β-Secondary and Solvent Deuterium Kinetic Isotope Effects and the Mechanisms of Base- and Acid-Catalyzed Hydrolysis of Penicillanic Acid

摘要

β-Secondary and solvent deuterium kinetic isotope effects have been determined at 25℃ for the alkaline and acid-catalyzed hydrolysis of penicillanic acid. In order to determine the former isotope effect, [6,6-~2H_2]dideuteriopenicillanic acid has been synthesized. In alkaline solution, the former isotope effect (corrected to the effect of a single hydrogen orthogonal to the plane of the carbonyl group and for the inductive effect of deuterium) was found to be 0.95 ± 0.01 and the latter 0.76 ? 0.01. These values support the B_(AC)2 mechanism of hydrolysis with rate-determining formation of the tetrahedral intermediate that has been proposed for other β-lactams. The measured β-secondary kinetic isotope for the acid-catalyzed reaction was 1.00 ± 0.01. This represents a value averaged from experiments in 0.45 M HCl, 0.97 M HCl, 4.5 M HCl, and 33.3 wt% H_2SO_4. All precedent suggests that this result would be very unlikely for an associative mechanism, such as that commonly observed (A_(AC)2) for amide hydrolysis at these acid concentrations. Semiempirical AM1 calculations suggest that bicyclic β-lactams are not only very weakly basic (in accord with previous experiment) but also protonate preferentially on nitrogen. This likelihood, taken with the secondary isotope effect, indicates that a likely pathway of acid-catalyzed hydrolysis would be that of an A_(AC)1 mechanism with an intermediate acylium ion. If this were so, the calculated β-secondary isotope effect per hydrogen coplanar with the breaking C—N bond and corrected for the inductive effect of deuterium would be 1.06 ± 0.01. This suggests an early A_(AC)1 transition state, which would be reasonable in this case because of destabilization of the N-protonated amide with respect to the acylium ion because of ring strain. The absence of specific participation by solvent in the transition state, as would be expected of an A_(AC)1 but not an associative mechanism, is supported by the strongly inverse solvent deuterium kinetic isotope effect of 0.25 ± 0.00 in 1 M HCl and 0.22 ± 0.01 in 33.3 wt % H_2SO_4.