Nucleophilic substitution reactions of phenyl chloroformates

Abstract

Methanolysis and aminolysis of phenyl chloroformates in acetonitrile have been investigated. The rates are slow due to initial-state stabilization by strong resonance electron donation from the phenoxy group. In both reactions the large positive values of ρ_Y= 0.8–1.6 and low ΔH^‡ and ΔS^‡ values show that the transition state is strongly associative with little bond breaking. This mechanism is supported by the relatively large solvent isotope effect, k_MeOH/k_MeOD≅ 2.3–2.5, and by the relatively strong inverse secondary kinetic isotope effect, k_H/k_D≅ 0.74–0.94, involving deuteriated aniline nucleophiles, in addition to a negative value of ρ_XY. The dependence on aniline basicity, β_X(β_nuc)≅ 0.8, and the ρ_X values of –2.3 are similar to those corresponding values for the reactions of benzoyl chlorides which have been predicted to react by an associative S_N2 mechanism. These observations are consistent with a concerted displacement mechanism for the methanolysis and aminolysis of phenyl chloroformates.