Kinetic Study on Nucleophilic Substitution Reactions of O‐Phenyl O‐Y‐substituted‐Phenyl Thionocarbonates with 1,8‐Diazabicyclo[5.4.0]undec‐7‐ene in Acetonitrile

Abstract

Second‐order rate constants (kN) for nucleophilic substitution reactions of O‐phenyl O‐Y‐substituted‐phenyl thionocarbonates (4a–4k) with 1,8‐diazabicyclo[5.4.0]undec‐7‐ene (DBU) in MeCN at 25.0 ± 0.1°C are reported. The reactivity of 4a–4k decreases as basicity of the leaving group increases except O‐2,4‐dinitrophenyl O‐phenyl thionocarbonate (4a), which is less reactive than O‐3,4‐dinitrophenyl O‐phenyl thionocarbonate (4b) although the former possesses 2 pKa units less basic nucleofuge than the latter. The Brønsted‐type plot for the reactions of 4b–4k is linear with βlg = −0.50, a typical βlg value for reactions reported to proceed through a concerted mechanism. The Hammett plot correlated with σY− constants for the reactions of 4b–4k results in a better linear correlation than that correlated with σYo constants. Besides, the Yukawa‐Tsuno plot exhibits an excellent linear correlation with ρY = 2.12, r = 0.68 and R2 = 0.990, indicating that a negative charge develops partially on the O atom of the leaving group in the rate‐determining step (RDS). Thus, the reactions have been concluded to proceed through a forced concerted mechanism. Effects of steric hindrance on reactivity and reaction mechanism are also discussed in detail.