Kinetics and Mechanism of the Anilinolysis of Diethyl Thiophosphinic Chloride in Acetonitrile

The nucleophilic substitution reactions of diethyl thiophosphinic chloride with substituted anilines (XC6H4NH2) and deuterated anilines (XC6H4ND2) are investigated kinetically in acetonitrile at 55.0 degrees C. The values of deuterium kinetic isotope effects (DKIEs; k(H)/k(D)) invariably increase from secondary inverse (k(H)/k(D) < 1) to primary normal (k(H)/k(D) > 1) as the nucleophiles change from the strongly basic to weakly basic anilines. The secondary inverse with the strongly basic anilines and primary normal DKIEs with the weakly basic anilines are rationalized by the gradual transition state (TS) variation from a predominant backside attack, via invariably increasing the fraction of a frontside attack, to a predominant frontside attack, in which the reaction mechanism is a concerted S(N)2 pathway. A frontside attack involving a hydrogen bonded, four-center-type IS is substantiated by the primary normal DKIEs.