Selective Syntheses Using Cyclodextrins as Catalysts

Abstract

With the use of cyclodextrins as catalysts, 4-hydroxybenzaldehydes, 4-hydroxybenzoic acids, 4-dichloromethyl-2, 5-cyclohexadienones, and indole-3-aldehyde are synthesized in virtually 100 % selectivities and in high yields. 4-Allyl-2, 5-cyclohexadienone is obtained also in 100 % selectivity using a modified cyclodextrin, in which all the primary hydroxyl groups are replaced by N-methyl-formamido groups. In addition, one-pot syntheses are successfully carried out with cyclodextrin as catalyst, yielding chalcones and β-nitrostyrenes in 100 % selectivity and in high yields. In the absence of cyclodextrins, however, both the yields and the selectivities for the desired products in all the reactions are quite low. The selective catalyses by cyclodextrins involve, a) regulation of mutual orientation of reactants, b) regulation of molecular size of intermediates and products, c) protection of unstable intermediates and products, and/or d) solubilization of reactants with poor solubilities. The mechanisms of selective catalyses are clarified using the conformations of the molecular complexes, formed from cyclodextrins and reactants in the reaction mixtures, which are determined by ¹H-and ¹³C-NMR spectroscopy. Furthermore, 4-hydroxybenzoic acid and 4-hydroxybenzaldehyde are synthesized in almost 100 % selectivity using cyclodextrins, immobilized with epichlorohydrin as crosslinking agent, as catalysts. The immobilized catalysts are easily separated from the reaction mixtures, and are successfully used many times without measurable decrease in catalytic activities.