Present results

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 KSub 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 KSub 1, dm3 mol-1 KSub 3, dm3 mol-1
Sr2+8 x 10212
Ba2+3 x 103-
Table 2
Substratek0, s- 1
11.76 x 10- 5
32.60 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 krel = kobs/ k0 on the analytical Sr2+ and Ba2+ concentration, respectively, where kobs 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, krel (1) ca 15 at [SrBr2] = 4 x 10-2 M and krel (3) ca. 32 at [SrBr2] = 4 x 10-2 M. Since KMeO is identical for both reactions, it derives that KTS / KSub (1) < KTS / KSub (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 KMeO , KSub and KTS . Nevertheless, since KMeO (Ba2+) ca KMeO (Sr2+), 1a , 3 , 4  it derives that (KTS / KSub) (Ba2+) > (KTS / KSub) (Sr2+). Taking into account the data of Table 1, it emerges that the larger and less solvated cation (Ba2+ 1.35 , Sr2+ 1.18 ) 6 is better accommodated by both the substrate and the transition state.


7-2',5',8',11',14'-pentaoxacyclopentadecyl-3,6-dioxaheptyl tosylate 1

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


Introduction

Previous results

References