Cation-templated synthesis

J.-P. Bourgeois, P. Seiler, M. Fibbioli, E. Pretsch, F. Diederich, L. Echegoyen, Cyclophane-Type Fullerene-dibenzo [18] crown-6 Conjugates with trans-1, trans-2, and trans-3 Addition Patterns Regioselective Templated Synthesis, X-ray Crystal Structure, Ionophoric Properties, and Cation-Complexation-Dependent Redox Behavior , Helv. Chim Acta 1999, 82,1572-1595. 

The template synthesis method involves the diffusion of ligand precursors into the pores of a zeolite where they can assemble around an intrazeolite metal ion that acts as a template. This approach was first used in 1977 for the intrazeoHte synthesis of Cu, Co and Ni phthalocyanines by the condensation of four molecules of dicyanobenzene around the metal cation within the cages of NaY [132, 174]. 

The binary representation is applicable to various other oxide materials. However, an important distinction can be made between borates and other main group element oxide systems, such as aluminates and silicates. In the latter systems cations predominantly reside at sites created by the demands of rigid anionic oxide frameworks. Although some degree of structural control may be obtained by varying cations or by use of template synthesis, the oxide frameworks of these systems tend to be relatively inflexible in comparison with... 

An early synthesis of [18]crown-6 (1) involved the cyclization of hydroxychloride (30) via a Williamson ether synthesis. The yield was 2%. In an adaptation of this approach ethylene oxide was oligomerized in the presence of an alkali cation template. By varying the cation template from lithium to sodium to potassium the major macrocyclic product changed from [12]crown-4 to [15]crown-5 to [18]crown-6, albeit only in about 10% yield (76CC295). 

Considerably higher yields of [18]crown-6 were obtained when dichloride (32) was condensed with diol (31). Similar procedures for the synthesis of [12]crown-4 and [15]crown-5 have also been developed and the dichloride component can readily be replaced by its ditosylate derivative. Once again the presence of an alkali cation template can considerably increase the yield of the cyclization (77OS30). 

Our template synthesis of knots implies that the target molecules are obtained as cationic dicopper(I) complexes. Therefore we considered the possibility of interconverting both enantiomers into a pair of diastereomeric salts [137, 138] by combining them with an optically active anion. Binaphthyl phosphate (BNP") [139] drew our attention because its chirality arises from the binaphthyl core, which is twisted. This helical structure is of the same type as that of die copper double helix, precursor of the knot. Besides, both compounds are aromatic and, thus, we could expect some potentially helpful stacking interactions [87],... 

Figure 3.45 shows the results of kinetic studies of the template synthesis of benzo [18] crown-6 according to the reaction shown, as a function of the templating cation. Clear rate enhancements... 

Figure 2.24. Schematic representation of the principle of transition-metal-templated synthesis of [2]-rotaxanes from macrocyclic chelate (A), metal cation (black disk) and open chelate (B). The latter bears functions X at its extremities, which will be used for anchoring or constructing the stoppers (represented as diamonds). (/) Threading step (//) stoppering step (in) removal of the metal template.

An example of r-stacking as the driving force in a template synthesis is that by Hiinig et al. [15] of the cationic host molecule (cf. 11), which displays r-acceptor properties. In this case an increase in yield from 3-5% to 16% was observed when an excess of phenanthrene (as 7r-donor guest, see 11) was added to the reaction mixture. This example may even be considered as the first template effect... 

The zeolites are aluminosilicate Framework minerals of general formula [Al,.Si,0,f.. 2j -zH 0.- They are characterized by open structures that permit exchange of cations and water molecules (Fig. 16.2). In the synthetic zeolites the aperture and channel sizes may sometimes be controlled by a sort of template synthesis—the zeolite is synthesized around a particular organoammonium cation. This yields channels of the desired size. The zeolite framework thus behaves in some ways like a clathrate cage about a guest molecule (Chapter 8). The synthesis of zeolites also involves several other factors such as the Al/Si ratio, the pH. the temperature and pressure, and the presence or absence of seed crystals -- ... 

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... 

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