The acid-catalyzed hydrolysis of phosphinates. III. The mechanism of hydrolysis of methyl and benzyl dialkylphosphinates

Abstract

The rates of the acid-catalyzed hydrolysis of methyl dimethyl-, diethyl-, diisopropyl-, and di-tert-butylphosphinate in D2SO4–D2O and of p-methoxybenzyl, p-chlaorobenzyl, p-nitrobenzyl, and benzyl diethylphosphinate in H2SO4 – 70% w/w DMSO-H2O have been studied. The bell-shaped pH-rate profiles, the Bunnett–Olsen Φ (0.91 to 1.28), Bunnett w (2.28 to 3.52) and w* (−0.94 to +0.17), and Yates-McClelland r (0.61 to 2.6) values, and the entropies of activation (−18 to −24 eu) all support an A2 mechanism for the methyl esters. The suggested mechanism for the benzyl esters is a unimolecular hydrolysis (Aa11). This conclusion is supported by the pH-rate profiles, the Φr values (−0.12 to −0.29) and the entropies of activation (−4 to −7 eu). The pKa's of the esters are also reported and they range between −2.4 to −3.3.

The acid-catalyzed hydrolysis of phosphinates. IV. Pentacoordinate intermediate formation during hydrolysis of a phosphinothionate

“…2 and this work), which hydrolyze by an A-2 pathway, there is a linear relationship between r and m (using eq. [2] Qualitatively this is very reasonable since esters which are poorly solvated (high m values) will require more additional water in the rate-determining step (higher r value).…”

“…1 and, for the sake of comparison, Fig. 2 gives the profiles for the hydrolysis reaction of 3 to 4 and of l a to 5 (2).…”

“…methyl dimethylphosphinodithioate, were determined by the nmr method (14) and the results are given in Table 2. Included in Table 2 are the previously determined results for ester l a (2). The value of m in Table 2 substrate as compared to Hammett bases (14).…”

“…All rates were followed on a Varian Model EM 360L nmr spectrometer. For acidities below 50% DZS04, where no sulfur exchange was observed, the reactions were monitored in a manner similar to that of our previous report (2). At higher acidities two regions of the spectra were examined, the downfield region represented by all OCH, groups and the upfield region represented by all P-CH, groups.…”

“…From our previous studies on the acid-catalyzed hydrolysis of phosphinates we reported that methyl dialkylphosphinates hydrolyze by an A-2 process and, based on relative rate data for the esters l a -d of 11 : 2.5: 1.2: 1, we suggested that water attacks the carbon of the protonated ester with subsequent C-0 bond cleavage (2). On the other hand, McClelland has shown by 0 -1 8 studies that compounds 2a-c undergo acidcatalyzed hydrolysis with approximately 90% P-0 bond cleavage (3).…”