Catalysis of Organic..

Moses et al. 212.510 3 k obs / s -12.01.51.00.50.00.0 0.5 1.0 1.5 2.0Re loading (wt. %)Figure 3 Dependence of pseudo-first-order rate constants measured at 0°C forpropylene homometathesis, on the Re loading in 10 mg samples of two kinds ofsupported Re catalysts: SnMe4-promoted perrhenate/silica-alumina (solid circles)and MeReO 3 on HMDS-capped silica-alumina (open circle).The rate of reaction of propylene over the MeReO3/HMDS/silica-aluminacatalyst (1.4 wt% Re) is shown in Figure 2b. The profile is similar to that of the Sn-perrhenate catalyst, with k obs = (1.78 ± 0.09) x 10 -3 s -1 , and the activitypromotedresponds similarly to subsequent additions of propylene. In fact, the pseudo-firstorderrate constant for the organometallic catalyst lies on the same line as the rateconstants for the Sn-promoted perrhenate catalyst, Figure 3. Therefore we infer thatthe same active site is generated in both organometallic and promoted inorganiccatalyst systems.ConclusionsThe molecular role of the SnMe 4 promoter, which activates supported perrhenatemetathesis catalysts and confers functional group tolerance, appears to be to generateMeReO 3 in situ. The promoted inorganic catalyst is kinetically indistinguishablefrom an organometallic catalyst made directly from MeReO 3 . The organotin reagentsimultaneously caps the surface hydroxyls, by a mechanism analogous to thereaction of HMDS with Brønsted sites. We conclude that a bimetallic (Sn/Re) activesite is not required for the metathesis of polar olefins; consequently design ofregenerable catalysts without Sn is feasible. Understanding the detailed mechanismof olefin metathesis by MeReO 3 will be key to creating highly active and robust solidcatalysts for the metathesis of functionalized olefins.AcknowledgementsThis work was funded by the U.S. Department of Energy, Basic EnergySciences, Catalysis ScienceGrant No. DE-FG02-03ER15467.