Transition state analysis of acid-catalyzed hydrolysis of an enol ether, enolpyruvylshikimate 3-phosphate (EPSP)

Proton transfer to carbon represents a significant catalytic challenge because of the large intrinsic energetic barrier and the frequently unfavorable thermodynamics. Multiple kinetic isotope effects (KIEs) were measured for acid-catalyzed hydrolysis of the enol ether functionality of enolpyruvylshikimate 3-phosphate (EPSP) as a nonenzymatic analog of the EPSP synthase (AroA) reaction. The large solvent deuterium KIE demonstrated that protonating C3 was the rate-limiting step, and the lack of solvent hydron exchange into EPSP demonstrated that protonation was irreversible. The reaction mechanism was stepwise, with C3, the methylene carbon, being protonated to form a discrete oxacarbenium ion intermediate before water attack at the cationic center, that is, an A*A (or A + A) mechanism. The calculated 3- C and 3,3-H KIEs varied as a function of the extent of proton transfer at the transition state, as reflected in the C3-H bond order, n. The calculated 3-C KIE was a function primarily of C3 coupling with the movement of the transferring proton, as reflected in the reaction coordinate contribution ( †/†), rather than of changes in bonding. Coupling was strongest in early and late transition states, where the reaction coordinate frequency was lower. The other calculated C and O KIEs were more sensitive to interactions with counterions and solvation in the model structures than n . The KIEs revealed a moderately late transition state with significant oxacarbenium ion character and with a C3-H bond order ≈0.6. © 2012 American Chemical Society.