“…New chiral N , N ‘-, N , P -, and P , P ‘-ligands for Rh, derived from l -proline and l - trans -4-hydroxyproline have been tethered to the internal surface of a mesoporous US-Y (Scheme ). − 33 …”
Section: Covalent Approachmentioning
confidence: 99%
“…Due to geometrical constraints, substrate specificity can be even more pronounced in the confinements of the zeolitic material. A remarkable promoter effect of the zeolite on the enantioselectivity was observed when prochiral olefins were used . The less bulky the prochiral olefin, the larger the difference in enantiomeric excess between the homogeneous and heterogeneous system.…”
instance, by Venuto, Ho ¨lderich, and Jacobs for zeolite reactions, 7 or by Brunel, Corma, and Ying for the mesoporous molecular sieves; 8 however, comparison with the state-of-the-art at the end of 2001 shows that the field has rapidly expanded over the past few years.
“…New chiral N , N ‘-, N , P -, and P , P ‘-ligands for Rh, derived from l -proline and l - trans -4-hydroxyproline have been tethered to the internal surface of a mesoporous US-Y (Scheme ). − 33 …”
Section: Covalent Approachmentioning
confidence: 99%
“…Due to geometrical constraints, substrate specificity can be even more pronounced in the confinements of the zeolitic material. A remarkable promoter effect of the zeolite on the enantioselectivity was observed when prochiral olefins were used . The less bulky the prochiral olefin, the larger the difference in enantiomeric excess between the homogeneous and heterogeneous system.…”
instance, by Venuto, Ho ¨lderich, and Jacobs for zeolite reactions, 7 or by Brunel, Corma, and Ying for the mesoporous molecular sieves; 8 however, comparison with the state-of-the-art at the end of 2001 shows that the field has rapidly expanded over the past few years.
“…Since the preparation of such materials has been so far unsuccessful, the accommodation of chiral guests inside the pores which either act themselves as catalytically active sites or induce chirality onto an adjacent site is required. Enantioselective catalysis on zeolite-supported systems has been demonstrated with chiral complexes anchored in ultrastabilized zeolite Y [219][220][221] or platinum-loaded zeolites modified by cinchonidine [222].…”
Section: Stereoselective Catalysis In Zeolitesmentioning
ScopeAmong the primary tasks of catalysis is the control of the selectivity in chemical reactions. In heterogeneously catalyzed reactions in zeolites and zeolite-related microporous solids, this can, inter alia, be achieved by exploiting the phenomenon of shape-selective catalysis. In a very simplified manner, shape-selective catalysis can be described as the combination of catalysis with the molecular sieve effect. Shape selectivity effects can occur, if the sizes and shapes of reactants, of products, of transition states or of reaction intermediates are similar to the dimensions of the pores and cavities of the zeolite.The first examples of shape-selective catalysis in zeolites were reported more than 35 years ago by Weisz and Frilette from the Mobil laboratories [1,2]. Since those days, research in the field of shape-selective catalysis has expanded both at universities and in industrial laboratories in an impressive manner. This has resulted in numerous examples of shape selectivity effects in the course of catalytic conversions over zeoli tic catalysts, which are nowadays no longer restricted to acid-catalyzed reactions as described in the early days. Rather, there are also examples for shape-selective catalysis on metals, on redox active sites, on basic guests in zeolites, and even for shape-selective photochemistry in a zeoli tic environment. Moreover, from the intensive research done in the field of shapeselective catalysis during the last decades, several large-scale commercial applications in the refining and the petrochemical industries have emerged, demonstrating the usefulness and importance of the phenomenon of shape-selective catalysis.In the past, numerous review papers on shape-selective catalysis, its principles and its commercial applications have been published [e. g., 3 -19]. In view of the vast amount of work that has been done in this field in recent years, it is obvious that a review on shape-selective catalysis cannot be exhaustive. Rather, the discussion has to be restricted to selected topics. In the present contribution, emphasis will be placed on a thorough discussion of the classical and of more recent principles of shape-selective catalysis and on how these principles can be used to rationalize the observed catalytic behavior in selected examples. In addition, it is the aim of the present contribution to discuss the available methods for tailoring the shape-selective properties of zeolite catalysts and for their characterization by catalytic methods. Finally, an attempt will be under-
“…Asymmetric catalysis, either chemical or enzymatic, has so far the greatest potential for obtaining optically active molecules. In the 1990s, Thomas [39,122,123] and Corma [38,[124][125][126] reported enhanced enantioselectivity in hydrogenation reactions using functionalized solid catalysts, which was attributed to the confinement of catalytically active organometallic species grafted in the pores of mesoporous silica or zeolites. Inspired by these pioneering examples, MOFs offer new opportunities to support catalytic species with the additional advantages of a possible discrete variation of the pore size as well as well-defined single sites, allowing a complete characterization of the catalytic species such as for molecular catalysts.…”
Section: Applications In Asymmetric Synthesismentioning
confidence: 99%
“…Intense efforts have been devoted to the discovery of aluminophosphates and zeolites with very large pores, such as VPI-5 [35], IM-12 [36] (IM, IFP-Mulhouse), and ITQ-33 (ITQ, Instituto de Tecnología Química) [37]. On the other hand, mesoporous silicate (MS) materials, such as MCM-41 (MCM, mobile crystalline material), have pores too large to impose confinement effects [38,39]. MOF materials actually bridge the gap between these two porous material types because MOFs with pore systems ranging from the ultramicroporous to mesoporous have been reported, as shown in the selection in Figure 10.2.…”
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