Chemistry Reference
In-Depth Information
This is an example of a Claisen rearrangement which is an electrocyclic reaction
where no charges are involved. While no charges are involved, like the Diels-
Alder reaction, electron pairs do move and their movement can be illustrated
using arrow pushing. The mechanism, illustrated below, involves moving a lone
pair of electrons from the oxygen into the aromatic ring. The aromatic ring then
adds electrons to the double bond. The double bond then migrates and the
carbon-oxygen bond is cleaved. While the expected product may be the illustrated
ketone, spontaneous conversion to the enol form is facilitated by the stability of the
resulting aromatic ring. Thus the illustrated product is formed.
When considering the above mechanistic description, it is important to recognize
that all of these steps occur concurrently. Furthermore, like the Diels-Alder
reaction (and all electrocyclic reactions), there is no net loss or gain of bonds.
This is an example of a Robinson annulation. The mechanism for the Robinson
annulation involves a sequence of conjugate addition reactions and aldol conden-
sations. As illustrated, the first step is deprotonation of cyclohexanedione with
sodium hydride. The resulting anion then participates in a 1,4-addition to
methyl vinyl ketone. The resulting enolate anion then tautomerizes through
