Superbasic Heterogeneous Catalysts

To begin we would like to define what we mean when we refer to a very strong base. It is appropriate to compare the notion of a very strong base with that of a very strong acid, a superacid. Superacids in solution or in the solid state are so strongly acidic that they are capable of protonating a hydrocarbon molecule their acid strength Hq — 17. 

The existence of very strong bases in solution has not been known for very long. The basic strength of such strong bases does not exceed 27. Some authors have called them hyperbases.  

In this Chapter, which is devoted to very strong solid bases, the term superbase is used to denote a base that is so strong that it can detach the proton from a hydrocarbon molecule. It should deprotonate triphenyl-methane, — 33, and at the upper limit methane, pK = 40. The basic strength of a superbase should therefore lie in the range 40 H 33. 

Caub re, Fortsch. Chem. Forsch., 1978, 73, 49 (and references therein). 

Tanabe, Solid Acids and Bases , Academic Press, New York and London, 1970. J. Kijehski and S. Malinowski,/. Chem. Soc., Faraday Trans. 1, 1978, 74, 250. 

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Malinowski, S Kijenski. J. Superbasic heterogeneous catalysts. Journal oj Catalysis, 1981 4, 130-140. 

S.Malinowski and J.Kijenski, Superbasic Heterogenous Catalysts, in Catalysis, ed. (Specialist Periodical Report), The Royal Society of Chemistry, London, 1981, Vol. 4, p. 130. 

Regarding the transesterification, the research of heterogeneous catalysts is less advanced because of the difficulty of finding superbase catalysts. This issue will be presented later in this chapter. To date the most interesting innovation comes from the French Petroleum Institute where a heterogeneous catalyst based on zinc and aluminum oxides was developed and is currently being applied in commercial plants [2],... 

Preparation and use of supported TMG (1) as a novel base catalyst is discussed in a review elsewhere [6]. Heterogeneous guanidines are provided as environmentally friendly base catalysts and, thus, precise discussion on supported superbases is given in Chapter 6. Guanidine chemistry has been excellently surveyed in books [7]. This chapter focuses on the synthetic utility of TMG (1) and its analogues in organic synthesis and application of modified guanidine to asymmetric reactions. 

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