Formation kinetics crown ether complexes

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). 

Secondary amines can be acylated with acyl groups bearing no a-hydrogens. Deprotonation next to the nitrogen atom and introduction of electrophiles allow the oxidation of that position. Early work showed that these anions underwent self-condensation. For example, lV,7 -dimethylbenzamide can be deproton-atcd with lithium 2,2,6,6-tetramethylpiperidide (LITMP) to give A -methyl-A -phenacylbenzamide in 60% yield. It is clear that lithium ion is crucial to the success of this reaction. Crown ethers prevent the re-action, and recent kinetic studies show intermediate lithium complex formation. ... 

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... 

The kinetic deuterium solvent isotope effects on the acid-catalyzed dissociation of Li(2,l,l) and Ag(2,2,2) have been reported, as have the exchange kinetics " of cryptand (2,2,1) on Tl(2,2,2) and T1(2b,2,2) . The kinetics of complex formation between Li and 18-crown-6 ether in 1,3-dioxalane and 1,2-dimethoxyethane solvents have also been reported. ... 

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