Zeolite in organic synthesis

Moreover, recently the application of zeolites in organic synthesis demonstrated their ability to achieve reactions necessitating usually superelectrophilic intermediates as in superacid media.144 These results cannot be rationalized on the basis of the acidic character alone and needs the understanding of confinement effects145 in which case the zeolite cage behaves like a nanosized reactor favoring the contact between reactant and specific acidic sites. The electrostatic effects may also be important. 

In our on-going studies on the applications of zeolites in organic synthesis, [. i] we have investigated the potential for modified zeolites in this reaction in combination with molecular oxygen, as compared to the previous studies on homogeneous systems. The goal of this work is to find conditions which result in high selectivities to either of the products (I) or (2). 

Holderich, W.F., and H. van Bekkum, 1991, Zeolites in organic synthesis, in Introduction to Zeolite Science and Practice, eds H. van Bekkum, E.M. Flanigen and J.C. Jansen, Vol. 58 of Studies in Surface Science and Catalysis (Elsevier, Amsterdam) pp. 631-726. 

The application of zeolites in organic synthesis can be classified in three distinct categories ... 

Zeolites as catalysts and promoters in organic bromide synthesis and conversion In the second part of this contribution the use of zeolites in the synthesis and conversion of organic bromides will be discussed (ref. 16). 

II. Zeolites, Clays, and Silica Gel as Heterogeneous Catalysts and Their Use in Organic Synthesis... 

Zeolites are, perhaps, the most commonly used catalysts in industry for oil refining, petrochemistry, and in organic synthesis for the manufacture of fine chemicals [122-126],... 

Pertinent examples of zeolite-catalyzed reactions in organic synthesis include Friedel-Crafts alkylations and acylations and other electrophilic aromatic substitutions, additions and eliminations, cyclizations, rearrangements and isomeriza-tions, and condensations. 

The present example takes advantage of the specific affinity between a substrate with polyfunctional groups and a metal ion in the zeolite, and this type of reaction represents a novel aspect of zeolite catalysis in organic synthesis. 

The present method, which requires only readily available reagents (bromine, zeolite SA, and organic base) and can be operated under mild reaction conditions, appears to be useful for the selective bromination of various aromatic amines in organic synthesis [Eq. (10)]. 

Hoelderich, W. F. Potential of Zeolites as Catalysts in Organic Synthesis. Stud. Surf. Sci. 

The use of microporous solid catalysts such as zeolites and related molecular sieves has an additional benefit in organic synthesis. The highly precise organization and discrimination between molecules by molecular sieves endows them with shape-selective properties [12] reminiscent of enzyme catalysis. The scope of molecular sieve catalysis has been considerably extended by the discovery of ordered mesoporous materials of the M41S type by Mobil scientists [13,14]. Furthermore, the incorporation of transition metal ions and complexes into molecular sieves extends their catalytic scope to redox reactions and a variety of other transition metal-catalyzed processes [15,16]. 

Topics which have formed the subjects of reviews this year include excited state chemistry within zeolites, photoredox reactions in organic synthesis, selectivity control in one-electron reduction, the photochemistry of fullerenes, photochemical P-450 oxygenation of cyclohexene with water sensitized by dihydroxy-coordinated (tetraphenylporphyrinato)antimony(V) hexafluorophosphate, bio-mimetic radical polycyclisations of isoprenoid polyalkenes initiated by photo-induced electron transfer, photoinduced electron transfer involving C o/CjoJ comparisons between the photoinduced electron transfer reactions of 50 and aromatic carbonyl compounds, recent advances in the chemistry of pyrrolidino-fullerenes, ° photoinduced electron transfer in donor-linked fullerenes," supra-molecular model systems,and within dendrimer architecture,photoinduced electron transfer reactions of homoquinones, amines, and azo compounds, photoinduced reactions of five-membered monoheterocyclic compounds of the indigo group, photochemical and polymerisation reactions in solid Qo, photo- and redox-active [2]rotaxanes and [2]catenanes, ° reactions of sulfides and sulfenic acid derivatives with 02( Ag), photoprocesses of sulfoxides and related compounds, semiconductor photocatalysts,chemical fixation and photoreduction of carbon dioxide by metal phthalocyanines, and multiporphyrins as photosynthetic models. 

Alumina, silica, clay, and zeolites are increasingly bring used as acidic or basic supports in organic synthesis [3f, 13c, 33]. Several groups have reported improve-... 

The aldol condensation is an important reaction in organic synthesis leading to the formation of C-C bonds. This reaction is catalyzed by zeolites (121-127). To understand the mechanisms involved, various spectroscopic methods (122-125,... 

Recently, for the first time, copper(I)-exchanged zeolites were developed as catalysts in organic synthesis. These sohd materials proved to be versatile and efficient heterogeneous ligand-free catalytic systems. Cheap and easy to prepare, these catalysts exhibited a wide scope and a good compatibility with functional groups. They are very simple to use, easy to remove (by simple filtration) and they are recyclable. 

The application potential of zeolite catalysts in organic synthesis is by no means exhausted, and base catalysis remains practically unexplored. Thus the zeolites still have huge potential for future research and development. 

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