Thiol-catalysed hydrolysis of cephalosporins and possible rate-limiting amine anion expulsion

The rates of thiolysis of cephalosporins were investigated by high-performance liquid chromatography and ¹H NMR spectroscopy. Thiols catalyse the hydrolysis through the formation of a thioester intermediate and the catalytically reactive form of the thiol is the thiolate anion. Variation of nucleophilic reactivity by changing the basicity of the thiolate anion generates a Brønsted β_nuc value of 1.22 with cephaloridine, indicating that the breakdown of the tetrahedral intermediate is the rate-limiting step. The effect of C3′ substituents on the rate of thiolysis of cephalosporins generates a large Hammett ρ of ca 12, which is compatible with C-N bond fission occurring without protonation of the β-lactam nitrogen. Solvent kinetic isotope effects k_H2O/k _D2O of ca 1.1 also indicate that solvent water probably does not act as a general acid catalyst facilitating breakdown of the tetrahedral intermediate by protonating the departing amine.