Transition State Variation in the Anilinolysis of O-Aryl Phenyl Phosphonochloridothioates in Acetonitrile

Abstract

The nucleophilic substitution reactions of Y-O-aryl phenyl phosphonochloridothioates with substituted anilines (<TEX>$XC_6H_4NH_2$</TEX>) and deuterated anilines (<TEX>$XC_6H_4ND_2$</TEX>) are kinetically investigated in acetonitrile at <TEX>$55.0^{\circ}C$</TEX>. The deuterium kinetic isotope effects (DKIEs) invariably increase from an extremely large secondary inverse (<TEX>$k_H/k_D$</TEX> = 0.439; min) to a primary normal (<TEX>$k_H/k_D$</TEX> = 1.34; max) as both substituents of nucleophile (X) and substrate (Y) change from electron-donating to electron-withdrawing. These results are opposite to the DKIEs on Y-O-aryl methyl phosphonochloridothioates, and can be rationalized by the gradual transition state (TS) variation from backside to frontside attack. The trigonal bipyramidal pentacoordinate TS is proposed for a backside attack, while the hydrogen-bonded, four-center-type TS is proposed for a frontside attack. The negative values of the cross-interaction constants (<TEX>${\rho}_{XY(H)}$</TEX> = -0.38 for <TEX>$XC_6H_4NH_2$</TEX> and <TEX>${\rho}_{XY(D)}$</TEX> = -0.29 for <TEX>$XC_6H_4ND_2$</TEX>) indicate that the reactions proceed by a concerted <TEX>$S_N2$</TEX> mechanism.