PH-RATE PROFILE FOR HYDROLYSIS OF SOME ESTERS OF A BICYCLIC PHOSPHINIC ACID . EVIDENCE FOR RATE-LIMITING PSEUDOROTATION

The ethyl esters 1 and 4 of substituted bicyclic phosphinic acids undergo rapid hydrolysis of one of the two ester groups in both base and acid. We had previously suggested that this rapid reaction is necessarily accompanied by pseudorotation about the phosphorus atom. We have now established by nmr spectroscopy that the rapid reaction occurs as predicted at the phosphorus atom in the bridge position, where the maximum strain is localized. The pH-rate profiles for the hydrolysis of the esters 1 and 4 show that the rate is proportional to the hydroxide ion concentration at pH > 8, is independent of acidity near pH 5, and is proportional to the hydrogen ion concentration around pH 2. However, the rate does not continue to rise in proportion to the acid concentration above 0.01 M, but reaches a maximum in about 0.2 M acid. The acidity and ionic strength at the maximum rate are too low to account for the deviation from a linear relationship between rate and acidity on the basis of a decrease in the act vity of water or a salt effect. We therefore suggest that, at high acidity, the pseudorotation process itself becomes rate limiting. © 1969, American Chemical Society. All rights reserved.