Hydrolysis of diarylphosphinic amides in acidic solution: steric inhibition and mechanism

Abstract

The pseudo-first-order rate constants for the hydrolysis of diphenylphosphinic amide Ph₂P(O)NH₂ and its di-p-tolyl, di-o-tolyl, and dimesityl analogues (10⁶kψ 3 630, 2 340, 81.0, and 3.27 s^–1 respectively with H⁺ 0.0662M and T 30.2 °C) in water–dioxan (9 : 1 v/v) containing perchloric acid show that reaction is sterically hindered by ortho-methyl substituents in the P-aryl groups. Steric inhibition is as great, or greater, in the hydrolysis of the corresponding (N-phenyl)diarylphosphinic amides (10⁶kψ 5 440, 4 470, 54.9, and 0.88 s^–1 respectively with H⁺ 1.36M and T 39.9 °C) and (N-p-nitrophenyl)diarylphosphinic amides (10⁶kψ 724, 702, 6.57, and 0.098 s^–1 respectively with H⁺ 2.58M and T 39.9 °C) even though the departing amine is less nucleophilic. Such sensitivity to steric hindrance is consistent with associative (A2) mechanisms for the hydrolysis of all the substrates.