Alkylphenylphosphinic amides: formation of stable, crystalline hydrochlorides, and hydrolysis in acidic solution

Abstract

Alkylphenylphosphinic amides RPhP(O)NH₂, having R = cyclopropyl (8), 1-methylcyclopropyl (9), ethyl (15), isopropyl (16), and t-butyl (17), have been synthesised, and their reactions with anhydrous hydrogen chloride in benzene investigated. At room temperature amides (8) and (15) liberated ammonium chloride as expected, but (9), (16), and (17) formed stable, crystalline amide hydrochlorides RPhP(O)NH₂,HCl. The stability of these hydrochlorides is attributed to steric inhibition of nucleophilic attack at phosphorus by their relatively bulky alkyl groups. In accord with this explanation, the pseudo-first-order rate constants for hydrolysis of amides (9), (16), and (17) at pH 1·85 and 25·6°(10⁵k_ψ= 2·48, 5·50, and <0·0032 s^–1 respectively) are substantially smaller than those for (8) and (15)(10⁵k_ψ= 111 and 153 s^–1).