Cyclizations zeolite-catalyzed

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. 

Other examples of zeolite-catalyzed cyclizations include the Fischer indole synthesis [80] and Diels-Alder reactions [81]. 

HZSM5 zeolites catalyze the transformation of propane aromatic products. This aromatization requires various steps of propene, oligomerization of propene, cyclization of oligomers and hydrogen transfer from naphtenes to olefinic compounds with formation of aromatics. The cracking of Cg-Cg oligomers leads to C2-C5 olefins which, like propene, participate in the formation of aromatics. 

The acidity of the phosphorus doped B-pentasil zeolite catalyze the nucleophilic substitution of ammonia to the carbonic acid and the elimination of water. The structural properties of the catalyst prevent the cyclization reactions. This reaction is a typical, almost classical example for restricted transition state shape selectivity. 

Sames et al. studied the cyclization of phenyl/propargyl ethers catalyzed by different metal salts since this Friedel-Crafts alkenylation had previously been reported to be catalyzed by metals such as Pd, Zr In and Sc [122-124] or even zeolites by hereogeneous catalysis [125]. In this preliminary research, the maximum yield of the desired product achieved by gold was only 6%, the best results being obtained with PtCl2 as catalyst [126]. 

The gas phase acid-catalyzed synthesis of pyridines from formaldehyde, ammonia and an alkanal is a complex reaction sequence, comprising at least two aldol condensations, an imine formation, a cyclization and a dehydrogenation (9). With acetaldehyde as the alkanal, a mixture of pyridine and picolines (methylpyridines) is formed. In comparison with amorphous catalysts, zeolites display superior performance, particularly those with MFI or BEA topology. Because formation of higher alkylpyridines is impeded in the shape-selective environment, the lifetime of zeolites is much improved in comparison with that of amorphous materials. Moreover, the catalytic performance can be enhanced by doping the structure with metals such as Pb, Co or Tl, which assist in the dehydrogenation. 

Zeolites have also been shown to catalyze a variety of acid-promoted cyclizations. Many of these involve the formation of N-heterocycles via intramolecular amination reactions [75-77]. Some examples are shown in Fig. 2.24. 

II-Beta zeolite, loaded with 3 wt % Ir.catalyzes the one-pot full conversion of citronellal to menthol by consecutive acid catalysed cyclization and Ir-catalyzed hydrogenation, with 95 % selectivity for the menthol isomers of which 75 % is the desired (-)-mcnthol [81]. Conditions The catalytic tests were performed at 80 °C in a stirred stainless steel autoclave using IB pressures in the range 0.5-1.5 Mpa, 50 mg catalyst, 1 ml citronellal and 7 ml solvent (cyclohexane or 2 propanol). 

Zeolites are known to catalyze the formation of various nitrogen-containing aromatic ring systems. Examples include the synthesis of pyridines by dehydrogenation / condensation / cyclization of C -Cg precursors [1], the formation of methylpyridines by high-temperature isomerization of anilines [2], the amination of oxygen-containing heterocyclic compounds [3] and the Fischer indole synthesis [4,5]. The latter synthesis consists (see Scheme 1) of a condensation towards a phenylhydrazone followed by an acid-catalyzed cyclization with elimination of ammonia. The two reaction steps are usually combined in a one-pot procedure. 

Olah et al. reported the triflic acid-catalyzed isobutene-iso-butylene alkylation, modified with trifluoroacetic acid (TEA) or water. They found that the best alkylation conditions were at an acid strength of about Ho = —10.7, giving a calculated research octane number (RON) of 89.1 (TfOH/TFA) and 91.3 (TfOH/HzO). Triflic acid-modified zeolites can be used for the gas phase synthesis of methyl ferf-butyl ether (MTBE), and the mechanism of activity enhancement by triflic acid modification appears to be related to the formation of extra-lattice Al rather than the direct presence of triflic acid. A thermally stable solid catalyst prepared from amorphous silica gel and triflic acid has also been reported. The obtained material was found to be an active catalyst in the alkylation of isobutylene with w-butenes to yield high-octane gasoline components. A similar study has been carried out with triflic acid-functionalized mesoporous Zr-TMS catalysts. Triflic acid-catalyzed carbonylation, direct coupling reactions, and formylation of toluene have also been reported. Triflic acid also promotes transalkylation and adaman-tylation of arenes in ionic liquids. Triflic acid-mediated reactions of methylenecyclopropanes with nitriles have also been investigated to provide [3 + 2] cycloaddition products as well as Ritter products. Triflic acid also catalyzes cyclization of unsaturated alcohols to cyclic ethers. ... 

SCHEME 8 Cyclization of bulky imsaturated alcohols (a) 6-methylhept-5-en-2-ol (15) and (b) 2-allylphenol (16) catalyzed by zeolites. 

Although the intramolecular cyclization of unsaturated alcohols can be catalyzed by both Lewis and Bronsted acid sites in zeolites, the reported experimental results interestingly indicated that the largest conversions to the corresponding tetrahydrofurans were achieved for zeolites showing an optimum concentration of Bronsted sites (0.15-0.20 mmol g" ). This circumstance was rationalized by computational methods as well it was proposed that the... 

Nevertheless, such reactions catalyzed by zeolites have been discussed in the review of 2001 (1) isomerization (double-bond shift, isomerization of tricyclic molecules, like synthesis of adamantane, isomerization of terpenes, diverse rearrangements, conversion of aldehydes into ketones), (2) electrophilic substitution in arenes (alkylation of aromatics, including the synthesis of linear alkylbenzenes, alkylation and acylation of phenols, heteroarenes and amines, aromatics nitration and halogenation), (3) cyclization, including the formation of heterocycles, Diels-Alder reaction, (4) nucleophilic substitution and addition,... 

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