Present results

The reaction between the lariat ether 1 with MeO^- ion , in MeOH, at 40 °C, to yield the corresponding methoxy derivative 2 , Scheme 3 , was studied under the same conditions as indicated for the reaction of compound 3 , (see Previous results) , but at variance with this reaction, was possible to determine also the effect of Ba²⁺.
In Table 1 and Table 2 are reported the values of the binding constants K_Sub determined by ¹H NMR for the crown ethers 1 and 3 at ionic strength = 0.2 M, and the pseudo-first-order rate constants k₀ determined for the substitution reactions of 1 and 3 in the absence of alkaline-earth salts, at [Me₄NOMe] = 0.1 M and at ionic strength = 0.2 M, respectively.

**Table 1**
Cation	K_Sub 1, dm³ mol^-1	K_Sub 3, dm³ mol^-1
Sr²⁺	8 x 10²	12
Ba²⁺	3 x 10³	-

**Table 2**
Substrate	k₀, s^{- 1}
1	1.76 x 10^{- 5}
3	2.60 x 10^{- 6}

The values reported in Table 1 for Sr²⁺ clearly indicate that the polyoxyethylene side arm participates in the binding of alkaline earth cations.

The increase in reactivity observed for compound 1 with respect to 3 (Table 2) can be rationalized by the effect of the different branching of the carbon atom adjacent to the reaction centre. ⁵

In Fig. 2 and Fig. 3 are plotted the dependence of k_rel = k_obs/ k₀ on the analytical Sr²⁺ and Ba²⁺ concentration, respectively, where k_obs are the pseudo-first-order rate constants measured in the presence of the alkaline earth cations.

The comparison with the reaction of compound 3 in the presence of Sr²⁺ (see Previous results) shows a slight reduction of the catalytic effect, k_rel (1) ca 15 at [SrBr₂] = 4 x 10^-2 M and k_rel (3) ca. 32 at [SrBr₂] = 4 x 10^-2 M. Since K_MeO is identical for both reactions, it derives that K_TS / K_Sub (1) < K_TS / K_Sub (3), i.e. the polyoxyethylene side arm stabilizes the transition state to a lower extent than ground state.

Barium cation is more effective than strontium cation in catalysing the substitution process. This result depends on the relative importance of K_MeO , K_Sub and K_TS . Nevertheless, since K_{MeO (Ba²⁺)} ca K_{MeO (Sr²⁺)}, ^1a ^, ³ ^, ⁴ it derives that (K_TS / K_Sub) _(Ba²⁺) > (K_TS / K_Sub) _(Sr²⁺). Taking into account the data of Table 1, it emerges that the larger and less solvated cation (Ba²⁺ 1.35 Å , Sr²⁺ 1.18 Å) ⁶ is better accommodated by both the substrate and the transition state.

7-2',5',8',11',14'-pentaoxacyclopentadecyl-3,6-dioxaheptyl tosylate

9-2',5',8',11',14'-pentaoxacyclopentadecyl-2,5,8-trioxanonane

Introduction

Previous results

References