Solid-Base Catalysis

Examples of the application of recyclable solid base catalysts are far fewer than for solid acids [103]. This is probably because acid-catalyzed reactions are much more common in the production of commodity chemicals. The various categories of solid bases that have been reported are analogous to the solid acids described in the preceding sections and include anionic clays, basic zeolites and mesoporous silicas grafted with pendant organic bases. 

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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. 

Many more chemical processes have been developed using solid acid catalysts than with solid base catalysis, in particular at an industrial scale. However, most of those examples correspond to processes involving inorganic materials [161]. The first reports on the use of polymer-supported basic catalysts can be traced back to the use of anion exchange resins containing basic anions. The simplest, most general structure for this kind of resins is given by 32 (X = OH-, Scheme 10.9)... 

The catalytic reactions of interest in organic synthesis can conveniently be divided into five categories solid-acid catalysis, solid-base catalysis, catalytic hydrogenation and dehydrogenation, catalytic oxidation, and catalytic C-C bond formation. 

The generation of new C-C bonds also occupies a prominent position in organic synthesis. This often involves condensation of carbonyl compounds referred to earlier in the context of solid-base catalysis (Section 1.5.2). There are, however, also many C-C bond-forming reactions that involve transition metal catalysis and some of these can be conveniently performed with heterogeneons catalysts,... 

Kabashima. H Katou. T llaitori, H. Conjugate addition of methanol to 3-buten-2-one over solid base catalysis. Applied Catalysis. 1 General. 2001 214. 121-124. 

Meyer. U Gor/awski. H HOlderich. WF. Michael addition of ethyl acrylate and acetone over solid bases. Catalysis Letters. 1999 59, 201-206. 

Conna, A Iborra. S. Solid Base Catalysis. 7.1. Zeolites and related materials in Knoevcnagcl condensations and Michael additions. In Sheldon. RG van Bekkum. H editors. Lute Chemicals throuj h Heterogeneous Catalysis, Weinhcim ((iennany). Wilev-VC H.2(MM 309-326. 

Ono, Y. Hattori, H. Solid Base Catalysis, Tokyo Institute of Technology Press Tokyo, 2011 pp 118-127. 

Catalytic processes from the viewpoint of green chemistry include catalytic reductions and oxidations methods, solid-acid and solid-base catalysis, as well as carbon-carbon bond formation reactions (31). 

Alkaline Earth Metals. In the early literature, similar to alkali earth metals, alkaline earth metals were usually used to form the enolates, especially magnesium enolates, in the aldol reaction (128,129). Inspired by heterogeneous MgO solid base catalysis (130), alkaline earth metals have been applied successfully as homogeneous catalysts in the aldol reaction (Scheme 23). In 1998, Corey and co-workers described a new magnesium-catalyzed diastereoselective anti-aldol reaction as a key step for the total synthesis of lactacystin (131). The metal requirement is strict, since several salts were tested, for example, TiCl4, SnCL, ZnCl2, and Cu(OTf)2, only Mgl2 functioned as an effective catalyst (Scheme 24). 

In contrast to the areas of heterogeneous oxidation catalysis and solid acid catalysis, the use of solid base catalysis in liquid phase reactions has not met the same level of breakthrough. 

The observation that sodium deposited on ttiumina acts as an effective catalyst for the isomerization of olefins triggered much research in the Held of solid base catalysis. However, the catalytic properties of alkali oxides themselves have not been studied so extensively. Many studies have been done on alkali metals doped on or intercalated in other materials. Some of them are known as superbases and described in section 3.10. 

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