Pinacol-pinacolone rearrangements

Conversion of an alcohol having two adjacent OH groups (pinacol) to a ketone (pinacolone) by the action of acid

Pinacol-pinacolone Rearrangements

+CH3

CH3CH3CH3

OHOH

CH3

CH3

CH3

CH3

OH

CH3

CH3

CH3O

CH3

H

CH3

CH3CH3CH3

OH2OH

CH3

CH3

CH3O

CH3

H2SO4OH2

H+_

_

pinacol

pinacolone

+

+

CH3

CH3

CH3O

CH3

H

+..

Mechanism Mechanism of the pinacol rearrangement of a

symmetrical glycol

1. Protonation of a hydroxyl group

2. Loss of water

Resonance-stablized carbocation

3. Methyl migration4. Deprotonation

Facts

1, 2 shift takes place in carbonium ion which is already a tertiary carbonium ion

It is probably due to stabilization of rearranged ion that can be affected by delocalization involving the electron pair on the oxygen atom, and ready loss of proton from the oxygen to produce a stable end product

The stable cation formed superior for stabilization

H+Ph2C

OHCH2OH

+Ph2C CH2

OH

H+Ph2CH CH

OH+Ph2CH CH

O

Ph2CH CH

OH

+

Bromohydrin to pinacolone

Hydroxyamine to pinacolone

Facts It seems that migration of alkyl group is

extremely fast on the loss of leaving group, taking place simultaneously

That migrating group is from the opposite side of the leaving group– Because rearrangement is much more easy in

compounds having trans configuration than that having cis configuration

Pinacol rearrangement of an asymmetrical glycol

OH

Me

OH

Me

Ph

Ph

OH

Me

Me

Ph

Ph

Me

Me

O

Ph

Ph

H2SO4

Me

Me

OH

Ph

Ph

B C

D

A

+

+

OH

Et

OH

Et

Ph

Ph

OH

Et

Et

Ph

Ph

Et

Et

O

Ph

Ph

H2SO4

H+_

FE

+

Which group migrates?

Phenyl group move first because of greater chances of stabilization due to delocalization

PhHCOH

CHPhOH

H+PhHC CHPh

OH+

Ph2CH CHOH+

Ph2CH CHOH+

Ph2CH CHOH

+

Which group migrates?

The steric chemistry of Pinacol Rearrangements

It is found that o-MeOPh migrate more than a 1000 time less readily that p-substituent and even much less than phenyl. This is due to interference in the transition state with non-migrating group

Solved problemExplain the following experimental facts：

OH

R

R

OH

RR

O

R

O

R

CH2SO4

+

(A) (B) (C)

When R=CH3, （ B ） and （ C ） are formed；When R=Ph, only （ C ） is formed.

+

+

- H

- H

:

:

+

+

+

+

+

+

R

R

OH (6)

C R

R

OH (3)

C R

R

OH (5)

C R

R

OH (2)

C R

R

OH2OH (4)

C R

R

OHOH2 (1)

(C)

(B)

(A)

+ +H

R

C R

O

O

R

R

OH OH

R

C R

When R=Me

When R=Ph