Bell-Shaped pH-Rate Profile in a Reaction Involving a Pentacoordinated Phosphorus Intermediate

摘要

Kinetics of the nucleophilic monosubstitution of imidazole by p-nitrophenolate (PNP~-) in hexaimi-dazolylcyclotriphosphazene (2) in water: THF 4:1 (5.5 < pH < 7.5) at 24℃ have been studied. The reaction was followed by the disappearance of the PNP~- under pseudo-first-order conditions using the initial-rate procedure. A constant excess of KCl is needed in order to avoid medium effects. A bell-shaped curve is obtained when the k_(obs) values (obtained from the leveling or the extrapolation to zero of k_(obs) vs [H_2PO_4~-]_T plots) are plotted against the pH. From this plot, a kinetically determined pK_a of 5.4 was obtained for 2. From the same plot, a 5.7 L mol~(-1)s~(-1) value for the second-order rate constant for the nucleophilic PNP~- attack to the protonated species of 2 was found. A two-step mechanism is proposed. The formed pentacoordinated intermediate cleaves to the (p-nitrophenoxy)pentaimidazolyl)cyclotriphosphazene (3) in a general-acid-catalyzed reaction. As expected of the proposed mechanism, a change in the rate-limiting step is observed at pH > 7.0 when the [buffer]_T increases and k_(obs) becomes independent of [buffer]. The implications of the present results on the controversial mechanism of hydrolysis of RNA and derivatives are discussed.