Catalysis of Organic..

Moses et al. 132. Supported Re Catalysts forMetathesis of Functionalized OlefinsAbstractAnthony W. Moses, a Heather D. Leifeste, a Naseem A. Ramsahye, aJuergen Eckert b and Susannah L. Scott aa Department of Chemical Engineering and b Department of Materials,University of California, Santa Barbara CA 93106-5080sscott@engineering.ucsb.eduThe molecular role of organotin promoters, which confer functional group toleranceon supported Re catalysts for olefin metathesis, was explored through spectroscopicand computational analysis, as well as kinetic studies. On dehydrated silica andsilica-alumina, the addition of SnMe 4 results in two surface reactions: (i) in situgeneration of MeReO 3 ; and (ii) capping of Brønsted acid sites. The former isresponsible for catalytic activity towards polar α-olefins; thus, an independentlypreparedsample of MeReO 3 /silica-alumina catalyzed the homometathesis of methyl-3-butenoate. The latter stabilizes the catalyst: in sequential batch reactor testsinvolving propylene homometathesis, MeReO 3 deposited on silica-alumina cappedwith hexamethyldisilazane (to eliminate Brønsted acidity) showed activity identicalto that of the perrhenate/silica-alumina catalyst promoted with SnMe 4 . Thus, acompletely Sn-free catalyst performs metathesis as efficiently as the organotincontainingperrhenate catalyst.IntroductionCatalysts for olefin metathesis are used in relatively few large-scale industrialprocesses (e.g., SHOP, OCT). A few more applications are found in specialtychemicals (e.g., neohexene) and engineering plastics (e.g., PDCD). The economicsof practicing metathesis on a commercial scale are impacted by the low activationefficiency and rapid deactivation of known heterogeneous catalysts, typically Mo, Wor Re dispersed as oxides on supports such as silica and alumina. Furthermore, thesecatalysts are intolerant of polar functional groups, making it impossible to extendmetathesis processing to biorenewable feedstocks such as seed oils. One notableexception is Re-based catalysts promoted by alkyltin or alkyllead reagents, whichshow modest activity for metathesis of functionalized olefins (1). However, oncethese catalysts deactivate, they are not regenerable by calcination. Thus there isconsiderable need for longer-lived, highly active heterogeneous catalysts that toleratepolar groups.The mechanism of the catalytic metathesis reaction proceeds via reaction of theolefin substrate with a metal carbene intermediate, which may be generated in situ