Aminolysis of Y-substituted phenyl 2-thiophenecarboxylates and 2-furoates: Effect of modification of nonleaving group from 2-furoyl to 2-thiophenecarbonyl on reactivity and mechanism

Abstract

Second-order rate constants (kN) have been measured for reactions of Y-substituted phenyl 2-thiophenecarboxylates (6a-h) with morpholine and piperidine in 80 mol % H2O/20 mol % DMSO at 25.0 ± 0.1 °C. The Brønsted-type plot for the reactions of 6a-h with morpholine is linear with βlg = -1.29, indicating that the reactions proceed through a tetrahedral zwitterionic intermediate (T±). On the other hand, the Brønsted-type plot for the reactions of 6a-h with piperidine exhibits a downward curvature, implying that a change in the rate-determining step occurs on changing the substituent Y in the leaving group. Dissection of kN into microscopic rate constants (i.e., k1 and k2/k -1 ratio) has revealed that k1 is smaller for the reactions of 6a-h than for those of Y-substituted phenyl 2-furoates (5a-h), while the k2/k-1 ratio is almost the same for the reactions of 5a-h and 6a-h. It is also reported that modification of the nonleaving group from the furoyl (5a-h) to the thiophenecarbonyl (6a-h) does not influence pKao (defined as the pKa at the center of the Brønsted curvature) as well as the k2/k -1 ratio.