Superbases characteristics

Superbases have been classified as those with gas-phaseproton affinities > lOOOkJ/mol, greater than the bases listed in Table 6.6. Examples of organic superbases are shown in Figure 6.1. These are weak bases in water but exhibit superbasic characteristics in organic solvents. 

The kinetic parameters of the oxidation of DMSO with benzoyl peroxide in some dipolar aprotic solvents and superbasic media were correlated with the main physicochemical characteristics of solvents. A two-step scheme of the oxidation involving preliminary cleavage of the peroxide to perbenzoate with the base has been suggested.166... 

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. 

In their extensive efforts to devise a new strong nonionic base, Verkade and coworkers found that a highly basic dendrimer containing a PAPT base fragment could act as an efficient catalyst for Michael addition reactions, nitroaldol (Henry) reactions and aryl isocyanate trimerization reactions [42] (Figure 6.3). In view of the characteristic nature of this dendrimer, which has sixteen catalytic sites per molecule, the attachment of other superbase functionalities might also be attractive. 

In this chapter, the synthetic utility of Proton Sponge (1) was reviewed. This superbase, although not a main player, is indispensable for various mild and selective transformations in organic synthesis. Despite the unique characteristics of superbases, their exploitation is still limited. Recently, various types of proton sponges, including chiral ones, have been developed, and are likely to have a wide range of applications in organic and asymmetric synthesis. 

In addition to these applications the nucleophilicity of calcium-based organometallics can be exploited in stoichiometric reactions. The enormous reactivity toward weak acids justifies cmisideration of these reagents as superbases [49] with the tendency to degrade ethers as a characteristic feature. Due to the fact that two recent and excellent review articles already focus on catalytic applications [132, 133] we limit the discussion on selected stoichiometric reactivity studies. 

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