Hydrolysis of (N-aryl)alkylphenylphosphinic amides in acidic solution: influence of different P-alkyl groups, including cyclopropyl, and of substituents in the N-aryl group

Abstract

The (N-phenyl)alkylphenylphosphinic amides RPhP(O)NHPh (R = ethyl, cyclopropyl, isopropyl, 1-methylcyclopropyl, or t-butyl) give the corresponding alkylphenylphosphinic acids without rearrangement on hydrolysis in aqueous hydrochloric acid. The pseudo-first-order rate constants at 31.2 °C in 1:1 v/v water–methanol 2.08M in hydrochloric acid (10⁵k ψ 690, 275, 11.0, 4.22, and <0.003 5 s^–1, respectively) show that replacement of an isopropyl group by cyclopropyl, and of a t-butyl group by 1-methylcyclopropyl, causes a substantial increase in rate. This is attributed to reduced steric inhibition of an associative (A2) hydrolysis mechanism rather than stabilisation of a dissociative (A1 or A1-like) transition state by the cyclopropyl groups. The hydrolysis reactions have large negative values of ΔS^‡. For the substituted amides RPhP(O)NH·C₆H₄·X-p(X = OMe, H, Br, or NO₂) reducing the nucleophilicity of the leaving group does not materially reduce the sensitivity of hydrolysis to steric hindrance nor, by implication, the associative nature of the mechanism.