Metals, activated with crown ethers

Earlier work in this field has been thoroughly reviewed [1,2]. However, to illustrate in a sensible and logical way the evolution from simple metal ion promotion of acyl transfer in supramolecular complexes to supramolecular catalysts capable of turnover catalysis, an account of earlier work is appropriate. The following sections present a brief overview of our earlier observations related to the influence of alkaline-earth metal ions and their complexes with crown ethers on the alcoholysis of esters and of activated amides under basic conditions. 

Introduction of Fluorine with Alkali Metal Fluorides, Including Ammonium Fluoride and Tetraalkylammonium Fluorides (Including Special Methods of Fluorinations, e. g., Phase Transfer Catalysis, Activation by Crown Ethers, Reagents... 

Two basically different strategies have been followed in combining dihydropyridines with crown ethers to obtain catalytically active systems. One approach is to attach one or more derivatives of 43) to or in the periphery of a crown ether, which has good complexing properties for metal ions. Those metal ions should be the electrophilic catalyst for coordination and activation of a carbonyl substrate (eq. 17). The success... 

The cis-isomers of azobis(benzocrown ether)s, 4 and 5, can form intramolecular 1 2 metal/crown sandwich-type complexes with Rb " and Cs ". Examination of Table I reveals that the cis-to-trans isomerization from these sandwich-type complexes requires extra free energies of activation by 0.6-1.7 kcal mol"l. The energies should be consumed to disrupt the interaction between the metal and the crown ethers. On the other hand, the rate inhibition was not observed for 6 which has methylene spacers between the benzenes and the crown rings. Conceivably, the geometrical change induced by the isomerization of the N=N bond may be absorbed by the rotational freedom of these methylene groups. Thus, the effective inhibition occurs only in the rigid cis-isomers. 

There is a review of selected topics in organometallic ion chemistry and evidence for the hollow cage structure of met-cars is collected together with a discussion of cluster-assisted O-bQnd activation.Metal containing ions, [ML]+ (M = Cr, Mn, La, Pr, Yb, Nd and others L = Ph) react with crown ethers in the gas phase by adduct formation or ligand substitution. s... 

The crown ethers and cryptates are able to complex the alkaU metals very strongly (38). AppHcations of these agents depend on the appreciable solubihty of the chelates in a wide range of solvents and the increase in activity of the co-anion in nonaqueous systems. For example, potassium hydroxide or permanganate can be solubiHzed in benzene [71 -43-2] hy dicyclohexano-[18]-crown-6 [16069-36-6]. In nonpolar solvents the anions are neither extensively solvated nor strongly paired with the complexed cation, and they behave as naked or bare anions with enhanced activity. Small amounts of the macrocycHc compounds can serve as phase-transfer agents, and they may be more effective than tetrabutylammonium ion for the purpose. The cost of these macrocycHc agents limits industrial use. 

By considering the stability constant and the lipophilicity of host molecules, Fyles et al. synthesized a series of carboxylic ionophores having a crown ether moiety and energetically developed the active transport of alkali metal cations 27-32). Ionophores 19-21 possess appropriate stability constants for K+ and show effective K+-selective transports (Fig. 5). Although all of the corresponding [15]crown-5 derivatives (22-24) selectively transport Na+, their transport rates are rather slow compared with... 

A certain crown ether having additional coordination sites for a trasition metal cation (71) changes the transport property for alkali metal cations when it complexes with the transition metal cation 76) (Fig. 13). The fact that a carrier can be developed which has a reversible complexation property for a transition metal cation strongly suggests that this type of ionophore can be applied to the active transport system. 

In the context of Scheme 11-1 we are also interested to know whether the variation of K observed with 18-, 21-, and 24-membered crown ethers is due to changes in the complexation rate (k ), the decomplexation rate (k- ), or both. Krane and Skjetne (1980) carried out dynamic 13C NMR studies of complexes of the 4-toluenediazo-nium ion with 18-crown-6, 21-crown-7, and 24-crown-8 in dichlorofluoromethane. They determined the decomplexation rate (k- ) and the free energy of activation for decomplexation (AG i). From the values of k i obtained by Krane and Skjetne and the equilibrium constants K of Nakazumi et al. (1983), k can be calculated. The results show that the complexation rate (kx) does not change much with the size of the macrocycle, that it is most likely diffusion-controlled, and that the large equilibrium constant K of 21-crown-7 is due to the decomplexation rate constant k i being lower than those for the 18- and 24-membered crown ethers. Izatt et al. (1991) published a comprehensive review of K, k, and k data for crown ethers and related hosts with metal cations, ammonium ions, diazonium ions, and related guest compounds. 

In a different approach three different structurally defined aza-crown ethers were treated with 10 different metal salts in a spatially addressable format in a 96-well microtiter plate, producing 40 catalysts, which were tested in the hydrolysis of /xnitrophenol esters.32 A plate reader was used to assess catalyst activity. A cobalt complex turned out to be the best catalyst. Higher diversity is potentially possible, but this would require an efficient synthetic strategy. This research was extended to include lanthanide-based catalysts in the hydrolysis of phospho-esters of DNA.33... 

In 1990 we reported the synthesis of new redox-responsive crown ether molecules that contain a conjugated link between the crown ether unit and a ferrocene redox-active centre (Beer et al., 1990a). Examples of some of the species synthesized are shown in Fig. 5. The electrochemical behaviour of these species was investigated and also the electrochemical behaviour of their analogues with a saturated link between the ferrocene unit and the crown ether. The changes in the CVs of [2a] upon addition of magnesium cations are shown in Fig. 6. The metal cation-induced anodic shifts of [2a], [2b] and also their saturated analogue [3] and vinyl derivatives [4a], [4b] are shown in Table 1. 

The solvents used in these rhodium-catalyzed reactions may also act as complexing agents for counterions of the anionic rhodium complexes. For example, tetraglyme is known to coordinate alkali metal cations. Such solvation decreases the possibility of the cation interacting with the anionic rhodium catalyst and lowering its activity or solubility. The crown ethers, such as [18]-crown-6... 

The findings that, both in ester and amide cleavage, an alkaline-earth metal ion is still catalytically active when complexed with a crown ether, and that a fraction of the binding energy made available by coordinative interactions with the polyether chain can be translated into catalysis, provide the basis for the construction of supramolecular catalysts capable of esterase and amidase activity. 

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