Kinetic studies on nucleophilic substitution reactions of O-Aryl thionobenzoates with azide, cyanide, and hydroxide: Contrasting reactivity and mechanism

Abstract

A kinetic study is reported for nucleophilic substitution reactions of O-Y-substituted phenyl thionobenzoates (1a-h) and O-4-nitrophenyl X-substituted thionobenzoates (2a-f) with N3- and CN- in 80 mol % H2O-2O mol % DMSO at 25.0 ± 0.1 °C. The Br0nsted-type plot for the reactions of 1a-h with N3- exhibits a downward curvature, i.e., the slope (βlg) changes from-1.10 to-0.33 as the leaving group basicity decreases, indicating that the reactions proceed through a stepwise mechanism with a change in rate-determining step (RDS). In contrast, the Br0nsted-type plot for the corresponding reactions with CN-is linear with a βlg value of-0.33. This value is similar to that found previously for the reactions of 1a-h with OH- (-0.35). Besides, o° constants result in much better Hammett correlation than o- constants. Thus, the reactions with CN- and OH- have been concluded to proceed through a stepwise mechanism in which departure of the leaving group occurs after RDS. Reactions of 2a-f with N3- and CN- result in nonlinear Hammett plots. However, the Yukawa-Tsuno plots for the same reactions exhibit excellent linearity with r = 0.5 ± 0.1, indicating that the nonlinear Hammett plots are not due to a change in RDS but are caused by ground state stabilization through resonance interactions between the electron-donating substituent and the thio carbonyl functionality. Calculation of the k1 values (nucleophile attack as RDS) for the reactions of 1a-h with N3- indicates that azide ion is more reactive than OH-toward the thione esters, although the former is over 11 pKa units less basic than the latter. The high polarizability of N3- has been suggested to be responsible for its great affinity for the polarizable thione esters 1a-h and 2a-f. © 2009 American Chemical Society.