A procedure identifying a polyacetylene initiator of olefin metathesis. The reactivities of metal carbenes toward alkenes and alkynes

Abstract

Abstract : FWhen phenylacetylene plus tungsten hexachloride initiate the metathesis of cyclopentene, poly(phenylacetylene) is found attached to the resulting polypentenamer chains. The average amount of this poly(phenylacetylene) does not vary as the growth of the polypentenamer chains is checked by added diphenylacetylene, showing it to be present as a block at their start. The implication is that the metathesis is initiated by the growning acetylene writing with the olefin, strong evidence for the hypothesis that the acetylene polymerizatrion is propagated by metal-carbenes. The variations that occur in the length of the chains and the amount of poly(phenylacetylene) attached to the average chain as the acetylene concentration is varied are analyzed for the underlying kinetic parameters. The stabilized metal-carbenes propagating the phenylacetylene polymerization react 3,000 times more quickly with phenylacetylene than with cyclopentene, whereas the less stabilized metal-carbenes propagating the cyclopentene, metathesis select 17-fold in the opposite direction. Diphenylacetylene is 26 times more effective than phenylacetylene in quenching the metathesis, and 2.4 times more effective than phenylmethylacetylene. The key to the experiments is the observation that diphenylacetylene in minute amounts quenches metathesis but does not initiate the reaction.