reaction.
• By “stereochemically different molecules” is meant
stereoisomers(enantiomers or diastereomers) and to “react
differently” means to show any difference in chemical behaviour.
• Such a reaction is said to display stereospecificity.
• A given isomer leads to one product while another stereoisomer
leads to the opposite product.
9. • A reaction is stereospecific provided the reactant can exist as
stereoisomers and each stereoisomeric reactant gives a different
stereoisomeric product.
• In stereospecific reaction, stereoisomer can
a) Yield different stereoisomers as product
b) React at different rates and path
• Stereospecificity towards enantiomers is called enantiospecificity
and stereospecificity towards diastereomers is called
diastereospecificity.
10. • Addition of bromine to (Z)-2-butene gives a racemic mixture of 2,3-
dibromobutane
• Addition of bromine to (E)-2-butene gives a meso stereoisomer
• Addition of bromine to alkene is both stereoselective and
stereospecific
1.Bromination of (E)-2-Butene and (Z)-2-Butene
11. • In cis-2-butene, bromine attaches from the top face to give intermediate
bromonium ion.
• The bromine ion then attacks the bromonium ion at either carbon from
bottom(SN2) type of displacement at equal rates to yield the two
enantiomers in equal amounts.
Racemic mixture
(±)-2,3-dibromobutane
12. • Trans-2-butene react with bromine from top face to give a chiral
bromonium ion and bromide ion attacks at bottom face to give meso
product.
meso-2,3-dibromobutane
• Stereospecificity is possible only if the inherent facial relationship of the
olefinic bond is maintained through the addition process.
• The bridged bromonium ion intermediate not only maintains the olefin
geometry but also forces the second bromine to add from the opposite
direction.
15. Stereoselectivity vs Stereospecificity
• The quality of stereoselectivity is concerned solely with the products
and their stereochemistry. Out of a number of possible
stereoisomeric products, the reaction selects one or two to be
formed.
• The quality of stereospecificity is focused on the reactants and their
stereochemistry. Each stereoisomeric reactants behaves in its own
specific way.
• In synthesizing a drug or a hormone a chemist wants to
use(stereoselective) reactions that produce just the correct
stereoisomer, since only that stereoisomer will show(stereospecific)
activity in biological system.
16. • All stereospecific reactions are necessarily stereoselective but the
converse is not true. If a reaction is carried out on a compound
which has no stereoisomers, it cannot be stereospecific but at most
it could be stereoselective.
Methylacetylene trans-1,2-dibromopropene
• Add of bromine to methylacetylene results in preferential formation
of trans-1,2-dibromopropene.
• This reaction can only be stereoselective and not stereospecific
because the substrate has no stereoisomer.
17. REFERENCE
• Advanced Organic Chemistry (Sixth Edition) by Jerry March and Michael
B.Smith
• Stereochemistry Conformation and Mechanism (Eighth Edition) by P.S. Kalsi
• Organic Chemistry(Sixth Edition) by Morrison and Boyd
