Syntheses of imidazolium-bridged cyclodextrin dimers and their catalytic properties in the hydrolytic cleavage of p-nitrophenyl alkanoates

Two imidazolium-bridged cyclodextrin dimers 3a and 3b were prepared by reac ting 6-deoxy-6-N-imidazolyl-beta-CD (2) with bis(bromoethyl)benzene. The ca talytic properties of 2, 3a and 3b in the hydrolytic cleavage of p-nitrophe nyl alkanoates, in the form. of acetate (PNPA), butanoate (PNPB), hexanoate (PNPH) and octanoate (PNPO), were examined. CD dimers showed middling rate enhancements around neutrality. Catalytic rate constants (k(c)) in the pre sence of 3a or 3b did not vary much with chain length of esters. In contras t, dissociation constants (K-d) and selectivity factors (k(c)/K-d) for "lon g-chain" esters were much smaller and significantly larger than those for " short-chain" ones respectively, indicating CD dimers 3a and 3b have good di mensional recognition ability and substrate selectivity in the hydrolytic c leavage of p-nitrophenyl alkanoates. Their kinetic consequences are briefly interpreted.