Pinacol-Pinacolone Rearrangement

Pinacol-Pinacolone Rearrangement was first described by Wilhelm Rudolph Fittig in 1860.
Pinacol Pinacolone Rearrangement is an approach to convert a 1,2-diol(i.e. vic diol or Pinacol) to a carbonyl compound (i.e. Pinacolone) under acidic conditions. The name of the reaction comes from the rearrangement of pinacol to pinacolone.

Mechanism of Pinacol-Pinacolone Rearrangement

The possible mechanism is given below-

Migratory Aptitude

Migration order in general is H > aryl > alkyl
As the migratory group or atom migrates with bond pair, the more nucleophilic group or atom might be expected to migrate. Thus the order of migration amongst the aryl groups is-
p-anisyl > p-tolyl > phenyl > p-chlorophenyl.
Migratory aptitude among the alkyl groups is-
Me₃C > Me₂CH > Me.

Stereochemistry of Pinacol-Pinacolone Rearrangement

In cyclic systems, the stereochemistry of the diol plays a crucial role in deciding the major product. An alkyl group which is situated trans- to the leaving –OH group alone may migrate. If otherwise, ring expansion occurs, i.e. the ring carbon itself migrates to the carbocation centre.
Moreover, if the migrating alkyl group has a chiral center as its key atom, the configuration at this center is retained even after migration takes place.

Examples of the Pinacol-Pinacolone Rearrangement

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