Effective Charge Distribution for Attack of Phenoxide Ion on Aryl Methyl Phosphate Monoanion: Studies Related to the Action of Ribonuclease

Article abstract of DOI:10.1021/jo00284a019

The reaction of phenoxide ion with aryl methyl phosphate monoanions and aryl diethyl phosphates obeys second-order kinetics in aqueous solution at 39 deg C and 1 M ionic strength.The second-order rate constants (M^-1s^-1) for these reaction obey the following Broensted equations: log k₂ = -0.51pK_lg + 0.72 (r = 0.970) (aryl diethyl phosphate) log k₂ = -0.64pK_lg - 2.53 (r = 0.898) (aryl methyl phosphate monoanion) The monoanion is some 10⁴-fold less reactive toward attack by phenolate ion than is the diethyl ester with 4-nitrophenol leaving group, consistent in part with the operation of an electrostatic effect.The similarity between the Broensted exponents in both reactions indicates that the effective charge change on the leaving oxygen from ground state to transition state is similar in both cases; this indicates that the oxyanion in the monoanion case does not assist leaving group expulsion.The date are consistent with little coupling between proton abstraction from the 2'-hydroxyl group and the leaving group expulsion in the ribonuclease reaction.