Elimination–addition. Part XVIII. Acylation of amines and alcohols with adducts of carboxylic acids and allenic ′onium salts

Dimethylprop-2-ynylsulphonium bromide has been prepared; it isomerises readily in neutral or basic medium to the allenic isomer. Base-catalysed nucleophilic additions of carboxylic acids to the allenic salt occur readily and the adducts, which are activated vinyl esters, react rapidly with amines and alcohols to give amides and esters. In such reactions, the sulphonium salt can be prepared and used as an acylation catalyst in situ. Under certain conditions, dealkylation at the sulphonium centre accompanies acylation.Prop-2-ynyltriphenylphosphonium bromide can be used in a similar way but triethylprop-2-ynylammonium bromide is inert in the isomerisation–addition–acylation sequence.