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

Abstract

Aliphatic alcohols that form host-guest complexes with "hydroxypropyl-P-cyclodextrin" retard the cleavage of tnnitrophenyl acetate by hydroxypropyl-P-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, k,, for the reaction of the (ester.hydroxypropy1-P-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-0-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-nitropheny I acetate and p-nitrophenyl hexanoate by hydroxypropyl-P-cyclodextrin in the presence of 14 potential inhibitors can occur with the ester largely outside of the hydroxypropyl-P-cyclodextrin cavity during the transition state, allowing the cavity to be occupied by a molecule of potential inhibitor.Key words: cyclodextrin, spectator catalysis, esterolysis.Resum6 : A cause d'une inhibition compCtitive, les alcools aliphatiques qui forment des complexes h6tes-invitCs avec l'cchydroxypropyl-P-cyclodextrinen retardent le clivage de I'acCtate de tn-nitrophknyle par l'hydroxypropyl-P-cyclodextrine en solution basique aqueuse. Par opposition, ces m&mes espkces n'inhibent pas la rCaction de I'acCtate de p-nitrophenyle et de I'hexanoate dep-nitrophenyle de la m&me faqon et, dans certains cas, I'addition d'alcools sert mCme i augmenter la vitesse de la reaction. Les rCsultats cinCtiques observCs suggkrent I'existence d'un processus qui comporte urze molCcule d'ccinhibiteur potentiel)) dans 1'Ctat de transition du clivage de l'ester. Les constantes de vitesse, k, pour les rCactions des complexes (ester.hydroxypropy1-P-cyclodextrine] avec une sCrie d'inhibiteurs potentiels montre une forte dCpendance sur I'habilitC de l'inhibiteur potentiel B se lier B la cyclodextrine. Par ailleurs, les constantes de vitesse pour la rCaction cinktiquement Cquivalente de l'ester avec le complexe (cyclodextrine.inhibiteur potentiel) ne prksente qu'une faible dCpendance sur la structure de l'alcool et leurs variations ne sont que trks faibles. Les logarithmes nCgatifs des constantes de dissociations apparentes des inhibiteur...

Elucidation of the Role of the Complex in Hydride Transfer Reaction between Methylene Blue and 1-Benzyl-1,4-dihydronictinamide by Effect of γ-Cyclodextrin

“…The values obtained for MB + and BNAH were 130 and 220 dm 3 mol -1 , respectively. These values were considerably smaller than those obtained for the formation of 1:1 complexes between MB + (BNAH) and β-CD in aqueous solution (values of 420 [19] and 460 dm 3 mol -1 [29] for MB + and 810 dm 3 mol -1 for BNAH [30] were reported).…”

Biomimetic Reactions Catalyzed by Cyclodextrins and Their Derivatives

“…Since this earliest work, a number of esters have been examined that bind into the cyclodextrin cavity and then react with a cyclodextrin hydroxyl group. These include derivatives of adamantane , and of various other hydrophobic species, − in some cases with enantioselectivity. − ,,− The structural selectivity of such hydrolyses has been used for the analysis of mixtures . Particularly interesting are some cases (e.g.…”

Catalysis of Ester Aminolysis by Cyclodextrins. The Reaction of Alkylamines with p-Nitrophenyl Alkanoates

“…Cyclodextrins (CDs) form inclusion complexes with a wide variety of species in aqueous solution . As a result they have effects on the rates of organic reactions, ranging from complete inhibition through mild retardation to significant catalysis, depending on the reaction and the CD. − These features have been observed for basic ester cleavage in the presence of CDs, which has been investigated by many groups, − including ours. − By contrast, the effect of CDs on the reactions of esters with nucleophiles besides hydroxide ion have received little attention. It was reported that nucleophilic attack of α-amino acid anions on p -nitrophenyl acetate (pNPA) can be catalyzed by CDs,9a but a later study in our laboratory showed that the catalytic effect is less than originally thought 9b…”