The mechanism of the catalytic metathesis reaction proceeds via reaction of the olefin substrate with a metal carbene intermediate, which may be generated in situ

(as is the case for heterogeneous catalysts based on supported metal oxides and early homogeneous catalysts based on mixtures of metal halide and a main group alkylating agent), or prior to addition of the substrate (as is the case for ‘welldefined’ homogeneous catalysts such as those of Grubbs’ and Schrock). A supported organometallic catalyst, MeReO3 on silica-alumina, has also been reported to show activity in olefin metathesis. In solution, MeReO3 does not react with α-olefins, nor does the silica-alumina support catalyze olefin metathesis. However, MeReO3 supported on silica-alumina is effective for the metathesis of both simple and functionalized olefins at room temperature, without further thermal or chemical activation (2-4). Deposition of white, air-stable MeReO3 either by sublimation or from solution onto calcined, dehydrated silica-alumina generates a brown, air-sensitive solid. Evidence from both EXAFS and DFT calculations suggest that Lewis acidic aluminum centers on silica-alumina represent the most favorable chemisorption sites (5). One Re=O bond is substantially elongated due to its interaction with a distorted four-coordinate Al site. Coordination of Re to an adjacent bridging oxygen also occurs, creating the rigidly-bound surface organometallic fragment shown in Scheme 1. Interaction with a Lewis acid is known to promote tautomerization of MeReO3 (6), leading (at least transiently) to a carbene. However, the participation of this carbene tautomer in initiating metathesis has not been established.