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

Base-promoted hydrolysis kinetics for N-(4-nitrobenzoyl)pyrrole (1) have be en measured as a function of buffer concentration at several pH values at 2 5 degrees C. In addition carbonyl-O-18 exchange kinetics have been determin ed at a single pH value (9.48) as a function of 1,4-diazobicyclo[2.2.2]octa ne (DABCO) concentration. At zero buffer concentration the measured ratio o f O-18 exchange to hydrolysis (k(ex)/k(hyd)) is approximately 0.04, and thi s value increases and finally levels off at about 0.23 as the DABCO concent ration is increased. These observations are consistent with the buffer acti ng as a general-base to catalyze both the attack of water to generate an an ionic tetrahedral intermediate (TO-) and the breakdown of TO- to give hydro lysis products.