Catalysis of the enolization of indan-2-one by cyclodextrins in aqueous solution

Abstract

In basic aqueous solution, enolate formation from indan-2-one (2, pK_a= 12.2) exhibits saturation kinetics when cyclodextrins (CDs) are added, consistent with the formation of 1 : 1 complexes between 2 and the CDs. With α-CD, β-CD, γ-CD, ‘hydroxyethyl-β-CD’ and ‘hydroxypropyl-β-CD’, the reaction is accelerated up to 22-fold, but ‘dimethyl-β-CD’ slows it down by ca. 46%. All of the CDs (pK_a= 12.2) are more reactive towards 2 than is trifluoroethanol (pK_a= 12.4). Kinetic parameters for the CD-catalysed deprotonation are discussed in terms of the differences between transition-state binding and initial-state binding, and of the structures of the various CDs. It is concluded that anions of the CDs act as general bases towards 2, facilitated by partial inclusion of the transition state in the CD cavity, the extent of which depends on the CD. Enolate formation catalysed by β-CD is slowed by simple alcohols (propan-1-ol to heptan-1-ol), but it is not really inhibited by them, even though they bind to β-CD. Apparently, deprotonation of 2 by an anion of β-CD can still take place with an alcohol in the CD cavity, albeit more slowly.