Catalysis of Organic..

Moses et al. 19(AlOSi) of the cube. This predicted structure is practically the same as thatdetermined experimentally by direct deposition of MeReO 3 on amorphous silicaalumina,as demonstrated by EXAFS (5).Since MeReO 3 chemisorbs irreversibly on silica-alumina, we did not expect tobe able to retrieve it as a soluble species upon SnMe 4 treatment of perrhenate/silicaalumina.However, exposure of this material to SnMe 4 resulted in an immediatecolor change from white to brown, similar to that observed upon treatment ofunmodified silica-alumina with MeReO 3 . Evidence for the possible involvement ofthe in situ-generated MeReO 3 in olefin metathesis was then sought via reactivitystudies.Metathesis of functionalized olefins. SnR 4 -promoted perrhenate catalysts areknown to promote the metathesis of olefins bearing polar functional groups (1). Theactivity of silica-alumina-supported MeReO 3 towards functionalized olefins was alsotested, in the homometathesis of methyl-3-butenoate, eq 3. The reaction wasconducted at 15°C in pentane. The progress of the reaction was followed bymonitoring the evolution of ethylene in the head space, Figure 1, yielding thepseudo-first-order rate constant k obs = (1.3 ± 0.3) x 10 -3 s -1 . The presence of the nonvolatiledimethyl ester of hex-3-enedoic acid in the liquid phase was confirmed at theend of the reaction by GC/MS.2OOOOOO+(3)P(C2H4)0 20 40 60 80 100time / minFigure 1 Time-resolved evolution of ethylene during homometathesisof methyl-3-butenoate, catalyzed by 10 mg MeReO 3 /SiO 2 -Al 2 O 3 (8.8 wt% Re) in pentane at15°C. The solid line is the curve-fit to the first-order integrated rate equation.