Aminolysis of 4-nitrophenyl phenyl carbonate and thionocarbonate: effects of amine nature and modification of electrophilic center from CO to CS on reactivity and mechanism

Abstract

A kinetic study is reported for the reactions of 4-nitrophenyl phenyl carbonate (5) and thionocarbonate (6) with a series of alicyclic secondary amines in 80 mol% H₂O–20 mol% DMSO at 25.0 ± 0.1 °C. The plots of k_obsdvs. amine concentration are linear for the reactions of 5. On the contrary, the plots for the corresponding reactions of 6 curve upward as a function of increasing amine concentration, indicating that the reactions proceed through two intermediates (i.e., a zwitterionic tetrahedral intermediate T^± and its deprotonated form T⁻). The Brønsted-type plot for the reactions of 5 with secondary amines exhibits a downward curvature, i.e., the slope decreases from 0.98 to 0.26 as the pK_a of the conjugate acid of amines increases, implying that the reactions proceed through T^± with a change in the rate-determining step (RDS). The k_N values are larger for the reactions of 5 with secondary amines than for those with primary amines of similar basicity. Dissection of k_N values for the reactions of 5 into the microscopic rate constants (i.e., k₁ and k₂/k₋₁ ratio) has revealed that k₁ is larger for the reactions with secondary amines than for those with isobasic primary amines, while the k₂/k₋₁ ratio is nearly identical. On the other hand, for reactions of 6, secondary amines exhibit larger k₁ values but smaller k₂/k₋₁ ratios than primary amines. The current study has shown that the reactivity and reaction mechanism are strongly influenced by the nature of amines (primary vs.secondary amines) and electrophilic centers (CO vs. CS).