Nucleophilic Substitution Reactions of Thiopheneethyl Arenesulfonafes with Anilines and N,N-Dimethylanilines

Nucleophilic substitution reactions of 2-thiopheneethyl benzenesulfonates (2-TEB) and 3-thiopheneethyl ben-zenesulfonates (3-TEB) with anilines and N,N-dimethylanilines (DMA) are investigated in acetonitrile at 60.0 oC. The cross-interaction constants r XZ determined for the reactions with anilines are large negative (- 0.50) which are comparable to those for the similar predominantly frontside-attack S N2 reactions of 1-phenylethyl (1-PEB), 2-phenylethyl (2-PEB) and cumyl benzenesulfonates. A large negative r XZ value (- 0.4 ~- 0.8) is con-sidered to provide a mechanistic criterion for the frontside-attack S N 2 mechanism with a four-center transition state. In agreement with this proposal the kinetic isotope effects, k H / k D, involving deuterated aniline nu-cleophiles are all greater than one reflecting partial N-H(D) bond cleavage in the transition state. The MO theoretical reactant structures of 1-PEB, 2-PEB and 2-TEB based on the PM3 calculation show that the ben-zene ring blocks the backside nucleophile approach to the reaction center carbon (C a) enforcing the frontside-at-tack S N 2 mechanism