Aminolysis of S-4-nitrophenyl X-substituted thiobenzoates: Effect of nonleaving-group substituents on reactivity and mechanism

Abstract

A kinetic study is reported for aminolysis of S-4-nitrophenyl X-substituted thiobenzoates 3a-g in 80 mol % H 2O/20 mol % DMSO at 25.0 ± 0.1 °C. Thiol esters 3a-g are 7.8-47.6 fold more reactive than the corresponding oxygen esters (i.e., 4-nitrophenyl X-substituted benzoates 1a-g). Such reactivity order appears to be in accordance with the expectation that 4-nitrothiophenoxide in 3a-g is a better nucleofuge than 4-nitrophenoxide in 1a-g since the former is 2.64 pK a units less basic than the latter. Hammett plot for the reactions of 3a-g exhibit poor correlation coefficients (R 2 = 0.977-0.986) with negative deviation by substrates possessing an electrondonating group (EDG), while the Yukawa-Tsuno plots result in excellent linear correlation (R 2 = 0.995-0.997) with ? = 0.93-1.23 and r = 0.57-0.67, indicating that the negative deviation shown by substrates possessing an EDG is caused by ground-state stabilization through resonance interactions but not due to a change in ratedetermining step upon changing the nonleaving-group substituent X. The p value increases as the incoming amine becomes more basic and more reactive, indicating that the RSP is not operative in the current reactions.