Microdielectric catalysis

In this form of catalysis, inclusion of the substrate in the CD cavity provides an environment for the reaction that is different from that of the bulk, normally aqueous, medium. In the traditional view, the catalytic effect arises from the less polar nature of the cavity (a microdielectric effect) and/or from the conformational restraints imposed on the substrate by the geometry of inclusion (Bender and Komiyama, 1978). However, catalysis may also arise as a result of differential solvation effects at the interface of the CD cavity with the exterior aqueous environment (Tee and Bennett, 1988a,b Tee, 1989). 

Catalysis by cyclodextrins does not always involve the catalytic functions of the hydroxyl groups. Sometimes, cyclodextrins simply provide the cavities as a reaction field. This effect is attributable to (a) a microdielectric catalysis due to the apolar character of the cavity or (b) a conformational catalysis due to the geometric requirements of inclusion. 

A typical example of the microdielectric catalysis by cyclodextrins is the decarboxylation of anions of activated acids such as a-cyano and y8-keto acids [15]. These reactions proceed unimolecularly via rate-determining heterolytic cleavage of the carbon-carbon bond adjacent to the carboxylate group (Scheme 3). 

Cyclodextrin can accelerate the first step through microdielectric catalysis, since the interior of the cavity has an apolar or ether-like atmosphere. In fact, the activation parameters for cyclodextrin-catalyzed reactions are almost identical to reactions in a 2-propanol-H20 mixture. 

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See also in sourсe #XX -- [ ]

See also in sourсe #XX -- [ ]

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