Kinetics and Mechanism of the Anilinolysis of Diethyl Isothiocyanophosphate in Acetonitrile

초록

To extend the kinetic studies on the phosphoryl transfer reactions involving a leaving group of isothiocyanate, the nucleophilic substitution reactions of diethyl isothiocyanophosphate with substituted anilines (XC6H4NH2) and deuterated anilines (XC6H4ND2) are investigated kinetically in acetonitrile (MeCN) at 55.0 ± 0.1 ℃. The Hammett and Bronsted plots for substituent X variations in the nucleophiles exhibit the two discrete slopes with a break region between X = H and 4-F. The obtained DKIEs (kH/kD) are secondary inverse (kH/kD = 0.74-0.87) with the strongly basic anilines while secondary and primary normal (kH/kD = 0.71-1.58) with the weakly basic anilines. A concerted SN2 mechanism (or a stepwise mechanism with a rate-limiting bond formation step) involving a backside attack is proposed for the weakly basic anilines, and a stepwise mechanism with a rate-limiting leaving group departure from the TBP-5C intermediate involving both backside TSb and frontside attack involving a hydrogen-bonded, four-center-type TSf is proposed for the strongly basic anilines.