Synthesis, structure and pyrolysis of stabilised phosphonium ylides containing saturated oxygen heterocycles

Abstract

A range of twelve stabilised phosphonium ylides containing tetrahydrofuran, tetrahydropyran or 2,2-dimethyl-1,3-dioxolane rings have been prepared and fully characterised, including one X-ray structure determination of each type. The X-ray structures confirm the PC and CO functions to be syn and all the compounds undergo thermal extrusion of Ph₃PO to give the corresponding alkynes. In some cases there is also competing loss of Ph₃P to give different carbene-derived products and evidence has been obtained for the generation of 2-phenyloxete in this way. Raising the pyrolysis temperature leads in several cases to new secondary reactions of the alkyne products involving a sequence of alkyne to vinylidene isomerisation, intramolecular CH insertion, and retro Diels Alder reaction.