Reactions of phosphines with acetylenes. Part XII. The mechanisms of 1,2-aryl migrations common to αβ-unsaturated phosphonium salts and “Wittig-type” phosphonium betaines

The reactions of (a) methylenetriphenylphosphorane with benzaldehyde in alcohol, (b) triarylphosphines with styrene oxide in alcohol, and (c) triarylphosphines with electrophilic acetylenes and water, and (d) the alkaline hydrolysis of αβ-unsaturated phosphonium salts, all involve the formation of an equilibrium mixture of a betaine, a hydroxyethylphosphonium salt, an αβ-unsaturated phosphonium hydroxide, and the corresponding quinquecovalent oxyphosphorane. Under mildly basic conditions the products arise mainly from the base-catalysed rearrangement of the vinyloxyphosphoranes, but under more basic conditions decomposition of the betaine predominates.