Classes of Concave Reagents

Concave reagents have been employed in many reactions but for the reasons mentioned above, most of the reactions were catalyses. Besides acid or base catalyses, especially transition metal ion catalysed reactions are of interest and have been investigated. The following chapters will hrst present major classes of concave reagents and will then discnss some reactions and catalyses and the influence of the concave shielding on rate and especially selectivity. 

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Figure 7.8 The first class of concave reagents were bimacrocyclic 2,6-disubstituted pyridines with amide bridgeheads building up the bimacrocycle. The bridges X and Y were polymethylene but also polyethylene glycol chains, the basicity of the pyridine could be tuned by respective substituents R in 4-position ...

Fig. 1 Important classes of concave reagents concave pyridines and concave 1,10-phenanthrolines and...

Not only cavity-containing molecules can be employed as concave reagents, but also, cleft molecules have been realized. The necessary stiffness usually is realized by an aryl-aryl backbone (Fig. 3). In particular, two classes of concave reagents have been successfully employed to enhance selectivities concave reagents based on 2 -substituted m-terphenyls and 2,9-diaryl-1,10-... 

Section 2 discusses the syntheses of different classes of concave acids and bases. Convergent synthetic strategies were chosen for an easy structural variation of the reagents (modular assembly). Section 3 characterizes the concave acids and concave bases and checks whether the acid/base properties of the parent compounds benzoic acid, pyridine and 1,10-phenanthroline are conserved in the bimacrocyclic structures. In Section 4, the influence of the concave shielding on the reactivity and selectivity of the concave reagents is measured in model reactions. In principle, the concave shielding should be able to influence inter- and intramolecular competitions as well as chemoselectivity and (dia)stereoselectivity. If the reagent is chiral, enantioselectivity should also be observable. 

As outlined in Sect. 2, various classes of concave acids and bases may be synthesized in gram quantities. But the syntheses are multistep sequences and the yields are often limited by the macrocyclization steps. Therefore, for a practical use, these reagents are quite expensive and recovery and recycling is necessary. 

The central benzene ring of cuppedophane 1 can be replaced by various other aromatic or heteroaromatic rings (for example, 9,10-phenanthroline) as in general representation 6. Cuppedophanes of this type possess basic sites in their concave cup. Various members of this class have recently been synthesized and studied by Liining and co-workers as a part of their effort to design concave reagents [8]. 

Three different classes of reactions have been investigated using concave reagents ... 

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