The kinetics of basic cleavage of nitrophenyl alkanoate esters by ‘hydroxypropyl-β-cyclodextrin’ in aqueous solution

Abstract

The kinetics of cleavage of m- and p-nitrophenyl alkanoates by ‘hydroxypropyl-β-cyclodextrin’(Hp-β-CD) in basic aqueous solution vary significantly with the chain length of the esters (C₂ to C₁₀). For both series of esters with short chains (C₂ to C₆) simple saturation kinetics are observed, indicative of 1 : 1 (ester : CD) binding and reaction of one molecule of ester with one molecule of Hp-β-CD. For longer chains, there is also a cleavage process involving two molecules of Hp-β-CD. This type of behaviour was not found previously for the same esters reacting with α-CD and with β-CD but it has been observed for some carboxynitrophenyl alkanoates. With the longest esters there is also evidence of productive 1 : 2 (ester : CD) binding. For the 1 : 1 binding, there is a linear dependence of the strength on acyl chain length and the close similarity of the dissociation constants for the two series of esters implicate inclusion of the alkyl chains of the esters' acyl groups. Transition-state binding is more complex: for the meta-nitro isomers aryl-group inclusion appears dominant whereas for the para isomers there seems to be a switch from aryl-group inclusion to acyl-group inclusion, occurring at a chain length of C₆–C₇.