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 N(3)(-) and CN(-) in 80 mol % H(2)O-20 mol % DMSO at 25.0 +/- 0.1 degrees C. The Brønsted-type plot for the reactions of 1a-h with N(3)(-) exhibits a downward curvature, i.e., the slope (beta(lg)) 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 Brønsted-type plot for the corresponding reactions with CN(-) is linear with a beta(lg) value of -0.33. This value is similar to that found previously for the reactions of 1a-h with OH(-) (-0.35). Besides, sigma(o) constants result in much better Hammett correlation than sigma(-) 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 N(3)(-) 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 k(1) values (nucleophile attack as RDS) for the reactions of 1a-h with N(3)(-) indicates that azide ion is more reactive than OH(-) toward the thione esters, although the former is over 11 pK(a) units less basic than the latter. The high polarizability of N(3)(-) has been suggested to be responsible for its great affinity for the polarizable thione esters 1a-h and 2a-f.