Nuclear magnetic resonance studies of proton exchange in imidate esters and amides in strong acid

Abstract

Saturation-transfer or lineshape measurements of kinetics of proton exchange show that HZ of ethyl acetimidate in 46% H2SO4 or ethyl N-methylformimidate in 40% H2SO4 exchanges faster than HE, and HE of protonated 2-iminotetrahydrofuran exchanges faster than HZ, as expected on the basis of the stabilities of the stereoisomeric imidate esters. In contrast, amide exchange is too fast to measure in aqueous H2SO4, but can be measured in 100% H2SO4, where HE and HZ exchange at equal rates. Also, there is intramolecular exchange occurring at this same rate. These results are interpreted as evidence for exchange in amides via the N-protonated intermediate, whose lifetime in H2SO4 is sufficiently long to permit the protons on nitrogen to become equivalent. By comparing OH exchange with NH exchange, it is concluded that N-protonation is the rate-limiting step. Even though this is a thermodynamically favorable proton transfer in H2SO4, its rate constant is significantly lower than the diffusion-controlled limit. The contrasts between imidate esters and amides and between dilute solutions and H2SO4 are discussed.