Reaction of phosphonated acetals. Acid-catalysed hydrolysis of dialkyl 2,2-dialkoxyethylphosphonates

Abstract

The kinetics of the acid-catalysed hydrolysis of diethyl 2,2-diethoxyethylphosphonate(I) of diethyl 2,2-dimethoxyethylphosphonate(II), and of dimethyl 2,2-dimethoxyethylphosphonate(III) haye been studied in aqueous dioxan solution containing different acid catalysts. Plots of log k₁ against H₀ had slopes of 0.98; Plots of (log k₁+H₀) against log a_H2O had slopes W=–0.1. The deuterium solvent isotope effect k_D/k_H for compound(II) in 1M-sulphuric acid at 30° was 2.8. According to the hypotheses of Zucker–Hammett and of Bunnett, the results obtained are in agreement with an A-1 mechanism. The measured second-order rate constants at 25° were: k₂ 3.7 × 10^–4 l mols^–1 for compound(I) and 1.7 × 10^–4 l mol s^–1 for compounds(II) and (III). Low values of the energy (15–17 kcal mol^–1) and entropy (–25 to –27 cal mol^–1 K^–1) of activation were observed. The substituents at the central carbon atom show the expected polar effects, without any measurable steric effects. The observed values of the energy and entropy activation are correlated with the polarity of the phosphoryl group. An effect of intramolecular catalytic influence is introduced. The products of hydrolysis were identified as the corresponding aldehydes; that of compounds(II) and (III) is the novel dimethyl formylmethyl phosphonate.