Reactions of Pyrazolylborate−Zinc−Hydroxide Complexes Related to β-Lactamase Activity

Simple β-lactams and their hydrolysis products, the β-amino acids, react with Tp*Zn−OH under deprotonation. The latter become semibidentate carboxylate ligands with a NH···O hydrogen bond, and the former become N-bound β-lactamide ligands. Likewise the antibiotic derivatives 6-aminopenicillanic acid and 7-aminocephalosporanic acid are incorporated as carboxylate ligands. β-Lactams bearing nitrophenyl or acyl substituents at the nitrogen atoms are opened hydrolytically by Tp*Zn−OH, and the resulting N-substituted β-amino acids are attached to zinc by their carboxylate functions. Only with trifluoroacetyl as the N-substituent does the hydrolytic cleavage occur at the external amide bond, yielding the free β-lactam and Tp*Zn−trifluoroacetate. The kinetic investigation of the opening reactions has shown them to be of second order like all other Tp*Zn−OH-induced hydrolytic cleavages, thereby supporting the four-center mechanism for the monozinc β-lactamases.