Pinacol rearrangement

The pinacol–pinacolone rearrangement is a method for converting a

This reaction was first described by

The driving force for this rearrangement step is believed to be the relative stability of the resultant oxonium ion. Although the initial carbocation is already tertiary, the oxygen can stabilize the positive charge much more favorably due to the complete octet configuration at all centers. It can also be seen as the -OH's lone pairs pushing an alkyl group off as seen in the asymmetrical pinacol example. The migration of alkyl groups in this reaction occurs in accordance with their usual migratory aptitude, i.e.phenyl carbocation > hydride > tertiary carbocation (if formed by migration) > secondary carbocation (if formed by migration) > methyl carbocation. {Why carbocation? Because every migratory group leaves by taking electron pair with it.} The conclusion is that the group which stabilizes the carbocation more effectively is migrated.

Example of asymmetrical pinacol rearrangement

When a pinacol is not symmetrical, there is a choice for which hydroxyl group will leave and which alkyl shift will occur. The selectivity will be determined by the stability of the carbocations. In this case although both choices are tertiary, the phenyl groups result in significantly higher stabilization of the positive charge through resonance.

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 may migrate to the carbocation center, but cis- alkyl groups migrate at a very low rate. In the absence of trans- alkyl groups, ring contraction may occur as the major product instead, i.e. the ring carbon itself may migrate.^[3][4] 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.

History

Although Fittig first published about the pinacol rearrangement, it was not Fittig but

Again Fittig was unable to assign a molecular structure to the reaction product which he assumed to be another isomer or a polymer. Contemporary chemists who had already adapted to the new atomic weight reality did not fare better. One of them,

• Benzilic acid rearrangement
• Semipinacol rearrangement
• Tiffeneau–Demjanov rearrangement, in which the leaving group is a diazo (from amine) rather than oxonium (from hydroxyl)

References