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

Second-order rate constants (k N ) have been measured for reactions of Y-substituted phenyl 2-thiophene-carboxylates (6a-h) with morpholine and piperidine in 80 mol % H 2 O/20 mol % DMSO at 25.0 ± 0.1 °C. The Bronsted-type plot for the reactions of 6a-h with morpholine is linear with β 1g = -1.29, indicating that the reactions proceed through a tetrahedral zwitterionic intermediate (T ± ). On the other hand, the Bronsted-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 k N into microscopic rate constants (i.e., k 1 and k 2 /k -1 ratio) has revealed that k 1 is smaller for the reactions of 6a-h than for those of Y-substituted phenyl 2-furoates (5a-h), while the k 2 /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 thiophene-carbonyl (6a-h) does not influence pK a o (defined as the pK a at the center of the Bronsted curvature) as well as the k 2 /k -1 ratio.