Transition-State Structures for Ester Aminolysis with and without Rate-Limiting Proton Transfer

Article abstract of DOI:10.1021/ja00311a024

The reaction of phenyl acetate (CH3CO2C6H5 and CD3CO2C6H5) with methoxyamine at 25 deg C in water exhibits β-secondary deuterium isotope effects k_3H/k_3D of 0.857 +/- 0.024 (general-acid catalyis by CH3O2CCH2(CO2CH3)NH3⁺, k_HOH/k_DOD =3.9 +/- 0.8) and 0.867 +/- 0.009 (general-acid catalysis by CH3ONH3⁺, k_HOH/k_DOD =1.75 +/- 0.06).Thesevalues are consistent with rate-limiting proton trasfer from general acid to a zwitterionic tetrahedral adduct and rate-limiting solvent reorganization in a complex of general acid and zwitterionic adduct (COX, M.M., Jencks, W.P.J.Am.Chem.Soc. 1981, 103, 572-580).The reaction of hydrazine with phenyl acetate, with general-base catalysis by hydrazine, exhibits k_3H/k_3D=0.970+/- 0.002 (H2O solvent) and 0.979 +/- 0.009 (D2O solvent) and k_HOH/k_DOD =1.44 +/- 0.002.This is not consistent with proton transfer from a tetrahedral adduct but rather suggests a near-trigonal structure at carbonyl in the transition state.Possibly leaving-group expulsion in a cyclic process with external stabilization by a base catalyst is rate-determinig.The uncatalyzed reaction of semicarbazide with 2,4-dinitrophenyl acetate has k_3H/k_3D = 0.975 +/- 0.009, also consistent with a near-trigonal transition state.