7431-74-5Relevant articles and documents
Immobilized Grubbs catalysts on mesoporous silica materials: Insight into support characteristics and their impact on catalytic activity and product selectivity
Dewaele, Annelies,Van Berlo, Boris,Dijkmans, Jan,Jacobs, Pierre A.,Sels, Bert F.
, p. 2580 - 2597 (2016/06/06)
Silica materials show a high ability to physisorb the 2nd generation Hoveyda-Grubbs catalyst (HG2) in organic solvents. The interaction with the complex, likely proceeding through hydrogen bonding, is particularly strong with surfaces rich in silanols, wherein geminal silanols show the highest affinity, and therefore mesoporous silicas are the supports of choice. As long as the silica material is sufficiently pure and free of cages, in which high HG2 concentrations can accumulate, the immobilization of HG2 occurs in a very stable manner. Despite the complex stability, exploration of HG2-loaded mesoporous silica supports in metathesis of cis-cyclooctene indicated significant diffusional and confinement effects, and therefore control of pore size, pore architecture and morphology in balance with the intrinsic catalytic activity is essential for catalyst design. As metathesis of cis-cyclooctene apparently proceeds through the initial formation of linear polymers, followed by backbiting forming cyclic oligomers, potential interference of mass transport and space restriction issues is not surprising. This study shows that the catalyst requirements are best met with the TUD-1 silica support (1.24 wt% HG2). Under such conditions, the heterogeneous catalyst performs as good as the homogeneous one, presenting a thermodynamic distribution of cyclic oligomers. The latter catalyst also showed high catalyst stability in a continuous fixed bed reactor, corresponding to a catalytic turnover number of 18 000. The catalytic rates and catalyst stability are lower when operating in a diffusional regime, therefore long reaction times are required to reach the thermodynamic product distribution. Water removal from the catalyst is also important, not because of HG2 stability reasons, but of lower reaction rates which were measured for hydrated samples, likely due to inhibition of cis-cyclooctene uptake in the pores. Mild removal of physisorbed water before immobilization is therefore advised, for instance by thermal treatments, but care has to be taken to keep the silanol density high for firm HG2 immobilization and also to avoid formation of reactive siloxanes, which chemically react with and destroy HG2. Surprisingly, reactive siloxane formation conditions strongly depend on the silica type, with TUD-1 being fairly sensitive to their formation. Finally, the best HG2-loaded TUD-1 catalyst is used successfully in a broad set of other metathesis reactions.
Ruthenium carbenes supported on mesoporous silicas as highly active and selective hybrid catalysts for olefin metathesis reactions under continuous flow
Bru, Miriam,Dehn, Richard,Teles, J. Henrique,Deuerlein, Stephan,Danz, Manuel,Mueller, Imke B.,Limbach, Michael
, p. 11661 - 11671 (2013/09/12)
In the search for a highly active and selective heterogenized metathesis catalyst, we systematically varied the pore geometry and size of various silica-based mesoporous (i.e., MCM-41, MCM-48, and SBA-15) and microporous (ZSM-5 and MWW) versus macroporous materials (D11-10 and Aerosil 200), besides other process parameters (temperature, dilution, and mean residence time). The activity and, especially, selectivity of such "linker-free" supports for ruthenium metathesis catalysts were evaluated in the cyclodimerization of cis-cyclooctene to form 1,9-cyclohexadecadiene, a valuable intermediate in the flavor and fragrance industry. The optimized material showed not only exceptionally high selectivity to the valuable product, but also turned out to be a truly heterogeneous catalyst with superior activity relative to the unsupported homogeneous complex. In the ring-opening/ring-closing metathesis (RO-RCM; see picture) of cis-cyclooctene, mesoporous silicas (i.e., MCM-41, MCM-48, SBA-15) are preferred supports for homogeneous ruthenium carbenes (see picture). The optimized "linker-free" support showed not only exceptionally high selectivity toward 1,9-cyclohexadecadiene, a valuable intermediate in the flavor and fragrance industry, but is also a truly heterogeneous catalyst with superior activity relative to the unsupported homogeneous complex. Copyright
Olefin metathesis on a TLC Plate as a Tool for a High-Throughput Screening of Catalyst-Substrate Sets
Cabrera, Jose,Padilla, Robin,Dehn, Richard,Deuerlein, Stephan,Gulajski, Lukasz,Chomiszczak, Ewa,Teles, J. Henrique,Limbach, Michael,Grela, Karol
experimental part, p. 1043 - 1051 (2012/05/20)
A methodology for screening either various catalysts for a given metathesis reaction, i.e., ring opening-ring closing alkene metathesis (RO-RCM) and cross-metathesis (CM), or various substrates for a given pre-catalyst on a thin layer chromatography (TLC) plate has been developed. As the substrates elute with the solvent, this TLC-based system acts as a heterogeneous catalyst bed ("TLC reactor"). Selected promising catalyst candidates were screened on a TLC plate and their initial catalytic potential as observed in the TLC test was later fully confirmed in a classical heterogeneous reaction set-up using standard commercially available silica (D11-10). Reacting polyfunctional, natural product-like substrates in our TLC reactor allows the simultaneous screening of various substrates and the convenient micro-scale preparation and isolation of potentially biologically active products. Copyright
