Solid Base Catalyzed Reactions

Solid base catalyzed reactions are not as commonly used in synthetic processes as are those reactions promoted by solid acids. While several so-called super bases have been developed, they have not been generally used for the types of reactions 

Of more synthetic interest is the Cs-X catalyzed liquid phase condensation of benzaldehyde with active methylene compounds such as ethyl cyanoacetate, ethyl malonate and ethyl acetoacetate but the yields in these reactions were only in the 40%-70% region. Higher yields were obtained using a germanium substituted faujasite 2 06 or a nitrided aluminophosphate as the basic catalyst. 

The condensation of 4-phenyl-2-butanone (45) with ethyl cyanoacetate took place over Cs-X to give the product, 46, in only a 39% yield but when a magnesium oxide containing hydrotalcite clay was used a 75% yield of 46 was obtained (Eqn. 22.43). 8 Formaldehyde was condensed with acetaldehyde at 425°C over a ZSM-5 zeolite containing MgO. Acrolein was obtained in almost 70% yield. 9 

3-methyl-cyclohexanone (49) with benzaldehyde gave almost exclusively the monobenzylidine compound, 50, regardless of the zeolite used (Eqn. 22.45).  

Cs-X has also been used to promote the selective O-methylation of phenol with dimethyl carbonate. 2 N-Monomethylaniline was obtained in 93% yield by reacting aniline with dimethyl carbonate over a K-Y catalyst at 180°C (Eqn. 22.46). 3 The reaction of alcohols with chloromethyl ether over Na-Y gave the resulting methoxymethyl ethers in 70%-90% yields (Eqn. 22.47). 4 

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Although this reaction was discussed earlier, it is mentioned here because it is catalyzed by solid acids such as NH4SCN, which provide H+ ions that bond to the pairs of electrons after breaking them loose from the metal. Over a rather wide range of catalyst concentrations, the rate is linearly dependent on the amount of solid acid. Once the NH4+ ion donates a proton, NH3 is lost and the protonated ethyl-enediamine molecule is the acid that remains and continues to catalyze the reaction. While base catalyzed reactions of complexes may be better known, there are many acid catalyzed reactions as well. 

This base-catalyzed reaction has been discussed in more detail by Snaith and co-workers <92CC1152, 93AG1801>. The reaction between 5-naphthylamino-1,2,3,4-thiatriazole (99) and 1 equivalent of solid Ba(OH)2 in toluene in the presence of 3 equivalents of HMPA gave rise to salt (100) in 60% yield (path (a)) for which an x-ray structure has been obtained (Equation (10)) <92CC1152>. 

Transesterification Reactions. The heterogeneous acid-catalyzed transesterification of TGs has not been investigated as much as its counterpart, the base-catalyzed reaction. Various solids are available with sufficient acid strength to be effective catalysts for the named reaction. Among the solid acids available are functionalized polymers, such as the acid forms of some resins, as well as inorganic materials, such as zeolites, modified oxides, clays, and others. Some of these solids have already been found to be effective in transesterification reactions of simple esters and (3-ketoesters. 

Different catalysts bring about different types of isomerization of hydrocarbons. Acids are the best known and most important catalysts bringing about isomerization through a carbocationic process. Brpnsted and Lewis acids, acidic solids, and superacids are used in different applications. Base-catalyzed isomerizations of hydrocarbons are less frequent, with mainly alkenes undergoing such transformations. Acetylenes and allenes are also interconverted in base-catalyzed reactions. Metals with dehydrogenating-hydrogenating activity usually supported on oxides are also used to bring about isomerizations. Zeolites with shape-selective characteristics... 

Reactions catalyzed by solid bases were obvious candidates for testing hypotheses on the nature and the mode of action of enzymes. Bredig [40] used aminated cellulose (B2) as a model because an enzyme was thought to consist of "a specific active function and a colloidal carrier". Indeed, cyanohydrin 40 was formed with an enantiomeric excess of 22% Fig. 3 and Table 3 contain a summary of the reported results for base-catalyzed reactions. It is not clear whether the ZnO/ffuctose catalyst (Bl) described by Erlenmeyer [39] is really heterogeneous but it is the first report on using sugars as modifiers. Some reactions are probably just curiosities (39, 41), but two... 

The a-diketone 1,2-cyclodecadione undegoes a base-catalyzed reaction to provide a high melting solid, C20H32O4, which was incapable of being dissolved in any solvent ... 

Mg-Al mixed oxides obtained by thermal decomposition of anionic clays of hydrotalcite structure, present acidic or basic surface properties depending on their chemical composition [1]. These materials contain the metal components in close interaction thereby promoting bifunctional reactions that are catalyzed by Bronsted base-Lewis acid pairs. Among others, hydrotalcite-derived mixed oxides promote aldol condensations [2], alkylations [3] and alcohol eliminations reactions [1]. In particular, we have reported that Mg-Al mixed oxides efficiently catalyze the gas-phase self-condensation of acetone to a,P-unsaturated ketones such as mesityl oxides and isophorone [4]. Unfortunately, in coupling reactions like aldol condensations, basic catalysts are often deactivated either by the presence of byproducts such as water in the gas phase or by coke build up through secondary side reactions. Deactivation has traditionally limited the potential of solid basic catalysts to replace environmentally problematic and corrosive liquid bases. However, few works in the literature deal with the deactivation of solid bases under reaction conditions. Studies relating the concerted and sequential pathways required in the deactivation mechanism with the acid-base properties of the catalyst surface are specially lacking. 

From the properties of zeolites mentioned, it is clear that they have always been regarded as solids that promote acid-catalyzed reactions. A spectacular new development has been their use in base-catalyzed reactions (Davis, 1993). Another development that merits equal attention is the synthesis of titanium silicalites (TS-1) with their unique oxidative potential. We shall refer to these applications later following a review of their more conventional use as Bronsted and Lewis acid zeolites. 

The first part of this chapter is intended to survey recent literature on new catalytic materials because the development of new types of metal oxides and layered- and carbon-based materials with different morphologies opens up novel acid-base catalysis that enables new type of clean reaction technologies. Mechanistic considerations of acid- and base-catalyzed reactions should result in new clean catalytic processes for Green and Sustainable Chemistry, for example, transformations of biorenewable feedstock into value-added chemicals and fuels [21-35]. The latter part of this chapter, therefore, focuses on biomass conversion using solid acid and base catalysts, which covers recent developments on acid-base, one-pot reaction systems for carbon-carbon bond formations, and biomass conversion including synthesis of furfurals from sugars, biodiesel production, and glycerol utilization. 

This way, metal hydroxide fluorides are accessible in which the metal site is coordinated by both anions, hydroxide and fluoride, thus presenting real hydroxide fluorides that are almost inaccessible via any other synthesis route [58], By varying the F to OH ratio, the whole series of M(OH)jF , from M(OH) to MF can be synthesized. As a result, the Lewis to Brpnsted acid site ratio of these materials can be tuned over a wide range resulting in optimized solid catalysts with high potential for a wide range of acid-base-catalyzed reactions [40,52],... 

Base catalyzed reaction has been covered with reference to transesterification to make biodiesel. A classical case of considerable practical importance is the side chain alkylation of toluene with propylene to give isobutyl benzene, apart from conversion of acetone to diacetone alcohol and isophorone and even here heterogenerous versions are in vogue or under consideration. Microwave assisted reactions are duly covered and it would have been useful to cover ultrasonically assisted reactions, particularly when solid reactant/ catalyst is involved. 

The hydrotalcite (HT) family is one class of solid base catalyst that has attracted much attention in solid base—catalyzed biodiesel production since it possesses good anion-exchange properties (Allada et al., 2002 Sels et al., 2001), and tunable base strength for the transesterification reaction (Sels et al., 2001 Xie et al., 2006 Debecker et al., 2009). 

The base-catalyzed reactions play a more and more important role in modem fine chemistry. Nevertheless reports about solid base catalysts are still far fewer than those about solid acid catalysts [1-3]. Therefore, new solid base catalysts are imminendy desirable to meet the fine chemical industry needs [4]. 

Base catalysis is most effective with alkali metals dispersed on solid supports or, in the homogeneous form, as aldoxides, amides, and so on. Small amounts of promoters form organoalkali comnpounds that really contribute the catalytic power. Basic ion exchange resins also are usebil. Base-catalyzed processes include isomerization and oligomerization of olefins, reactions of olefins with aromatics, and hydrogenation of polynuclear aromatics. 

Dibenz[c,e]azepine (30) is obtained in excellent yield as a stable crystalline solid by base-catalyzed elimination of methylsulfinic acid from 6-mesyl-6,7-dihydro-57/-dibenz[c,e]azepine (29. R = Ms).5 An analogous reaction is noted with 6-nitro-6,7-dihydro-5//-dibenz[c,e,]azepine (29, R = N02). 

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