Kinetic Study on Reactions of O‐Y‐substituted Phenyl Thionobenzoates with Quinuclidine: Factors Governing Reactivity and Reaction Mechanism

Abstract

Second‐order rate constants (k quin) for the reactions of O‐Y‐substituted phenyl thionobenzoates (3a–3i) with quinuclidine have been measured spectrophotometrically. Comparison of k quin with the rate constants reported previously for the corresponding reactions with benzylamine (k BzNH2) has revealed that quinuclidine is less reactive than benzylamine toward 3a–3i although the former is 2.1 pK a units more basic than the latter. Steric hindrance exerted by quinuclidine has been suggested to be responsible for the decreased reactivity of the tertiary amine. The Brønsted‐type plot for the reactions of 3a–3i with quinuclidine is linear with βlg = −0.37. The Hammett plot correlated with σ Y o constants exhibits many scattered points ( R 2 = 0.982). In contrast, the Yukawa‐Tsuno plot results in an excellent linear correlation ( R 2 = 0.9992) with ρ Y = 0.96 and r = 0.51, indicating that a negative charge develops partially on the O atom of the leaving group. Thus, the reactions of 3a–3i with quinuclidine have been concluded to proceed through a concerted mechanism in which expulsion of the leaving group is advanced only a little in the rate‐determining transition state.

Kinetic Study on Quinuclidinolysis of O‐4‐Nitrophenyl X‐substituted Thionobenzoates and O‐3,4‐Dinitrophenyl Thionobenzoate: Effect of Amine Nature on Reactivity and Reaction Mechanism

“…Thus, one might suggest that the reactions of 1a with quinuclidines proceed through a stepwise mechanism in which expulsion of the leaving group occurs in the RDS on the basis of the linear Brønsted‐type plot with β nuc = 0.89. However, this idea is inconsistent with our previous report that quinuclidinolysis of O ‐Y‐substituted phenyl thionobenzoates including 1a proceeds through a concerted mechanism on the basis of the kinetic result that the Yukawa–Tsuno plot exhibits an excellent linear correlation with ρ Y = 0.96 and r = 0.51 …”

“…We have recently reported that the Brønsted‐type plot for the reactions of quinuclidine with a series of O ‐Y‐substituted phenyl thionobenzoates ( 1a–1i ) is linear with β lg = −0.37, which is typical for reactions reported previously to proceed through a stepwise mechanism, with formation of an intermediate being the RDS. However, the Hammett plot correlated with σ o constants has exhibited highly scattered points . In contrast, the Yukawa–Tsuno plot for the same reactions has resulted in excellent linearity ( R 2 = 0.9991) with ρ Y = 0.96 and r = 0.51 .…”

“…However, the Hammett plot correlated with σ o constants has exhibited highly scattered points . In contrast, the Yukawa–Tsuno plot for the same reactions has resulted in excellent linearity ( R 2 = 0.9991) with ρ Y = 0.96 and r = 0.51 . Thus, the reactions have been proposed to proceed through a forced concerted mechanism .…”

“…In contrast, the Yukawa–Tsuno plot for the same reactions has resulted in excellent linearity ( R 2 = 0.9991) with ρ Y = 0.96 and r = 0.51 . Thus, the reactions have been proposed to proceed through a forced concerted mechanism . Steric hindrance exerted by the tertiary amine has been suggested to be an important factor that governs the reaction mechanism as well as reactivity …”

“…Thus, the reactions have been proposed to proceed through a forced concerted mechanism . Steric hindrance exerted by the tertiary amine has been suggested to be an important factor that governs the reaction mechanism as well as reactivity …”