Catalysis of Organic..

Moses et al. 15perrhenate, followed by α-H elimination. The formation of tin perrhenates has alsobeen discussed (7). The question of the presence (or not) of tin in the active site iscrucial, in view of the detrimental effect of tin on the ability to regenerate thedeactivated catalyst.Experimental SectionMaterials. Methyltrioxorhenium, NH 4 ReO 4 and Re 2 O 7 were purchased from Aldrichand used as received. The silica-alumina was Davicat 3113 (7.6 wt.% Al, BETsurface area 573 m 2 /g, pore volume 0.76 cm 3 /g), provided by Grace-Davison(Columbia, MD). For reactions involving MeReO 3 , silica-alumina was pretreated bycalcination for 12 h under 350 Torr O 2 at 450°C to remove adsorbed water,hydrocarbons, and carbonates, then allowed to cool to room temperature underdynamic vacuum. The silica was Aerosil 200 (BET surface area 180 m 2 /g, with nosignificant microporosity) from Degussa (Piscataway, NY).Hexamethyldisilazane (>99.5%, Aldrich) and tetramethyltin (>99%, Aldrich)were both subjected to several freeze-pump-thaw cycles to remove dissolved gasesand stored over P 2 O 5 in evacuated glass reactors. A solution of 0.25 mL methyl-3-butenoate (Aldrich) in 2 mL dry hexanes (Fisher) was prepared under N 2 and driedby stirring over P 2 O 5 before use. Propylene (99.5%, polymer grade, Matheson Tri-Gas) was purified by passing through a trap containing BTS catalyst (Fluka) andactivated molecular sieves (4 Å, Aldrich), and was stored in a Pyrex bulb overactivated sieves.Catalyst preparation. All operations were performed with strict exclusion of airand moisture, either on a high vacuum line (base pressure 10 -4 Torr) or in a N 2 -filledglove box equipped with O 2 and moisture sensors. Silica-alumina-supportedperrhenate catalysts were prepared by stirring silica-alumina with aqueousammonium perrhenate, air-drying at 80 o C for >1 hr then drying under dynamicvacuum at 450 o C for 4 hrs, followed by calcination in 250 Torr O 2 at 450°C for 16 h,heating under dynamic vacuum at 450 o C for 1 hr, and cooling to room temperatureunder vacuum. MeReO 3 was deposited on calcined silica-alumina either by vapordeposition under reduced pressure, or from a solution of dry hexanes (1 mg/mL)prepared under N 2 . In both methods, the white silica-alumina acquired a light browncolor and became highly air-sensitive.Re loadings were determined by quantitative extraction, followed by UVspectrophotometric analysis. Samples containing ca. 30 mg silica-alumina were firstweighed in an inert atmosphere. (The mass of calcined silica-alumina increases up to15% upon exposure to air, due to adsorption of atmospheric moisture.) Re wasextracted as perrhenate by stirring overnight in air with 5 mL 3 M NaOH. Sampleswere diluted to 25 mL with 3 M H 2 SO 4 and filtered. The Re concentration wasdetermined at 224 nm, using a calibration curve prepared with NH 4 ReO 4 .