1. Chapter 7 Rearrangement Reactions （重排反应）

2. What is Rearrangement Reactions?
The term of “rearrangements” is used to describe organic reactions which involve the migration of an H atom or of a larger molecular fragment.
Nucleophilic Rearrangements
Electrophilic rearrangements
Radical rearrangements

3. 1. Nucleophilic Rearrangements

4. Wagner-Meerwein rearrangements
Wagner-Meerwein Rearrangements are [1,2]-rearrangements of H atoms or alkyl groups in carbenium ions that do not contain any heteroatoms attached to the valence-unsaturated center C-1 or to the valence-saturated center C-2.

7. Wagner-Meerwein rearrangements
Carbocation Stability
CH3+
<
CH3CH2+
(CH3)2CH+
CH2=CH-CH2+
C6H5CH2+
Carbenium ions: 1 °→2 °，1 °→3 °
2 °→3 °
Reactions include Wagner-Meerwein rearrangement step:
1. Electrophilic additions of alkenes
2. Nucleophilic substitutions (SN1)
3. E1 elimination
4. Friedel-Crafts alkylation reactions, etc

8. Example: Friedel-Crafts Alkylation
1-Bromopropane isomerizes quantitatively to 2-bromopropane under Friedel-Crafts conditions. The [1,2]-shift A→B involved in this reaction again is an H-atom shift.

9. Wagner-Meerwein rearrangement as part of an isomerizing E1 elimination
Example:
Wagner-Meerwein rearrangement as part of an isomerizing E1 elimination
Methyl shift

10. Example: Nucleophilic Substitution
HNO2
Methyl shift
Mechanism
HNO2 C H 3 + C H C C H 3 2 C H 3 + H O C H C C H C H 2 3 2 3 + C H H 3

11. Example: E1 and Nucleophilic Substitution
Mechanism

12. Wagner-Meerwein rearrangement as part of an HCl addition to a C=C double bond
Alkyl shift
Ring expansion

13. An E1 elimination involving five Wagner-Meerwein rearrangements
Alkyl shift
Ring expansion

14. (2) Pinacol Rearrangements
Mechanism of the pinacol rearrangement of a symmetrical glycol
1. Protonation of a hydroxyl group
2. Loss of water
3. Methyl migration
4. Deprotonation
Resonance-stablized carbocation

15. • Which hydroxyl group is lost as water
• Which hydroxyl group is lost as water? or Which carbrnium ion forms first? • What is the inherent shifting tendency (migratory apptitude) of different substituent groups? or Which group migrates? • What is the influence of steric hindrance and other strain factors on the rearrangement? (the steric chemistry) • Are epoxides formed as intermediates in the pinacol rearrangement? • Do the reaction conditions (i.e. type of acid, concentration, solvent and temperature) influence the course of rearrangement?
?????????

16. Which carbenium ion forms first?
Regioselectivity of the pinacol rearrangement of an unsymmetrical glycol
Why?

17. The stable cation formed superior+
Which intermediate carbocation is more stable?

18. Which group migrates? Phenyl group move first+ P h C H C H 2 O H
Phenyl group move first
Group moved as the following order:
tertiary alkyl＞aryl＞H＞secondary alkyl
＞primary alkyl＞methyl

19. Solved problem Explain the following experimental facts：
When R=CH3,（B）and（C）are formed；
When R=Ph, only（C）is formed.

20. When R=Me

21. When R=Ph

22. The steric chemistry of
Pinacol Rearrangements

23. The steric chemistry of Pinacol Rearrangements ?
Leaving group and shifting group must be anti-coplanar

24. (3)Tiffeneau-Demjanov Rearrangements ----Semipinacol Rearrangements
Primary amine Nitrosonium ion
Protonated
N-nitrosoamine
Diazonium ion
N-nitrosoamine

25. Reaction Mechanism Loss of nitrogen gas Alkyl migration Deprotonation
Resonance-stablized carbocation

26. Semipinacol Rearrangements
Useful reaction for Ring expansion

27. More examples

28. The steric chemistry of
Pinacol Rearrangements

29. The steric chemistry of
Pinacol Rearrangements

33. Summary (1)Wagner-Meerwein rearrangements (2) Pinacol Rearrangements
(3)Tiffeneau-Demjanov Rearrangements Semipinacol Rearrangements

34. Finish the following reactions and write reasonable mechanisms