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. For example, reaction of 1-chloro-2-butene with sodium hydroxide gives 2-buten-1-ol and 3-buten-2-ol:

In the similar substitution of 1-chloro-3-methyl-2-butene, the secondary 2-methyl-3-buten-2-ol is produced in a yield of 85%, while that for the primary 3-methyl-2-buten-1-ol is 15%.

Allylic shifts occur because the transition state is an allyl intermediate. In other respects they are similar to classical nucleophilic substitution, and admit both bimolecular and monomolecular mechanisms (respectively the S_N2' and S_N1'/S_Ni' substitutions).

Scope

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. The effects of substitution at the vinyl group are less clear.^[1]

Although rarer still than S_N', allylic shifts can occur vinylogously, as a "butadienylic shift":^[2]

S_N2' reduction

In S_N2' reduction, a

The

Electrophilic allyl shifts

Allyl shifts can also take place with

The active catalyst system in this reaction is a combination of a

Examples

Repeated allylic shifts can "flip-flop" a double-bond between two possible locations:^[5]

An S_N2' reaction should explain the outcome of the reaction of an aziridine carrying a methylene bromide group with methyllithium:^[6]

In this reaction one equivalent of acetylene is lost.

Named reactions

• Ferrier rearrangement
• Meyer–Schuster rearrangement

References