Zeolites heterocyclic synthesis

In the same domain of heterocyclic synthesis, much effort has been devoted to understanding the selectivities in the synthesis of indoles starting from phenylhydrazine and ketones, i.e. the Fischer indole synthesis. With phenyl-hydrazine, l-phenyl-2-butanone and zeolite H-Beta, the slimmer product 2-phenyl-3-methylindole dominates over the more bulky isomer (13) ... 

Although zeolites have been known for their adsorption properties for over a century, it was not until 1952, when the first synthetic zeolite was prepared, that their utility in chemical transformations was explored. Since that time, zeolites have been used for a multitude of purposes, and to this day, they are essential catalysts in the petroleum industry, converting large and small hydrocarbons into high-octane compounds. As an outgrowth of this work, zeolites have found utility in industrial fine chemical synthesis for the construction of aromatics, heterocycles, aliphatic amines, and ethers, and the photochemistry within zeolites has already grown out of its infancy. 

The aeylative cyclization of phenol with acetic anhydride was carried out over CeY type zeolites. The reaction mechanism is given in Fig. 4. At 380°C, the yield of 4-methyl coumarin was 75 % at 81% conversion of phenol. N-Methylpyrrolidine was synthesized from 1,4-butanediol and methylamine over Cr ZSM-5 and modified ZSM-5 catalysts at 300 C. The reaction mechanism is given in Fig. 5. The synthesis of a number of five- and six- membered heterocyclics have been depieted in Table 1. The reaction was carried out at 250-400°C at 30-80 hydrogen atm., under down-flow fixed bed conditions. The yield of N-methyl piperazine was 90% at 95% conversion over ZSM-5 catalysts. Similarly 2-methyl pyrazine and piperazine were synthesized from propylene glycol and ethylenediamine over HZSM-5 (Fig. 6). 

Heterocyclic compounds can also be obtained in the vapor phase, with evident advantages in comparison with conventional liquid-phase reactions. For example, the synthesis of alkylquinolines has been recently reported using either zeolites... 

The synthesis of pyridine bases with solid-acid catalysts is of considerable commercial importance. In the future, zeolites and related catalysts can be expected to have an impact on other areas of heterocyclic chemistry, because they bring the combined benefits of high yields and environmentally clean processes. The use of these catalysts to introduce new functions into pre-formed heterocycles or to manipulate their side chains-a topic not addressed here-is an area worthy of more research activity. Hopefully, this brief review provides some insight into heterocyclic chemistry and encourages readers to pursue their own catalysis research in this fascinating and fruitful area. 

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. 

Katkar SS, Mobile PH, Gadekar LS, Arbad BR, Lande MK (2010) ZnO-beta zeolite mediated simple and efficient method for the one-pot synthesis of quinoxaline derivatives at room temperature. Cent Eur J Chem 8(2) 320-325. doi 10.2478/sll532-009-0151-7 Katoh A, Yoshida T, Ohkanda J (2000) Synthesis of quinoxaline derivatives bearing the styryl and phenylethynyl groups and application to a fluorescence derivatization reagent. Heterocycles 52 (2) 911-920. doi 10.3987/COM-99-S61... 

This chapter is aimed to offer an update of the most relevant and interesting catalytic processes involving zeolites, clays, mesoporous materials, particularly fxmctionalized mesoporous silicates and metallosilicates, MOFs, and carbon materials in the synthesis of bioactive heterocycles and related compoxmds. 

A. Jackowski, S.l. Zones, S.-J. Hwang, A.W. Burton, Diquaternary ammonium compounds in zeolite synthesis cyclic and polycyclic N-heterocycles connected by methylene chains, J. Am. Chem. Soc. 131 (2009) 1092-1100. 

V.V. Krishna, M. Kandepi, N. Narender, Synthesis of N-heterocyclic compormds over zeolite molecular sieve catalysts an approach towards green chemistry, Catal. Sci. Technol. 2 (2012) 471 87. 

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,... 

---