Solid superbase

It can be expected that solid bases could be successful for commercializing the alkylation of toluene with methanol as a route to styrene, or for selective alkene coupling. There is no doubt that achieving success in several important commercial processes will boost the field of solid base catalysis. Because it appears to be difficult to achieve superbasic organic resins, much more attention should be paid to enhancement of the base strengths of solid superbases. Further work should be done on supported alkali metals and mixed metal oxides. Development of new solid superbases will be improved by increasing our understanding of how alkali metal clusters (302-304) interact with supports and become stabilized. 

Solid bases with H >26are called superbases. Suitable color indicators for solid superbases are 4-chloroaniline (pKa=26.5) [4], diphenylmethane 1 =35), or cumene (pTCa= 37) [7]. Another method of characterization of superbasic sites is the use of test reactions [8], spectroscopic techniques [9], or C02 adsorption [10]. 

Solid superbases can deprotonate alkenes in the allylic position and alkyl benzenes (toluene) in the benzylic position. Typical pK, values of some organic molecules are given in Scheme 1 [11], However, these values are only approximations, because plQ values of such highly basic compounds are indirectly adapted to the... 

In comparison with molecular catalysts, solid catalysts can be isolated from the reaction mixtures by filtration or used in continuous processes this is both environmentally friendly and useful in laboratory-scale experiments. The most important reactions catalyzed by solid superbases are isomerization reactions and the alkylation of substituted arenes in the side chain (Scheme 2). They proceed at room temperature or below with high yield (typically >99%). The surperbase-cata-lyzed alkylation of aromatic compounds complements the acid-type Friedel-Crafts alkylation and acylation, because the latter results in ring alkylation, whereas the former results in side-chain alkylation. 

Heterogeneous C-H Transformation with Solid Superbases 409 Stefan Kaskel... 

Solid superbases, which have been defined as materials with base strengths, H > 26, represent a highly desirable class of solid materials. This is because they offer the possibility of activation of relatively unreactive species under mild reac-hon conditions. 

In this paper, we will focus on the zeolite basic catalysts, other solid superbases will be considered in detail elsewhere [6]. 

Thus, the data obtained show that chloroform and acetylene and its derivatives are suitable IR-spectroscopic probe-molecules for basic centers in zeolites. These probes exhibit the following advantages as compared to the conventionally used molecules, like CO2 and pyrrole (1) the wride ranges of the frequency shifts, which allows one to differentiate the centers of different nature and strength, (2) the easiness and reversibility of adsorption/desorption of these molecules, and (3) the favorable spectral range where the spectral features attributed to adsorbed probes appear. The use of such an approach allows us to shed some light on the nature and properties of basic sites in zeolites. The similar technique will be applied in our future studies devoted to other solid superbases. 

Isomerization of Alkenes. - Solid superbasic catalysts show remarkable activity in the isomerization of alkenes. At room temperature in the liquid phase the double-bond shift in pent-l-ene and hex-l-ene, and the isomerization of the initially formed 2-alkenes takes place. The highest activity in both types of isomerization is shown by catalysts that have been prepared by deposition of alkali metal by evaporation on MgO calcined below 973 K. In this case the isomerization proceeds on very strong superbasic centres. On MgO-alkali metal calcined at temperatures above 973 K, one-electron donor centres prevail this system causes double bond shift only (Table 7). The superbasic systems are also active in the isomerization of ( )-pent-2-ene to the mixture of (Z)-pent-2-ene and pent-l-ene. The systems for which one-electron donor character prevails are completely inactive in this transformation. 

Preparation of Solid Superbase Catalyst and Its Application to the Synthesis of Fine Chemicals... 

A solid superbase whose basicity (H ) is higher than 37 has been prepared by the successive treatment of y -alumina with alkali metal hydroxide and alkali metal at 200-600t under nitrogen. The solid base has been found to be highly effective in the catalytic isomerization of olefinic double bonds and in the side-chain alkylation of alkylbenzenes with olefins. 

In comparison to intense activities in the field of the solid superacids, not much work has been done on solid superbase catalysis[l]. Here we report first the preparation of a solid superbase which exhibits remarkably strong basicity, and then its synthetic application to the olefin isomerizations[2] and side-chain alkylations of alkylbenzenes with olefins[3]. The reactions proceed smoothly under mild reaction conditions to give the products quantitatively. 

Isomerization of olefinic double bonds We have applied the solid superbase B as catalyst for the isomerization of olefinic double bonds such as shown in the equations (1), (2), (3), and (4). 

The solid superbase catalysts are air-, moisture-, and CO2- sensitive. Nevertheless, they can be handled even in industrial scale. 

These materials have been referred to as solid superbases with an estimated Hammett basicity constant, H , of >37. They are capable of promoting reactions of hydrocarbons such as the isomerisation of 5-vinylbicyclo[2.2.1]-hept-2-ene to 5-ethylidenebicyclo[2.2.1]hept-2-ene, which is used as a comonomer in the production of synthetic rubber.132 Solid superbases have been used in commercial scale manufacturing, where they are typically generated in situ so as to overcome their great sensitivity towards the atmosphere. 

Catalytic behaviors of solid base catalysts for fine chemicals synthesis as well as the fundamental reactions are described. The reactions included are double bond isomerization of olefins, addition of hydrogen and amines to conjugated dienes, dehydration, dehydrogenation, reduction, alkylation, aldol addition and condensation, Wittig-Horner and Knoevenagel reactions, dehydrocyclodimerization, and ring transformation. The characteristic features of different types of solid base catalysts, zeolites, metal oxides, solid superbases and non metal-oxides, are summarized. 

Suzukanio, G Fukao. M Minobe. M. Preparation of new solid superbase and its catalytic activity. Chemistry Letters. 1987 585-588. 

Tanaka. K Yanashima. II Minobe, M Su/ukamo, G. Characterization of solid superbases prepared from gamma-alumina and their catalytic activity. Applied Surface Science. 1997 121.461-467. 

Decomposition of Methyl Formate.- Methyl formate decomposes to CO and CH OH over solid superbases, like magnesium oxide upon which metallic sodium has been evaporated. This reaction could be used as a simple way to obtain very pure carbon monoxide. 

In contrast to solid acid catalysts (including solid superacids) described earlier, solid base catalysts (including superbases) have received much less attention. A common basic catalyst used in organic synthesis is Ba(OH)2. The use of alumina-supported sodium in the double bond isomerization of olefins, first studied by Pines et al. (1955), is also widely known. More recent developments include the use of K, Na/K, MgO, Zr02, Th02, and K/KOH/AI2O3 zeolites containing different metal ions to control their acid-base properties, solid superbases, and... 

On the other hand, a solid superbase is defined as a solid whose base strength expressed by the basicity function, //, is higher than -h 26. The basis of the definition has been described in the literature. The kinds of solid superbases are shown in Table 1.4 together with their preparation method and pretreatment temperature. 

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