Complexation between tert-butyl ketones and beta-cyclodextrin. Structural study by NMR and MD simulations

A structural study (NMR and MD) of the complexation between tert-butyl ketones and beta-cyclodextrin has been performed. A priority order for the alkyl and phenyl groups composing the ketones has been determined based on association constants: Ph- > C(6)H(11)- = t-Bu- > Bu-, Pr-, Me-. Geometries for the complexes are proposed based on NOE values and on the MD simulations. Bimodal complexation occurs in all the compounds studied.

Ester hydrolysis and enol nitrosation reactions of ethyl cyclohexanone-2-carboxylate inhibited by beta-cyclodextrin

Both the ester hydrolysis and the nitrosation reactions of the enol tautomer of ethyl cyclohexanone-2-carboxylate (ECHC) are investigated in the absence and presence of beta-cyclodextrin (beta-CD). The ester hydrolysis reaction is studied in dilute H2O and D2O solutions of hydrochloric acid and in aqueous buffered solutions of carboxylic acids (acetic acid and its chloro derivatives). The pseudo-first-order rate constant increases with both the [H+] and the total buffer concentration, indicating that the hydrolysis is subject to acid and general base catalysis. Substantial solvent isotope effects in the normal direction (kH/kD > 1) for the acid-catalyzed hydrolysis was observed. Addition of beta-CD strongly slows the hydrolysis reaction. The variation of the observed rate constant (k(o)) with [beta-CD] exhibits saturation behavior, consistent with 1:1 binding between the enol of ECHC and beta-CD. The binding is quite strong, and bound ECHC-enol is unreactive. The nitrosation reaction of ECHC in aqueous acid medium, using sodium nitrite in great excess over the concentration of ECHC, yields perfect first-order kinetics, indicating that the slow step is the nitrosation of the enol tautomer. This finding suggests that a great percentage of the total ECHC concentration must exist in the enol form. The nitrosation reaction is of first order in [nitrite] and is catalyzed by the presence of Cl-, Br-, or SCN- ions, which indicates that the attack of the nitrosating agent is the slow step. The nitrosation reaction is also strongly inhibited by the presence of beta-CD because of the formation of unreactive inclusion complexes between the host, beta-CD, and the guest, the enol of ECHC. In alkaline medium, the formation of the enolate ion is observed, which absorbs at higher wavelengths (lambda(max) = 256 nm in acid medium shifts to lambda(max) = 288 nm in alkaline medium). This anion also undergoes ester hydrolysis spontaneously, but shows neither specific basic catalysis nor appreciable effect by the presence of beta-CD. From kinetic and spectroscopic measurements the pKa of the enol of ECHC has been determined as 12.35.

Spectator catalysis in the cleavage of p-nitrophenyl acetate and p-nitrophenyl hexanoate by "hydroxypropyl-β-cyclodextrin"

Aliphatic alcohols that form host–guest complexes with "hydroxypropyl-β-cyclodextrin" retard the cleavage of m-nitrophenyl acetate by hydroxypropyl-β-cyclodextrin in basic aqueous solution, due to competitive inhibition. By contrast, these same species do not inhibit the reaction of p-nitrophenyl acetate and p-nitrophenyl hexanoate to the same extent and, in some cases, the addition of alcohols serves to increase the rate of reaction. The observed reaction kinetics require the presence of a process that has one molecule of the "potential inhibitor" in the transition state for ester cleavage. Rate constants, ka, for the reaction of the {ester•hydroxypropyl-β-cyclodextrin} complexes with a series of potential inhibitors show a strong dependence on the ability of the potential inhibitor to bind to the cyclodextrin. On the other hand, rate constants for the kinetically equivalent reaction of the ester with the {cyclodextrin•potential inhibitor} complex show little dependence on the alcohol structure and they vary over a very limited range. The negative logarithms of the apparent dissociation constant of the potential inhibitor from the transition state show a strong dependence on the ability of the potential inhibitor to bind to hydroxypropyl-β-cyclodextrin, indicating that the binding of the potential inhibitor in the initial state and the transition state is similar. It is concluded that the cleavage of p-nitrophenyl acetate and p-nitrophenyl hexanoate by hydroxypropyl-β-cyclodextrin in the presence of 14 potential inhibitors can occur with the ester largely outside of the hydroxypropyl-β-cyclodextrin cavity during the transition state, allowing the cavity to be occupied by a molecule of potential inhibitor. Key words: cyclodextrin, spectator catalysis, esterolysis.