Wittig and Stevens Rearrangement

Three examples of an unusual 1,2 alkyl shift from oxygen to carbon, known as the [1,2]-Wittig rearrangement, are shown below. Here a powerful base generates a reactive carbanion alpha to an ether. An intramolecular shift of an alkyl or aryl group then creates a much more stable alkoxide anion, which in the last example eliminates cyanide anion. Many studies of the mechanism of this rearrangement have been conducted, and it has been established to be intramolecular. The initially created negative charge weakens the other carbon-oxygen bond, enabling a rapid radical-radical anion dissociation-recombination process to take place (green-shaded box).

A related reaction involving a 1,2-shift from nitrogen to carbon is known as the Stevens rearrangement. Many aspects of this transformation are similar to the Wittig rearrangement.