Examination of P-OR Bridging Bond Orders in PhosphateMonoesters using 18O Isotope Shifts in 31P NMR

Document Type

Article

Journal/Book Title

Journal of Organic Chemistry

Publication Date

2005

Volume

70

Issue

12

First Page

4805

Last Page

4809

Abstract

Evidence indicates that phosphate monoesters undergo hydrolysis by a loose transition state with extensive bond fission to the leaving group. It has been proposed that part of the high dependence of the rate on the leaving group pKalg = −1.23) arises from weaker ester bonds in the reactants as the pKa of the leaving group decreases, on the basis of X-ray structures and calculations. In contrast, IR and Raman studies suggest that the leaving group has little effect on the length of the P−OR bridging bond in solution. To gather additional data on this issue, we have used 18O isotopic shifts in 31P NMR to monitor the bond order of P−O bonds in a range of phosphate esters with different leaving groups. Using this technique, we have been able to evaluate whether significant changes are observed in the P−O bond orders for the bridging and nonbridging positions of methyl, ethyl, phenethyl, propargyl, phenyl, and p-nitrophenyl phosphate using [16O18O] labeled species in deuterium oxide. The results indicate that the bridging and nonbridging bond orders to phosphorus in phosphate monoesters are not significantly altered by differences in the pKa of the leaving group or by the counterion of the phosphate ester dianion.

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