Nucleophilic substitution reactions of aryl N-phenyl thiocarbamates with benzylamines in acetonitrile.
The aminolysis reactions of aryl N-phenythiocarbamates (PhNHC(=O)SC(6)H(4)Z; 3b) with benzylamines (XC(6)H(4)CH(2)NH(2)) in acetonitrile are studied. Rates are much faster than the corresponding reactions of aryl N-phenylcarbamates (PhNHC(=O)OC(6)H(4)Z; 3a). The rate increase from 3a to 3b is greater than that expected from substitution of thiophenoxide for phenoxide leaving group in the stepwise aminolysis reactions of esters. This large rate increase and the similar change in the aminolysis rates that are reported to occur from aryl ethyl carbonate (EtOC(=O)OC(6)H(4)Z; 2a) to aryl ethylthiocarbonate (EtOC(=O)SC(6)H(4)Z; 2b) lead us to conclude that the aminolysis of 3b proceeds by a concerted mechanism in contrast to a stepwise process for 3a. The negative rho(XZ) values (-0.63) and violation of the reactivity-selectivity principle (RSP) support the proposed mechanism. The large beta(X) values (1.3-1.5) obtained are considered to indicate a large degree of bond making in the transition state, which is consistent with the relatively large kinetic isotope effects (k(H)/k(D) > 1.0) observed.
Oh
Hyuck Keun
HK
Department of Chemistry, Chonbuk National University, Chonju 560-756, Korea.
Park
Jie Eun
JE
Sung
Dae Dong
DD
Lee
Ikchoon
I