Diastereoselective acyclic aza-[2,3] Wittig sigmatropic rearrangements

Abstract

The scope of anion-stabilising groups in promoting and controlling the diastereoselection of the aza-[2,3] Wittig sigmatropic rearrangement has been assessed by the syntheses of allylic amines 1c–g that have incorporated SiPhMe₂, Ph, SPh, SOPh and SO₂Ph respectively at the C-2 position. The subsequent anionic [2,3]-sigmatropic rearrangements are analysed with respect to the extent of diastereoselection of the product homoallylic amines 2 and 3. It appears that silicon not only is the most efficient at electronically facilitating this rearrangement, but its steric bulk controls the diastereoselectivity of the process exclusively. Phenyl and phenylthio substituents also have a similar, but decreased, effect on diastereoselection that mirrors their lower steric bulk in comparison to the silicon derivative. Sulfoxide and sulfone substituents are incompatible with the reaction conditions required for rearrangement.