Asymmetric Induction in Photochemical Reactions Conducted in Zeolites and in the Crystalline State

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-

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“…5.19) [225,226]. This was possible by employing a strategy which involved the use of chirally modified zeolites as the reaction medium.…”

Section: Host/guest Chemistry In Zeolitesmentioning

confidence: 99%

Shape-Selective Catalysis in Zeolites

Weitkamp

Puppe

1999

Catalysis and Zeolites

119

129

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“…For this purpose, the approach referred to as CIM, the nonchiral interior surface of the zeolite is rendered " locally chiral " by adsorption of a chiral inductor such as ephedrine. 49,52 This simple method affords easy isolation of the product, as the chiral inductor and the reactant are not connected through either a covalent or an ionic bond. The chiral inductor that is used to modify the zeolite interior will determine the magnitude of the enantioselectivity of the photoproduct.…”

Section: Chiral Inductor Methods ( Cim )mentioning

confidence: 99%

Controlling Photoreactions Through Noncovalent Interactions Within Zeolite Nanocages

Ramamurthy¹,

Sivaguru²

2011

Supramolecular Photochemistry

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Drawing inspiration from nature, chemists have employed confi nement as a tool to achieve selectivity in chemical reactions. 1 Confi ning reactants has the power to drive chemical reactions with astonishing precision as seen in the remarkable selectivities exhibited by enzymes in chemical processes, the phenomenon of photosynthesis, and so on. Within a confi ned environment, 2 the familiar electronic and steric effects of solution chemistry are replaced by structural and topological factors that frequently result in products that display a level of selectivity or specifi city that has hitherto been unobtainable in solution. Upon confi nement, inherent reactivity of confi ned guests often becomes of secondary importance compared with features such as symmetry, geometric considerations, and noncovalent interactions that develop within the confi ned environments. This review will highlight the confi nement effect offered by zeolite Y supercages, 3,4 the noncovalent interaction (viz., cation -π and cationcarbonyl interactions) 5,6 that develops upon inclusion of substrates within faujasite zeolite X/Y supercages, and the effect of such interactions in photophysical and photochemical processes). 7,8 ZEOLITE Y SUPERCAGES: NANOCAVITY THAT OFFERS NONCOVALENT INTERACTIONSZeolites 3 are inorganic crystalline aluminosilicate microporous materials (particle size ranges 0.1 -10 μ m, 40 naturally occurring and more than 100 synthetic 389Supramolecular Photochemistry: Controlling Photochemical Processes, First Edition. Edited by V. Ramamurthy and Yoshihisa Inoue.

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“…Performing the reaction in organized media such as zeolites or cyclodextrins. Applications have been reported in the field of Norrish II reactions (Scheme 14) [62][63][64], in the tropolone rearrangement (Scheme 14) [65], in the Paternò-Büchi reaction [66], photodimerization [67], and in photoisomerization reactions (Scheme 15); in this case the role of (-)-ephedrine in order to induce a chiral environment into the zeolite cavity has to be noted [68]. 4.…”

Section: Molecules With Prevented Mobilitymentioning

confidence: 99%

Concepts of Stereoselective Photochemistry and a Case Study: the Paterno-Buchi Reaction

D’Auria¹,

Racioppi

2009

COC

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The main methods reported in literature to obtain stereoselective photochemical reaction is reviewed. The most important approached attempted to obtain stereoselective photochemical reaction are:1.) the use of diastereoselective photochemical reactions, 2.) the use of molecules with prevented mobility (this approach can be obtained 1. introducing chiral auxiliary, 2. introducing chiral molecules in the reaction mixture able to give complexes with a reduced mobility, 3. performing the reaction in organized media such as zeolites or cyclodextrins, 4. performing the reaction on single crystal, 5. performing the reaction on single crystals of molecules able to give crystal in chiral space group, and, 6. finally performing the reaction in solid phase on inclusion complexes of the substrate with chiral molecules); 3.) using chiral solvents and chiral light, and 4.) using chiral photosensitizers.The case of the diastereoselective Paternò-Büchi reaction on furan derivatives is also discussed.

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Asymmetric Induction in Photochemical Reactions Conducted in Zeolites and in the Crystalline State

Cited by 83 publications

References 8 publications

Shape-Selective Catalysis in Zeolites

Shape-Selective Catalysis in Zeolites

Controlling Photoreactions Through Noncovalent Interactions Within Zeolite Nanocages

Concepts of Stereoselective Photochemistry and a Case Study: the Paterno-Buchi Reaction

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