R.S. Downing

Catalytic syntheses of aromatic amines

Abstract The most important manufacturing processes for aniline and other bulk arylamines are based on the continuous catalytic hydrogenation of nitro compounds, employing heterogeneous copper, nickel or platinum-group metals. For more complex amines produced on a smaller scale, homogeneous catalysis with its greater possibilities for chemo- and regioselective hydrogenation is increasingly the method of choice. A newer alternative process of increasing importance for aniline and phenylenediamines is the nucleophilic amination of phenols. The catalytic amination of haloarenes has also been employed commercially. Amination processes also find application in the production of pyridines and pyrroles.

Stereoselective Meerwein–Ponndorf–Verley and Oppenauer reactions catalysed by zeolite BEA

Abstract Zeolite BEA has been studied as catalyst in the Meerwein–Ponndorf–Verley and Oppenauer (MPVO) reactions of substituted cyclohexanones and cyclohexanols. In the MPV reduction of 4- tert -butylcyclohexanone to 4- tert -butylcyclohexanol a high stereoselectivity (>95%) to the thermodynamically less stable cis -isomer was obtained while in the complementary Oppenauer oxidation the cis -alcohol was preferentially converted. This stereoselectivity is explained by transition-state selectivity imposed by the zeolite structure. The catalytic activity is related to Lewis-acid aluminium sites which are located in the micropores. FT-IR results indicate that these sites are related to aluminium atoms which are only partially bonded to the framework. The mechanism is proposed to involve a six-membered transition state in which both the alcohol and the carbonyl are coordinated to the same aluminium.

Application of zeolite titanium Beta in the rearrangement of α-pinene oxide to campholenic aldehyde

Zeolite titanium Beta is found to be an effective catalyst for the industrially relevant rearrangement of α-pinene oxide to campholenic aldehyde, giving selectivities of up to 89% in the liquid phase. In the gas phase, up to 94% selectivity is obtained at conversions above 95%, which exceeds the best values obtained with homogeneous Lewis acid catalysts. The selectivity obtained is profoundly influenced by the solvent or co‐adsorbate applied.

Shape-selective hydrogenation and hydrogen transfer reactions over zeolite catalysts

Abstract A review of the application of zeolite systems in shape-selective hydrogenation and hydrogen transfer reactions is presented. Two different types of catalytic systems are discussed. The first consists of metal clusters or coordination complexes encapsulated in the micropores of a zeolite employing hydrogen as the reductant. Recent developments, such as the application of zeolite-containing composite catalysts are included. In the second system, zeolites are applied as catalyst in hydrogen-transfer reactions using a secondary alcohol as hydrogen donor in the Meerwein–Ponndorf–Verley reduction.

Zeolite-catalysed rearrangements in organic synthesis

Results are presented on the use of zeolites as catalysts in organic rearrangements of interest in the synthesis of fine chemicals viz., the rearrangement of epoxides and α,β-epoxy ketones, the Claisen rearrangement of allyl aryl ethers and an intramolecular hydroxyalkylation of an aromatic with an epoxide.

New multifunctional probe for testing outer surface activity of zeolites: application to surface-located platinum clusters and acid sites

Abstract Allyl 3,5-di-tert-butylphenyl ether is being developed as a multifunctional probe for testing the outer surface activity of zeolites. Its use is demonstrated by the hydrogenation of the double bond over platinum/H-BEA, in which comparison with various non zeolite-based platinum catalysts enabled the proportion of platinum clusters located on the outer surface to be quantified. The Bronsted acid-catalysed Claisen rearrangement of the probe molecule, followed by the cyclization of the primary product to 4,6-di-tert-butyl-2-methyldihydrobenzofuran illustrates the use of the probe to provide information regarding the outer surface acidity of H-MOR and H-BEA. This test reaction proved also to be useful for the investigation of the effects of various modifications of H-MOR on the outer surface acidity.

On the potential of zeolites to catalyse aromatic acylation with carboxylic acids

Summary The intramolecular acylation of 4-phenylbutyric acid was investigated as a model reaction for Friedel-Crafts acylation catalysed by zeolites in the liquid phase. This reaction is catalysed by zeolite H-Beta in 4-chlorotoluene as solvent. The catalytic ability of H-Beta was demonstrated by the fact that at reflux temperature the total turnover number (TON) was found to be 35. However, the acylation of toluene or butylbenzene with carboxylic acids was slow; the unbalanced adsorption equilibrium between the two reactants on the H-Beta (Si/Al = 12) may be a contributing factor in this case. The acylation of anisole by carboxylic acids or acid anhydrides however, was readily catalysed by zeolite H-Beta.

Synthesis of 7-hydroxycoumarins catalysed by solid acid catalysts

Syntheses of substituted 7-hydroxycoumarins via reactions of 1,3-dihydroxybenzene with ethyl acetoacetate (Pechmann reaction) and with propenoic acid and propynoic acids are reported, in which the production of environmentally harmful waste streams is minimized by the use of solid acid catalysts.

Zeolite-catalysed Claisen rearrangement of allyl aryl ethers

The catalytic activity of various zeolites in the Claisen rearrangement wasinvestigated. It was found that H-mordenite and Hβ catalyse the rearrangement of allyl phenyl ether to 2-allylphenol and the cyclisation of latter compound to 2-methyldihydrobenzofuran. The reaction was accompanied by the formation of dimers and oligomers, which could be suppressed by triphenylphosphine treatment of the catalysts. This treatment led in the case of Hβ to better selectivities to the major reaction products.

Zeolite catalyzed regioselective synthesis of indoles

Publisher Summary The indole nucleus is present in many biologically active molecules. Examples include plant growth regulators, proteins, and pharmaceuticals. The selective synthesis of indole derivatives, therefore, constitutes an important area of drug research. A versatile method for synthesizing substituted indoles is the well known “Fischer Indole Synthesis” (FIS). It consists of a condensation of a phenylhydrazine and a ketone toward a phenylhydrazone followed by a Bronsted or Lewis acid catalyzed tautomerization to an enehydrazine. The enehydrazine tautomer undergoes a fast [3,3]-sigmatropic rearrangement followed by the elimination of ammonia yielding the indole product. The use of nonsymmetrical ketones will result in two isomeric enehydrazine tautomers and hence two isomeric indoles. In view of the continuing interest in the use of zeolites as shape selective catalysts in fine chemical synthesis, a more detailed study on the zeolite catalyzed FIS is undertaken in this chapter. The main interest was to elucidate the parameters that determine the selectivity when zeolites are used as catalysts in FIS. The parameters of interest are the strength and concentration of the acid sites and the geometric constitution of the zeolites imposing steric constraints on the reactants, transition states, and products. Also the contribution of the external surface is investigated in the chapter.