Previous results

We have already shown that the Sr²⁺ cation accelerates the reaction between the 2,5,8,11,14-pentaoxacyclopentadecylmethyl tosylate 3 and MeO^- ion, in MeOH at 40 °C, to yield the corresponding methoxy derivative 4, Scheme 1.
In Fig. 1 the dependence of k_rel = k_obs/ k₀ on the analytical Sr²⁺ concentration is plotted, where k₀ = 2.6 x 10^{- 6}s^{- 1} is the pseudo-first-order rate constant for the reaction 3 ^_ 4 measured at [Me₄NOMe] = 0.1 M and at ionic strength = 0.2 M, and k_obs is the pseudo-first-order rate constant measured under the same conditions in the presence of the alkaline-earth cation. ^1b

The existence of a remarkable catalysis indicates that strontium stabilizes the transition state of this reaction more than the corresponding ground state. ^1>2 To the ground state stabilization concur the coordination of metal cation by the substrate and by the methoxy cation, that are expressed by the substrate binding constant K_Sub = 12 dm³ mol^-1 (determined by ¹H NMR) and by the methoxide binding constant K_MeO , Scheme 2, whose value is not firmly determined, but seems to vary between 20-100 dm³ mol^-1. ^1a,3,4

The greater transition state stabilization by the metal cation indicates that K_TS K_Sub and K_TS K_MeO , Scheme 2, and appears due both to the cooperative effects of the electrostatic interaction with the negative charge, that is delocalised between the entering and leaving groups, and to the binding to the polyether moiety.

2,5,8,11,14-pentaoxacyclopentadecylmethyl tosylate

4 Fig. 1

Introduction

Present results

References