ACYL TRANSFER-REACTIONS MEDIATED BY CYCLODEXTRINS - THE REACTION OF EXTERNAL NUCLEOPHILES WITH ENCAPSULATED ALKANOATE ESTERS OF VARYING CHAIN-LENGTH

The kinetics of the reaction of p-nitrophenyl alkanoates (acetate to decanoate, C-2 to C-10) with trifluoroethanol (TFE) in the presence of alpha-, beta-, or hydroxypropyl-beta-cyclodextrin (alpha-, beta-, or Hp-beta-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 (k(N) and k(cN), respectively). Generally speaking, the values of k(N) and k(cN) 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 k(cN) values such that transition-state stabilization increases in a biphasic manner as the alkanoate chain is lengthened from C-2 to C-10. For short chains (< C-7) the rise in transition stabilization is gentle but for longer chains (> C-6) 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 double dagger) for the short esters to acyl-group inclusion (4 double dagger) when the acyl chain is lengthened beyond C-6.