Allylic rearrangement

An allylic rearrangement or allylic shift is an organic chemical reaction in which reaction at a center vicinal to a double bond causes the double bond to shift to an adjacent pair of atoms: It is encountered in both nucleophilic and electrophilic substitution, although it is usually suppressed relative to non-allylic substitution.

Allylic shifts become the dominant reaction pathway when there is substantial resistance to a normal (non-allylic) substitution.

For nucleophilic substitution, such resistance is known when there is substantial steric hindrance at or around the leaving group, or if there is a geminal substituent destabilizing an accumulation of positive charge.

[1] Although rarer still than SN', allylic shifts can occur vinylogously, as a "butadienylic shift":[2] In SN2' reduction, a hydride allylically displaces a good leaving group in a formal organic reduction, similar to the Whiting diene synthesis.

Only when the cyclohexane ring is properly substituted will the proton add trans to the adjacent methyl group.

SN2 accent reaction mechanism
reaction of 1-chloro-but-2-ene with sodium hydroxide
A macrocyclization: a 1,5-pentanedithiol terminus attacks the butadiene tail of a 1-substituted 2,4-pentadien-2-yl aryl ketone. Instead of forming an enol, the compound undergoes an allylic shift, expulsing the 1-substituent and leaving a 5-thioether 1,3-pentadien-2-yl ketone. The other end of the thiol then adds to the ketone in conjugate.
MeOH is methanol solvent; ( i ‑Pr) 2 EtN is catalytic diisopropylethylamine
Electrophilic allyl shift
A diene epoxide (from Jacobsen epoxidation) adds a pyrazole with an allylic shift. Then methylmagnesium bromide expulses the pyrazole with another allylic shift, returning the remaining double-bond to its original position.