Kinetics and mechanism of the reaction of mercuric acetate with substituted phenylhydrazones. Part 4. An internal cyclometallation. Reactions of mercury(II) acetate with nitrogen compounds

Abstract

Treatment of substituted benzaldehyde phenylhydrazones with mercury(II) acetate in acetic acid resulted in exclusive mercuriation at the ortho-site of the N-phenyl ring. Kinetic measurements at 25 °C in acetic acid gave Hammett ρ values for methine phenyl substituents of –0.81 for the p-bromophenylhydrazone series and –1.02 for the p-nitrophenylhydrazone series (σ⁺ correlation). The ρ values for amino-N-phenyl substituents were –2.30 for the p-tolualdehyde series and –2.99 for the p-nitrobenzaldehyde series (σⁿ correlation). Thermodynamic parameters ΔE_act 17.5, ΔH_act 16.9 kcal mol^–1, and ΔS_act–14.1 cal K^–1 mol^–1 were measured for benzaldehyde p-bromophenylhydrazone. The ortho-mercuriation occurred by an internal cyclometallation process involving the hydrazone imino-moiety. The comparison between the reactions of mercuric acetate and lead tetra-acetate with phenyl-hydrazone systems is discussed.