| The mechanism for the initiation of olefin metathesis over the heterogeneous catalyst rhenium heptoxide on {dollar}gamma{dollar}-alumina was studied, using a pulse catalytic microreactor. Of the currently proposed mechanisms for initial carbene formation, only a mechanistic scheme which requires the reactant olefin to contain an allylic hydrogen, and proceeds via an organometallic {dollar}pi{dollar}-allyl complex, predicts the first-formed olefins found in the reaction product mixture. The proposed {dollar}pi{dollar}-allyl mechanism is further substantiated by the finding that isotopically labeled ethylene, which contains no allylic hydrogen, did not self-initiate metathesis, but immediately underwent metathesis after the catalyst was activated with cis-2-pentene. |
