Catalysis by cyclodextrins

Jerry Atwood liquid clathrates from alkyl aluminium salts 1969 - Ron Breslow catalysis by cyclodextrins... 

From all this we concluded that the catalysis by cyclodextrin involves the reversible formation of a cyclodextrin hypochlorite (64), which can transfer chlorine to the anisole para position but cannot reach the ortho position. The reaction is faster when the RO-Cl group is held closer and more rigidly in the smaller a-cyclodextrin. 

As shown in Table 2, cyclodextrins exhibit catalysis in many organic reactions. A typical rate vs. concentration plot for the catalysis by cyclodextrin is shown in Fig. 6, which is reminiscent of enzymatic saturation kinetics. A double reciprocal plot (of the same data) shows a straight line, just as an enzymatic reaction does, as shown in Fig. 7, This double reciprocal plot is a direct analog of a Lineweaver-Burk plot in enzymatic kinetics. 

Catalysis by cyclodextrins may be divided into two categories (1) Catalysis by the hydroxyl groups, in which the hydroxyl groups of the cyclodextrin function as intracomplex catalysts toward the substrates included in the cayity of the cyclodextrin. (2) Effect of the reaction field, in which the cavity of the cyclodextrin serves as an apolar and sterically restricted reaction field. Both of these catalyses are important in enzymatic reactions. 

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). 

Catalysis by cyclodextrins often shows specificity which is characteristic of enzymatic catalysis. Here, the specificities are divided into 3 categories (1) substrate specificity, in which subtle changes in the structure of the substrates have large effects on the catalysis (2) product specificity, in which the products of the catalyzed reactions are highly selective and (3) d,l specificity, in which enantiomeric recognition is made by a-cyclodextiin. 

R. Breslow, Selective binding and catalysis by cyclodextrins, Minutes of The Sixth International Symposium on Cyclodextrins, Editions de Sante, Paris, 625-630 1992. 

Enzymatic process is the main method to produce cyclodextrins (CDs), and so far, chemical methods have been reported. CD is industrially produced from starch, glucogen, malto-oligosaccharides, and other dextrins through catalysis by cyclodextrin glucosyl transferase (CGTase). More and more researchers focus on the essential CGTase to prepare CD due to its wide applications in food, medicine, cosmetics, environmental protection, and analytical chemistry. 

The structure of the ternary inclusion complex composed of 3-cyclodextrin, phenol, and, chloroform or carbon tetrachloride, formed in the reaction mixture, is determined by NMR spectroscopy. The selective catalysis by cyclodextrin was attributed to the regulation of molecular conformation of substrates with respect to dichlorocarbene, to trichloromethy1 cation, or to allyl cation in the ternary molecular complex. 

Shimizu, S. Sasaki, Y. Hirai, C. (1990) Inverse phase-transfer catalysis by cyclodextrins. Palladium-catalyzed reduction of bromoanisoles with sodium formate. Bull Chem. Soc. // ., 63,176-8. 

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