Aminolysis of 4-nitrophenyl phenyl carbonate and thionocarbonate: Effects of amine nature and modification of electrophilic center from C=O to C=S 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% H 2O-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., k 1 and k 2/k -1 ratio) has revealed that k 1 is larger for the reactions with secondary amines than for those with isobasic primary amines, while the k 2/k -1 ratio is nearly identical. On the other hand, for reactions of 6, secondary amines exhibit larger k 1 values but smaller k 2/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 (C=O vs. C=S). © The Royal Society of Chemistry 2008.