Kinetics and mechanism of the anilinolyses of aryl dimethyl, methyl phenyl and diphenyl phosphinates

Abstract

The reactions of Z-aryl dimethyl (1), methyl phenyl (2), and diphenyl (3) phosphinates with X-anilines in dimethyl sulfoxide at 60.0 °C are studied kinetically. Kinetic results yield the primary normal deuterium kinetic isotope effects (DKIEs) involving deuterated aniline (XC₆H₄ND₂) nucleophiles, k_H/k_D = 1.03–1.17, 1.15–1.29, and 1.24–1.51, and the cross-interaction constants (CICs), ρ_XZ = 0.37, 0.34, and 0.65 for 1, 2, and 3, respectively. The steric effects of the ligands (R₁ and R₂) on reaction rates play a role, but are relatively much smaller compared to other phosphinate systems. A stepwise mechanism with a rate-limiting leaving group expulsion from the intermediate is proposed on the basis of the CICs positive signs. The dominant frontside nucleophilic attack through a hydrogen-bonded, four-center-type transition state is proposed on the basis of primary normal DKIEs and large magnitudes of the CICs for 2 and 3, while both frontside and backside attack are proposed on the basis of relatively small primary normal DKIEs for 1.