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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. |
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