CHAIN-LENGTH EFFECTS IN THE CLEAVAGE OF ARYL ESTERS BY CYCLODEXTRINS - DIFFERENT TRANSITION-STATES FOR META-NITROPHENYL AND PARA-NITROPHENYL ALKANOATES

The kinetics of cleavage of m- and p-nitrophenyl esters (acetate, propanoate, butanoate, pentanoate, and hexanoate) in a basic aqueous phosphate buffer containing a- or β-cyclodextrin (a- or β-CD) have been measured. For these two series of esters the m-nitro derivatives undergo more efficient cleavage than their p-nitro isomers, indicating that the former react via a transition state in which the aryloxy moiety is included in the CD cavity, even though substrate binding probably occurs through the alkyl group. For the p-nitrophenyl esters kinetic parameters vary significantly with the length of the alkanoate chain, and in a manner that suggests that substrate binding and transition-state binding both involve inclusion of the alkyl moiety. In contrast to the above, kinetic parameters for the cleavage of 2-carboxy-4(or 5)-chlorophenyl esters by a- and β-CD show little or no variation with the chain length of the alkanoate, implying that both series of esters react solely by aryloxy group inclusion. The present studies illustrate the usefulness of the pseudo constants pKTS (= -log KTS),where KTS is the apparent dissociation constant of the transition state of the CD-mediated reaction into the transition state of the normal reaction and CD. The variation of these constants with structure can be a useful probe of mechanism, particularly with regard to the mode of binding of transition state of the CD-mediated reaction. © 1990, American Chemical Society. All rights reserved.