Catalytic properties of novel cyclodextrin dimers in the hydrolytic cleavage of p ‐nitrophenyl alkanoates

Novel cyclodextrin (CD) dimers ( 3a–c ) linked by multidentate ligands were prepared by reacting 6‐deoxy‐6‐(hydroxyethylamino)‐β‐CD ( 2 ) with p ‐ and m ‐bis(bromomethyl)benzene and 2,6‐bis(bromomethyl)pyridine, respectively. The catalytic properties of 2, 3b and 3c in the hydrolytic cleavage of p ‐nitrophenyl alkanoates, namely acetate (PNPA), butanoate (PNPB), hexanoate (PNPH) and octanoate (PNPO), were examined. CD dimers 3b and 3c showed modest rate enhancements around neutrality. Although the catalytic rate constants ( k c ) in the presence of 3b or 3c did not vary significantly with the chain length of the esters, the Michaelis constants, K M , for ‘long‐chain’ esters (PNPH and PNPO) were much smaller than those for ‘short‐chain’ esters (PHPA and PNPB), and consequently the selectivity factors ( k c / K M ) for ‘long‐chain’ esters were much larger than those for ‘short chain’ esters, indicating that the CD dimers had good dimensional recognition ability and substrate selectivity in the hydrolytic cleavage of p ‐nitrophenyl alkanoates. Addition of Cu 2+ to the reaction media did not have much impact on K M , but led to an appreciable increase in k c , and therefore increases in k c / k u and k c / K M . The monomeric CD compound 2 showed essentially no selectivity in the hydrolysis of these esters. Kinetic consequences are briefly interpreted. Copyright © 2001 John Wiley & Sons, Ltd.