Alkaline earth metal cations templates

Greene observed that the formation of 18-crown-6 from a ditosylate and a diol in the presence of f-butoxide salts was enhanced when a potassium cation was used (Greene, 1972). This template effect was operative for the synthesis of other polyether crown compounds using alkali or alkaline-earth metal cations. Template effects have also been observed for the preparation of aza-crown ethers, although the effect is less pronounced because the softer A-donor atoms form weaker complexes with the alkali metal cations (Frens-dorff, 1971). Richman and Atkins reported that high-dilution techniques were not required for the cyclization reaction of the disodium salt of a pertosylated oligoethylenepolyamine with sulfonated diols to form medium and large polyaza-crown compounds (Richman and Atkins, 1974 Atkins et al., 1978). 

Template effects play an important role in most of the cyclizations leading to alloxygen macrocycles. Usually, alkali or alkaline earth metal cations serve as a template ion. Transition metal cations are useful in the syntheses of macrocyclic polyamines. The template effect is less pronounced in the case when formation of mixed nitrogen/ oxygen macrocycles are formed in comparison with their all-oxygen analogues. 

Since the free SBs are not always stable, many SB complexes are prepared by template synthesis which implies carrying out the condensation reaction in the presence of a metal ion. This is a very common method for the preparation of macrocyclic SB complexes.2,3 In this case large alkaline earth metal cations (i.e., Ba11, lanthanoids, or Pb11) have proved to be efficient templates. The size... 

The simplest type of template directed cyclization is metal cation induced macrocycliza-tion, as discussed in Section 1.1. Mandolin and co-workers [37] have made a detailed study of the influence of alkali and alkaline earth metal cations on the cyclization of 2-hydroxyphenyl-3,6,9,12-tetraoxa-14-bromotetradecyl ether 28 in methanol and in dimethyl sulfoxide (Scheme 1-8). They studied different sizes of macrocycle and metal cation to determine the most effective combination for cyclization the results from this study will be summarized in Section 1.4.2.3. 

In zeolite synthesis, sUicate and alummate ions are reacted with base under hydrothermal conditions. Basic components such as inorganic alkali or alkaline earth metal cations can also be used in the synthesis, in addition to organic cations. The organic cations can act as a structme-directing template. 

The calcium, strontium, barium, and lead 80) complexes of 160 and 161 have also been reported. In these two ligands the six donor atoms are essentially confined in a plane these complexes thus permit study of unusual coordination geometries in species of high coordination number. Attempts to form alkali metal complexes with 160 and 161 under the same conditions as employed for the alkaline earth metal complexes have failed. The successful syntheses of complexes of the latter type indicate that the higher charge to radius ratio is of consequence when spherically charged cations are employed. Such metal ions have no apparent coordinative discrimination as the template ion 87). 

In the context of crown ether hosts, non-covalent bonds of pole-pole, pole-dipole, and dipole-dipole types can all be employed [3-6] in the formation of host-guest complexes. Where the guest species is an alkali metal (i.e. Li, Na", K", Rb, Cs ), alkaline earth metal (i.e. Mg, Ca, Sr, Ba ), or harder transition or post-transition metal (e.g. Ag", TT, Hg, Pb, La, Ce ) cation [3-6,14], an electrostatic (M" O) pole-dipole interaction binds the guest to the host whilst the (M" X ) pole-pole interaction with the counterion (X ) is often retained. The features are exemplified by the X-ray crystal structure [15] shown in Fig. la for the 1 2 complex (1) (NaPF jj formed between dibenzo-36-crown-12 (1) and NaPF. Molecular complexes involving metal cations have considerable strengths even in aqueous solution and a template effect involving the metal cation is often observed during the synthesis of crown ether derivatives. 

Crown ethers are excellent ligands for alkali and alkaline earth metals. As a rule, they form the most stable coordination compounds with those cations that are the best template agents for the assembly of the corresponding macrocycles. 

The addition of various alkali and alkaline earth cations to the cyclooligomerization of ethylene oxide by Dale and Daasvatn also provides strong presumptive evidence for the template effect. Recently, Reinhoudt, de Jong and Tomassen utilized several metal fluorides to effect crown formations . The reaction rates were found to be in the order Cs > Rb > > Na LE. Such an order would be expected on the basis of binding... 

We have not attempted to cover all or even most aspects of crown chemistry and some may say that the inclusions are eclectic. We felt that anyone approaching the field would need an appreciation for the jargon currently abounding and for the so-called template effect since the latter has a considerable bearing on the synthetic methodology. We have, therefore, included brief discussions of these topics in the first two chapters. In chapters 3—8, we have tried to present an overview of the macrocyclic polyethers which have been prepared. We have taken a decidedly organic tack in this attempting to be comprehensive in our inclusion of alkali and alkaline earth cation binders rather than the compounds of use in transition metal chemistry. Nevertheless, many of the latter are included in concert with their overall importance. 

The interaction of the lone pair on the nitrogen atom of an amine with a metal ion spans a considerable range from the very weak template assisted association of alkali and alkaline earth cations " to the very robust Co" and Ru" metal nitrogen bonds. In aqueous solution, most amines are basic (Chapter 4) and there will always be competition between the lone pairs on oxygen (OH" or HjO) and the lone pair on the nitrogen, for the metal center. This competition can be overcome to a certain extent by using non-aqueous systems or the free amine as a solvent." " ... 

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