Stereoselective Reactions and Modern Reagents in Chemistry.pdf

Stereoselective Reactions and Modern Reagents in Chemistry
Topics covered: Bisecting conformations, Felkin Anh model, Houk model, Distereoselective reactions, mechanistic approach, among others.

Dr. Anand S. Burange
Assistant Professor, Department of Chemistry, Wilson College (Autonomous) Mumbai

1. STEREOSELECTIVE REACTIONS AND
MODERN REAGENTS IN CHEMISTRY
Dr. Anand S Burange
Ph.D. (Sci), CSIR-NET, GATE,
CSIR-RA and CSIR-Nehru pdf
Assistant Professor at Wilson College, Mumbai
[email protected]
ORGANIC CHEMISTRY
22/01/2017

2. PART I
STEREO-SELECTIVE REACTIONS

3. Inversion/Retention
R
S
R
S
S
S
R
R
?
?
?
?
Inversion of configuration of a molecule containing chiral centre does not necessarily require that
stereochemical descriptors (R,S) for absolute configuration should undergo change. Same is true for
retention of configuration.

4. Conformation of carbonyl compounds
Eclipsed conformation
More stable
Bisecting conformation
Acetaldehyde
Acetone
Stable
Diethylketone
Stable

5. Diastereoselective Reactions of carbonyl compounds
Crams’ Rule
Preferred conformations
L: Large group
M: Medium group
S: Small group
Larger group of asymmetric
alpha carbon prefers
periplanar
orientation with carbonyl.

6. Predict product (s)

7. Here –Me and –OH
are on same side while
–Me and –Et are anti;
--H & -Ph in plane.

8. Chelation and diastereoselective reactions
Non chelation control
Chelation control

9. Stereoselective reactions of acyclic alkenes
The Houk Model
Lowest energy
Preferred
-H eclipising with C=C
Slightly higher energy
-Me eclipising with
C=C

10. Predict product (s)
Epoxidation takes place from less
hindered site (Below the plane)

11. Hydrogen bonding changes the
selectivity

12. Epoxidation problems
50:50 both stereo-isomers are formed
Stereoselectivity

13. Stereochemistry in Aldol Reaction

14. Predict product (s)
Can only form trans-enolate
Cis enolate avoids steric
repulsion between t-butyl and
methyl group.

15. PART II
REAGENTS IN MODERN ORGANIC SYNTHESIS
PROBLEM SOLVING APPROACH

16. What is K-Selectride?
Potassium trisecondarybutylborohydride.

17. Suggest suitable Reagents
i. NBS
ii. OH
-
m-CPBA

18. Q.Which of the following compound will show decarboxylation reaction in
presence of base?

19. Thank you