Reaction of phosphonated acetals. Part 2. Synthesis and acid-catalysed hydrolysis of the cyclic acetals 2-diethylphosphonomethyl-1,3-dioxolan, 2-diethylphosphonomethyl-1,3-dioxan, and bis-O-(diethylphosphonoethyl)pentaerythritol

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Abstract
Phosphonated cyclic acetals were prepared by transacetalation of diethyl 2,2-diethoxyethylphosphonate (I) with ethylene glycol, propane-1,3-diol, or pentaerythritol, to form 2-diethylphosphonomethyl-1,3-dioxolan (III), 2-di-ethylphosphonomethyl-1,3-dioxan (IV), and bis(diethylphosphonomethyl)pentaerythritol (V), respectively. The structures of the products were confirmed by 1H and 13C n.m.r. and by mass and i.r. spectrometry. The kinetics of acid-catalysed hydrolysis of the cyclic acetals were measured in water containing 4% dioxan (v/v) as a function of acid concentration and temperature, and with respect to the deuterium solvent isotope effect. The results are compared with those of the open-chain acetals (I) and diethyl 2,2-dimethoxyethylphosphonate (II). The Zucker–Hammett hypothesis gives for compounds (III)–(V) a linear dependence of log kobs on log CH3O+ , with slopes between 1.7 and 3.2. The Bunnett W parameters have values of 0.7–2.7, and kD2O /kH2O is ca. 2. The reaction of the dioxan derivative (IV) is five times slower than that of the corresponding dioxolan derivative (III). These criteria are not in accordance with an A1 reaction, as found for acetals (I) and (II), nor do they agree too well with an A2 reaction. In view of these results we tentatively suggest for the cyclic acetals (III) and (IV) an A–SE2 mechanism. In the proposed mechanism the initial protonated substrate is partially stabilised by hydrogen bonding between the acetal oxygen and the phosphoryl oxygen atoms. As a result the oxonium ion is involved in a reversible reaction and hence proton transfer to the oxygen atom is the rate-determining step of the hydrolysis.

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