Pinacol Rearrangement - Stereochemistry of The Rearrangement

Stereochemistry of The Rearrangement

In cyclic systems, the reaction presents more features of interest. In these reactions, 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. This reveals another interesting feature of the reaction, viz. that it is largely concerted. There appears to be a connection between the migration origin and migration terminus throughout the reaction.

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.