Pinacol itself is produced by magnesium reduction of acetone, probably by way of a ketyl intermediate. Since the diol is symmetrical, protonation and loss of water takes place with equal probability at either hydroxyl group. The resulting 3ยบ-carbocation is relatively stable, and has been shown to return to pinacol by reaction in the presence of isotopically labeled water. A 1,2-methyl shift generates an even more stable carbocation in which the charge is delocalized by heteroatom resonance. Indeed, this new cation is simply the conjugate acid of the ketone pinacolone, which is the product of repeated rearrangements catalyzed by proton transfer. Each step in this rearrangement is potentially reversible, as demonstrated by the acid catalyzed dehydration of pinacolone (and pinacol) to 2,3-dimethyl-1,3-butadiene under vigorous conditions.
Many factors must be considered when analyzing the course of a pinacol rearrangement. These include:
? Which hydroxyl group is lost as water? or Which intermediate carbocation is more stable?
? What is the inherent shifting tendency (migratory apptitude) of different substituent groups?
? What is the influence of steric hindrance and other strain factors on the rearrangement?
? Are epoxides formed as intermediates in the pinacol rearrangement?
? Does product stability govern the outcome of competing rearrangements?
? Do the reaction conditions (i.e. type of acid, concentration, solvent and temperature) influence the course of rearrangement
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