Pinacol Rearrangement

• General Characteristics

Vicinal diol compounds (pinacols) undergo dehydrative rarrangement under strongly acidic conditions to produce carbonyl compounds.

Since alkyl, aryl, and hydride are all potential migrating groups, pinacols containing different R¹~R⁴ groups are likely to give a mixture of products.

In general, more electron rich groups migrate more easily. A rough order for the propensity to migrate is: aryl > H > alkenyl > tert-alkyl >> cyclopropyl > sec-alkyl > prim-alkyl.

Vicinal diols with one of the hydroxyl groups activated as a leaving group undergo the same rearrangement and this version is called the semi-pinacol rearrangement. This reaction proceeds under milder conditions and is synthetically valuable if the starting materials are appropriately designed.

• General References

・ Fittig, R. Liebigs Ann. Chem. 1860, 114, 54.
・ Rickborn, B. Comp. Org. Syn. 1991, 3, 721.

• Reaction Mechanism

The reaction proceeds by the cationic 1,2-shift mechanism similar to the Wagner-Meerwein rearrangement. The rearrangement is promoted by the presence of a lone pair of the neighboring hydroxyl group. When the substrate forms a stable carbocationic intermediate and bond rotation competes with rearrangement, the stereochemistry of the product becomes less predictable.

• Examples

Pinacols containing cyclic groups can be considered as precursors of spiro compounds. The pinacol rearrangement of these cyclic substrates is a typical example of ring-expansion reactions.

• Experimental Procedure

• Experimental Tips

• References

• Related Books

cationic rearrangement, pinacol, vicinal diols