General base-catalyzed hydrolysis and carbonyl-18O exchange of N-(4-nitrobenzoyl)pyrrole

Base-promoted hydrolysis kinetics for N-(4-nitrobenzoyl)pyrrole (1) have been measured as a function of buffer concentration at several pH values at 25 degrees C. In addition carbonyl-O-18 exchange kinetics have been determined at a single pH value (9.48) as a function of 1,4-diazobicyclo[2.2.2]octane (DABCO) concentration. At zero buffer concentration the measured ratio of O-18 exchange to hydrolysis (k(ex)/k(hyd)) is approximately 0.04, and this value increases and finally levels off at about 0.23 as the DABCO concentration is increased. These observations are consistent with the buffer acting as a general-base to catalyze both the attack of water to generate an anionic tetrahedral intermediate (TO-) and the breakdown of TO- to give hydrolysis products.