“Vinylogous Pinacol Rearrangement” and "Traditional Pinacol Rearrangement"

Vinylogous means that you have one or more vinylene bridges between functional atoms in an organic molecule. A vinylene group is a functional group with the formula –CH=CH-. A pinacol rearrangement usually involves a 1,2-diol that is converted to a carbonyl compound, causing a molecule of water to be lost during the reaction, and a methyl group shift from one of the central carbons to another. A vinylogous pinacol rearrangement also forms a carbonyl group in the compound, but the alcohol groups are separated by more bonds, which usually include a vinylene group. Additionally, a vinylogous pinacol rearrangement usually involves the intramolecular shift of a bulkier substituent than methyl.

In this diagram, Tanino-Kuwajima illustrates the mechanism of pinacol rearrangement. Vinylogous pinacol rearrangement is similar to this mechanism, except there is a double bond that further separates the two oxygen containing groups, which are the alcohol on the cyclopentene and the oxygen attached to Si(CH3)3 on molecule 17.

The oxygen that attacks the lewis acid is the oxygen that forms the double bond to carbon as shown in Tanino-Kuwajima’s mechanism. The alcohol will leave as water to form a carbocation that will become neutral when the double bond moves, which then creates a partially positive carbon that becomes closed shell when the cyclohexane group opens up to close the shell. The last carbocation, the carbon attached to the oygen-lewis acid complex, becomes closed shell by becoming a carbonyl compound from the oxygen’s electrons.