Entropy and Enthalpy Contributions to Solvent Effects onPhosphate Monoester Solvolysis. The Importance of Entropy Effects in the Dissociative TransitionState
Journal of Organic Chemistry
The solvolysis reactions of a series of aryl phosphates in tert-butyl alcohol and in tert-amyl alcohol have been examined. The dianion of p-nitrophenyl phosphate reacts 7500- and 8750-fold faster in these solvents, respectively, than the corresponding aqueous reactions. The monoanion reacts 14- and 16-fold slower respectively in tert-butyl alcohol and in tert-amyl alcohol. Analysis of the activation parameters shows that the rate enhancement for the dianion is due solely to entropic factors, while the slower reaction of the monoanion is due to increased enthalpy of activation. The significantly more positive entropy of activation for the solvolysis of p-nitrophenyl phosphate dianion in tert-butyl alcohol supports the original proposal that racemization at phosphorus in this reaction is caused by a switch to a DN + AN mechanism, rather than subsequently proposed mechanisms which avoid the formation of metaphosphate. Rate enhancements of similar magnitudes are seen for the dianion reactions of all of the aryl phosphates examined; the slope of a plot of the rate constants for solvolysis versus the aqueous pKa of the leaving phenols has a slope of −1.1, within experimental error of the value for the aqueous reaction. However, in the reactions in tert-amyl alcohol, para-substituted and meta-substituted aryl phosphates fall on separate but parallel lines with para-substituted compounds reacting faster than meta-substituted reactants with leaving groups of similar pKa. The pKa values for a series of para- and meta-substituted phenols in tert-butyl alcohol and in tert-amyl alcohol were determined and were found to have a linear relationship with the aqueous pKa values, with no distinction between para and meta substitution. Thus the different Brønsted behavior of para- and meta-substituted aryl phosphates in these solvents is not due to differential solvent-induced perturbations of the pKa values of the leaving groups. The mechanistic implications of these results and their relevance to enzymatic phosphoryl transfer are discussed.
R. H. Hoff and A. C. Hengge. “Entropy and Enthalpy Contributions to Solvent Effects on Phosphate Monoester Solvolysis. The Importance of Entropy Effects in the Dissociative Transition State,” J. Org. Chem., 1998, 63, 6680-6688.