Acyl transfer reactions mediated by cyclodextrins. The reaction of external nucleophiles with encapsulated alkanoate esters of varying chain length
Abstract
The kinetics of the reaction of p-nitrophenyl alkanoates (acetate to decanoate, C2 to C10) with trifluoroethanol (TFE) in the presence of α-, β-, or hydroxypropyl-β-cyclodextrin (α-, β-, or Hp-β-CD) in basic aqueous solution have been measured. The results are analysed to afford rate constants for nucleophilic attack on the free and CD-bound esters (kN and kcN, respectively). Generally speaking, the values of kN and kcN are not very different, so that binding the esters to CDs has only modest effects on their reactivities towards TFE, reacting as its anion. However, there is a general trend in kcN values such that transition-state stabilization increases in a biphasic manner as the alkanoate chain is lengthened from C2 to C10. For short chains (< C7) the rise in transition stabilization is gentle but for longer chains (> C6) is quite steep. This same behaviour is observed for all three CDs, with only minor differences between them, and also when the nucleophile is the anion of 2-mercaptoethanol. It is suggested that there is a change in the mode of transition-state binding of the esters from aryl group inclusion (3‡) for the short esters to acyl-group inclusion (4‡) when the acyl chain is lengthened beyond C6.