5806-90-6Relevant articles and documents
Transition-State Structures for Ester Aminolysis with and without Rate-Limiting Proton Transfer
Kovach, Ildiko M.,Belz, Michael,Larson, Mark,Rousy, Steven,Schowen, Richard L.
, p. 7360 - 7365 (2007/10/02)
The reaction of phenyl acetate (CH3CO2C6H5 and CD3CO2C6H5) with methoxyamine at 25 deg C in water exhibits β-secondary deuterium isotope effects k3H/k3D of 0.857 +/- 0.024 (general-acid catalyis by CH3O2CCH2(CO2CH3)NH3+, kHOH/kDOD =3.9 +/- 0.8) and 0.867 +/- 0.009 (general-acid catalysis by CH3ONH3+, kHOH/kDOD =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 k3H/k3D=0.970+/- 0.002 (H2O solvent) and 0.979 +/- 0.009 (D2O solvent) and kHOH/kDOD =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 k3H/k3D = 0.975 +/- 0.009, also consistent with a near-trigonal transition state.
Concerted Bifunctional Proton Transfer and General-Base Catalysis in the Methoxyaminolysis of Phenyl Acetate
Cox, Michael M.,Jencks, William P.
, p. 580 - 587 (2007/10/02)
The bifunctional acid-base catalysts cacodylic acid, bicarbonate, and the monoanions of phosphate, substituted phosphonates, and methylarsonate are up to 102-103 more effective than monofunctional acids or bases of comparable pK for catalysis of the methoxyaminolysis of phenyl acetate.The absence of the downward break in the Broensted plot and the solvent isotope effect maximum that are observed with monofunctional acid catalysts when proton transfer becomes partially rate limiting indicates that these bifunctional catalysts avoid this stepwise proton-transfer step.It is concluded that the two proton transfers occur through a mechanism with no detectable barrier or isotope effect, which appears to be concerted and is so fast that proton transfer never becomes kinetically significant; the rate-limiting step is amine attack with hydrogen bonding by the catalyst.Glycine and water show smaller rate increases that probably represent stepwise bifunctional proton transfer through a nine-membered ring or two water molecules in a one-encounter mechanism.Pyrazole and triazole show little or no enhancement of catalytic activity, indicating that bifunctional proton transfer through a seven-membered ring is relatively unfavorable in aqueous solution.Catalysis by monofunctional bases follows a nonlinear Broensted plot and is attributed to a preassociation mechanism analogous to that for general-acid catalysis.
