Intramolecular reactions. Part IX. Stereochemistry and mechanism of the rearrangement of acetylenic esters of sulphinic and sulphenic acids to allenes

Abstract

Sulphinic esters of prop-2-ynyl alcohols rearrange thermally to allenyl sulphones. Evidence for a cyclic mechanism for the rearrangement is discussed. (+)-(R)-1-Methylprop-2-ynyl toluene-p-sulphinate rearranges to (–)-buta-1,2-dienyl p-tolyl sulphone, whose absolute configuration, predicted on the basis of a cyclic mechanism for the rearrangement, agrees with that calculated from the polarisability sequence of substituents attached to the chiral system.

Sulphenic esters of prop-2-ynyl alcohols rearrange rapidly to allenyl sulphoxides, and the chirality of the allene group derived from an α-substituted prop-2-ynyl alcohol is the same as that generated in the sulphinate → sulphone rearrangement.

Sulphinic and sulphenic esters of but-2-yne-1,4-diol rearrange to 2,3-bis-sulphonyl- or sulphinyl-buta-1,3-dienes. Structures of the products obtained have been proven by synthesis.

Prototropic equilibration of four-carbon acetylenic–allenic aryl sulphones shows the following decreasing order of stability: ArSO₂·CH₂·C⋮CMe > ArSO₂·CH:C:CHMe > ArSO₂·C⋮C·CH₂Me.