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 reacting 6‐deoxy‐6‐N‐imidazolyl‐β‐CD (2) with bis(bromomethyl) benzene. The catalytic properties of 2, 3a and 3b in the hydrolytic cleavage of p‐nitrophenyl alkanoates, in the form of acetate (PNPA), butanoate (PNPB), hexanoate (WH) and octanoate ( PNPO), were examined. CD dimers showed middling rate enhancements around neutrality. Catalytic rate constants ( kc) in the presence of 3a or 3b did not vary much with chain length of esters. In contrast, dissociation constants (Kd) and selectivity factors (kc/Kd) for “long‐chain” esters were much smaller and significantly larger than those for “short‐chain” ones respectively, indicating CD dimers 3a and 3b have good dimensional recognition ability and substrate selectivity in the hydrolytic cleavage of p‐nitrophenyl alkanoate. Their kinetic consequences are briefly interpreted.