Pinacol pinacolone rearrangement

Pinacol pinacolone rearrangement involves conversion of 1,2 - diols to carbonyl compounds in presence of acid catalyst with change in carbon skeleton. It is an example of whitmore shift.

1. PINACOL PINACOLONE REARRANGEMENT SAVITA K.

2. Introduction: Conversion of 1,2- diol ( vicinal diols ) into carbonyl compounds in presence of acid catalyst with change in carbon skeleton. Example of 1,2- shift ( Whitmore shift)

3. General reaction:

4. Mechanism:

5. Migratory Aptitude: The most electron-rich alkyl group (more substituted carbon) migrates first. The general migration order: Tertiary alkyl > cyclohexyl > secondary alkyl > benzyl > phenyl > primary alkyl > methyl >> H For substituted aryls: p- MeO - Ar > p-Me- Ar > p- Cl - Ar > p-Br- Ar > p- MeOAr > p-O2N-Ar

6. Migratory Aptitude: The OH group lost should be the one that leaves behind the more stable carbocation Order of migratory aptitude : Ar > H > CH 3 In case of alkyl group : tert . Alkyl > sec. > pri . > CH 3 In case of aryl group: Eg . p- Cl Phenyl< Phenyl < p- tolyl < p- methoxy phenyl

7. Migratory Aptitude: In case of electron rich aryl ring, the position of EDG will decide the migratory aptitude. para > meta > ortho In case of electron deficient ring, the position of EWG will decide migratory aptitude. meta preferred Reaction occurs in anti manner i.e. migrating group should be anti to leaving group.

8. Example 1.

9. Example 1. mechanism

10. Example 2.

11. Example 2. mechanism

12. Example 3.

13. Example 3. mechanism

14. Example 4. Of the two OH groups that one departs which leaves behind more stable carbocation . This factor takes precedence over migratory aptitude.

15. Example 4. mechanism

16. Example 5.

17. Example 6. ring contraction

18. Example 7. spiro ketone formation

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