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

摘要

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% H2O/20 mol% DMSO at 25.0±0.1°C.The Bronsted-type plot for the reactions of 6a-h with morpholine is linear with beta_(lg)=-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.,k1and 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 thiophene-carbonyl(6a-h)does not influence pK_a~°(defined as the pK_a at the center of the Bronsted curvature)as well as the k2/k_(-1)ratio.