Kinetics and Mechanism of the Aminolysis of Aryl N-Isopropyl Thiocarbamates in Acetonitrile

We propose a concerted mechanism with a hydrogen bonded cyclic transition state for the aminolysis of aryl N-isopropyl thiocarbamates with benzylamines in acetonitrile based on the negative cross-interaction constant, failure of RSP and the kinetic isotope effects greater than unity. The four conducive factors for the concerted aminolysis mechanism for the present reaction series are: strong (strongest) push provided to expel ArS^- by the nonleaving group, (CH₃)₂CHNH, destabilization of the intermediate, T^±, by a powerful expulsion rate of the benzylamine from T^±, greater leaving ability of -SAr than -OAr due to the greater electron acceptor ability of σ^*_C-S than σ^*_C-O bond orbital, and instability of the intermediate, T^±, in a less polar solvent, MeCN than in water due to the ionic nature of T^±. The stepwise aminolysis mechanism of 2a through a tetrahedral intermediate, however, shifts to a concerted process where the leaving group is changed to a thiophenoxide (EtOC(=O)-SAr) instead of a phenoxide (^-OAr). The push provided by an EtO group to expel ^-SAr in T^± is now strong enough to make the intermediate so unstable that the intermediate can not exist. In view of the similar strong push expected from a (CH₃)₂CNH group to expel the ^-SAr group in T^±, it is of interest to see whether the aminolysis mechanism of the thiol analog of aryl N-isopropyl thiocarbamates, (CH₃)₂CHNHC(=O)SC₆H₄Z, also shifts to a concerted mechanism or not