Complexation kinetics crown ethers

De Jong et al. (1976c, 1977) have studied the rate of decomplexation of t-BuNHjPFj (AM) complexes of crown ethers. The exchange broadening of the t-butyl nmr signal was used to determine the rate of cation exchange between a kinetically stable (X.AM) and a kinetically unstable (Y.AM)... 

Kinetic and thermodynamic data for t-BuNH3PF6 complexes of crown ethers in CDC13 at 20°C ... 

Substituent effect on the kinetic stability (4(7 in kcal mol-1 at Tc) of RNH3C104 complexes of crown ethers [257] in CD2C12 ... 

Obtained kinetic regularities of the oxidation of ethylbenzene testify to formation presumably of (Fe(II)(acac)2)p (18K6) complexes and products of their transformation in the course of oxidation. It is known that Fe(II) and Fe(III) halogens form complexes with crown-ethers of variable composition (1 1, 1 2, 2 1) and stmcture dependent on type of crown-ether and solvent [87]. 

When the organic solvent containing crown ether is used in the form of a liquid membrane between two aqueous solutions (see Section 5.4.4), an alkali metal salt can be selectively transferred through the organic liquid membrane from one aqueous feed solution to an aqueous strip solution. Illustrative treatment of the kinetics and mechanisms of formation and dissociation of the metal complexes with crown ethers is available in Burgess... 

Evidence that the cation plays an essential role, at least in some cases, is that when the Li was effectively removed from L1A1H4 (by the addition of a crown ether), the reaction did not take place. The complex 19 must now be hydrolyzed to the alcohol. For NaBH4 the Na" " does not seem to participate in the transition state, but kinetic evidence shows that an OR group from the solvent does participate and remains attached to the boron... 

The relative order of the catalytic activities of the crown ethers ([20] + [21]) > [9] > [ 11 ] > [ 13] > [8]) is the same as the relative order of their capacities to bind K+ (Table 4). However, the intrinsic reactivities of the ion pairs were also dependent on crown-ether structure, as was shown by experiments in which the alkylation rates were determined at various crown/phenoxide molar ratios. The curve obtained (Fig. 2) is similar to the curves found in titration experiments (Live and Chan, 1976 De Jong et al., 1976b), and shows that the rate constant reaches a maximum (called plateau kinetics in the literature) when all of the salt is complexed. 

Whereas Gold and Sghibartz showed that cation complexation depressed the rate of crown-ether disrupture, there is convincing evidence that crown ether formation is facilitated by the presence of cations. The template effect, presumably due to complexation of the open-chain precursor and formation of a crown-type conformation, clearly emerged from studies in which the yield of crown ethers was related to the type of cations present (Reinhoudt et al., 1976). Kinetic evidence for the template effect was presented by Mandolini and Masci (1977), who showed that the rate of cyclization of the precursor of benzo-18-crown-6 [2061 decreased in the order Ba2+ > SrJ+ > K+ > Na+ > Li+. This sequence is the same as the one found for the stability constants of the 1 1 complexes of these cations with 18-crown-6 in water (Table 3). 

So far only a few quantitative data on the thermodynamic stability of arenediazonium salts and crown ethers have been reported. Bartsch et al. (1976) calculated the value of the association constant of the complex of 18-crown-6 and 4-t-butylbenzenediazonium tetrafluoroborate from kinetic data on the thermal decomposition of the complex, Kt = 1.56 x 105 1 mol-1 in 1,2-dichloroethane at 50°C. Compared with the corresponding linear polyether this is at least a factor of 30 higher (Bartsch and Juri, 1979). 

The thermal decomposition of arenediazonium tetrafluoroborates is slowed down when the salt is complexed by 18-crown-6 (Bartsch et al., 1976). The kinetic data obtained for the 4-t-butylbenzenediazonium salt at 50°C in 1,2-dichloroethane revealed that the rate of complexed to uncomplexed salt is more than 100. Other crown ethers such as dibenzo-18-crown-6 and dicyclohexyl-18-crown-6 exhibited the same effect but smaller molecules such as 15-crown-5 did not influence the rate at all. It is not only the rate of the Schiemann reaction that is affected by the crown ether nucleophilic aromatic substitutions by halide ions (Cl-, Br-) at the 4-positions in arenediazonium salts are retarded or even entirely inhibited when 18-crown-6 is added. This is attributed to the attenuation of the positive charge at the diazonio group in the complex (Gokel et al., 1977). 

The observed catalytic effect of the crown ether appears to be dependent on the nucleophile employed in both polymerization and corresponding model reactions. Not surprisingly, it appears that the stronger the nucleophile employed, the smaller the catalytic influence of the crown ether. For example, with potassium thiophen-oxide yields of polymer or model products were almost quantitative with or without catalyst. By contrast, the reaction of PFB with potassium phthalimide, a considerably weaker nucleophile, affords 6 in 50% with catalyst and in 2-3% without catalyst under identical conditions. However, it may be that this qualitative difference in rates is, in fact, an artifact of different solubilities of the crown complexed nucleophiles in the organic liquid phase. A careful kinetic study of nucleophilicity in catalyzed versus non-catalyzed reactions study is presently underway. 

The study of the complexing of macrocycle ligands should be considered for its intrinsic importance rather than for its value in illuminating the mechanism of substitution. Kinetic (but much more thermodynamic °) data are available for the reactions of the different macrocycle ligand types, shown in Fig. 4.5, including azamacrocycles,crown ethers and cryp-tands, and porphyrins. ... 

Lehn and coworkers have profitably employed tartaric acid-containing crown ethers as enzyme models. The rate of proton transfer to an ammonium-substituted pyridinium substrate from a tetra-l,4-dihydropyridine-substituted crown ether was considerably enhanced compared to that for a simple 1,4-dihydropyridine. The reaction showed first order kinetic data and was inhibited by potassium ions. Intramolecular proton transfer from receptor to substrate was thus inferred via the hydrogen bonded receptor-substrate complex shown in Figure 16a (78CC143). 

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