Nucleophilic Substitution Reactions of Aryl Dithioacetates with Pyridines in Acetonitrile

Abstract

Kinetic studies of the pyridinolysis (XC(5)H(4)N) of aryl dithioacetates (CH(3)C(=S)SC(6)H(4)Z) are carried out in acetonitrile at 60.0 degrees C. A biphasic Brönsted plot is obtained with a change in slope from a large value (beta(X) congruent with 0.9) to a small value (beta(X) congruent with 0.4) at pK(a) degrees = 5.2, which is attributed to a change in the rate-limiting step from breakdown to formation of a zwitterionic tetrahedral intermediate, T(+/-), in the reaction path as the basicity of the pyridine nucleophile increases. A clear-cut change in the cross-interaction constants rho(XZ) from a large positive value (rho(XZ) = +1.34) to a small negative value (rho(XZ) = -0.15) supports the mechanistic change proposed.

Nucleophilic Substitution Reactions of Aryl Thiophene-2-carbodithioates with Pyridines in Acetonitrile

“…Such decrease in the magnitude of the ρZ values from large (ρ Z = 3.0) to small values (ρ Z = 2.3) with the mechanistic change is also reported in the pyridinolysis of aryl dithioacetates. 13 A rough estimate 13 of the β Z (=β lg ) values shows a decrease from β Z ≅ -0.5 to -0.3 at the breakpoint in agreement with the change in the rate-determining step.…”

Kinetic Studies on the Structure-Reactivity of Aryl Dithiomethylacetates

“…These values, together with those of the pK a of the conjugate acids of the pyridines, are summarized in the Table 1 . 15 The higher rates observed with CH 3 than C 2 H 5 can be explained by the steric effect of the ethyl group. The Brönsted plots using the kN and pK a values in Table 1 were obtained as presented in Figure 1.…”

“…On the other hand, in the reactions of 1 the βx values were 1.5-2.8 and 0.9-1.2 with benzylamines (at -35.0 o C) and anilines (at 45.0 o C), respectively 15 and no breakpoints were observed. This means that the breakpoints are at pK a o ≥ 9.7 (the highest pK a used; 4-methoxybenzylamine) and pK a o ≥ 5.4 (the highest pK a used; 4-methoxyaniline) for the reactions with benzylamines and anilines, respectively.…”

“…(2). The aim is to complete the previous studies 15 on the aminolysis mechanism of aryl dithiomethylacetates and to further clarify the influence of the amine nature on the pKa o value. As an additional criterion for the elucidation of the mechanism, we determined the crossinteraction constant, 16 ρXZ, in eqs.…”

Kinetics and Mechanism of the Anilinolysis of Aryl Ethyl Isothiocyanophosphates in Acetonitrile

“…with the conventional criteria, where nonlinear free energy correlation of the concave downward plot is diagnostic of a rate-limiting step change from bond breaking with weakly basic nucleophiles to bond formation with strongly basic nucleophiles. [11][12][13][14][15][16][17][18][19][20] In this mechanism for anilines with high pK a values, k -a << k b , meaning that the k a step is the ratedetermining step, i.e., k H(D) = k a with smaller Brönsted coefficients β X in Eq. (3).…”