Kinetics and mechanism of the aminolysis of thiophenyl methylacetates in acetonitrile

The aminolysis of Z‐thiophenyl methylacetates (C₂H₅C(O)SC₆H₄Z) with X‐benzylamines in acetonitrile has been investigated at 45°C. The reaction is found to proceed by a stepwise mechanism in which the rate‐determining step is the breakdown of the zwitterionic tetrahedral intermediate, T^±, with possibly a hydrogen‐bonded four‐center‐type transition state. These mechanistic conclusions are drawn based on (i) the large magnitude of β_X (= 1.2 ∼ 2.5) and β_z (= −0.9 ∼ −1.5), (ii) the normal kinetic isotope effects (k_H/k_D ≅ 1.2) involving deuterated benzylamines (XC₆H₄CH₂ND₂), (iii) a large positive ρ_xz (= 2.4) and (iv) adherence to the reactivity‐selectivity principle in all cases. The extremely large β_X (β_nuc) values can be accounted for by the loss of a strong localized cationic charge on the N atom of benzylamines in the expulsion from the T^±. The pK_a^o (≥ 10.0) is high due to a large ratio of the expulsion rates of the amine (k_−a) to thiophenolate (k_b) (k_−a/k_b) from the T^±. © 2000 John Wiley & Sons, Inc. Int J Chem Kinet 32: 485–490, 2000