Previous results

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 krel = kobs/ k0 on the analytical Sr2+ concentration is plotted, where k0 = 2.6 x 10- 6s- 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 kobs 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 KSub = 12 dm3 mol-1 (determined by 1H NMR) and by the methoxide binding constant KMeO , 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 KTS KSub and KTS KMeO , 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 3

3-2',5',8',11',14'-pentaoxacyclopentadecyl-2-oxapropane 4 Fig. 1


Present results