Aminolysis of O-Aryl Thionobenzoates: Amine Basicity Combines with Modulation of the Nature of Substituents in the Leaving Group and Thionobenzoate Moiety to Control the Reaction Mechanism

Abstract

A kinetic study is reported for aminolysis of O-Y-substituted phenyl thionobenzoates (1a-f) and O-4-nitrophenyl X-substituted thionobenzoates (2a-f) in 80 mol % H2O/20 mol % DMSO at 25.0 +/- 0.1 degrees C. The reaction proceeds through one or two intermediates (i.e., a zwitterionic tetrahedral intermediate T(+/-) and its deprotonated form T(-)) depending on the basicity difference between the nucleophile and nucleofuge, that is, the reaction proceeds through T(+/-) when the leaving aryloxide is less basic than the attacking amine, but through T(+/-) and T(-) when the leaving group is more basic than the amine. However, the reaction mechanism is not influenced by the electronic nature of the substituent X in the nonleaving group. The Hammett plot for the reactions of 2a-f with benzylamine is consisted of two intersecting straight lines, which might be interpreted as a change in the rate-determining step (RDS). However, the Yukawa-Tsuno plot for the same reactions exhibits an excellent linear correlation, indicating that the nonlinear Hammett plot is not due to a change in the RDS but caused by stabilization of the ground-state of the substrate through resonance interaction between the electron-donating substituent X and the thionocarbonyl moiety.

Nonlinear Brønsted and Hammett Correlations Associated with Reactions of 4‐Chloro‐7‐nitrobenzofurazan with Anilines in Dimethyl Sulfoxide Solution

“…Deviations from Hammett plots based on standard σ galues have been rarely reported in S N Ar reactions, but have been commonly observed in the S N 2 process 33,34. Koh and coworkers, for example, investigated the reactions of substituted phenacyl bromides with pyridines in acetonitrile at 45°C and the curved Hammett plots have been interpreted in terms of a change in the rate‐determining step from breakdown to formation of a tetrahedral intermediate [33a].…”

Single‐Electron Transfer in σ‐Complexation Reactions of 2,6‐Dimethoxy‐3,5‐dinitropyridine with Para‐X‐phenoxide Anions in Aqueous Solution

“…Literature survey reveals that deviations from Hammett plots based on standard σ‐values have absolutely no report of any systematic study on σ‐complexation reactions, but have been commonly observed in S N Ar or S N 2 process . Um and Bae investigated the reactions of 4‐pyridyl X‐substituted‐benzoates (X = 3,5‐(NO 2 ) 2 , 4‐NO 2 , 3‐NO 2 , 4‐CN, 4‐Cl, H, 4‐Me, 4‐OMe, and 4‐NMe 2 ) with piperidine in acetonitrile at 25°C.…”

Kinetic Study on Aminolysis of O‐2‐Pyridyl Thionobenzoate in Acetonitrile: Effect of Changing Electrophilic Center from CO to CS on Reactivity and Reaction Mechanism

“…TS III ) is responsible for the unusually small β nuc value found for the uncatalyzed reaction of 7 to a certain degree. Another plausible reason for the small β nuc value can be found from the report that β nuc is smaller for reactions of substrates possessing a CS electrophilic center than for those of substrates bearing a CO electrophilic site …”

“…As shown in Scheme , aminolysis of esters has been reported to proceed either through a concerted mechanism with a second‐order rate constant ( k c ) or via a stepwise pathway with one or two intermediates, i.e . a zwitterionic tetrahedral intermediate T ± and its deprotonated form T − . Factors suggested to affect the reaction mechanism include type of the reaction medium (protic or aprotic), nature of the electrophilic center (CO, CS, PO, PS and SO 2 ), basicity of the leaving group and incoming amine, etc …”