Template effect transition metal complexes

Finally, Sargeson and co-workers have prepared a series of thioether-containing cage ligands, typically using template reactions. Transition metal complexes of these compounds exhibit interesting properties and the extremely robust nature of the complexes is attributed to the cryptate effect. 50,51... 

Polymeric pseudocrown ether networks have been generated in situ by the photopolymerization of poly(ethylene glycol) diacrylate transition metal complexes <00CM633>, and the effect of metal ion templation was evaluated. The 1,6,13,18-tetraoxa[6.6]paracyclophane-3,15-diyne (termed pyxophanes) was prepared from hydroquinone and l,4-dichlorobut-2-yne it forms size-selective 7i-complexes with alkali metal cations <00CC2377>. Dibenzo[ ]crown-m have been used in numerous elegant studies in which they were the needles that were threaded by diverse reagents the resultant... 

Until now syntheses along path d) of Figure 3 are known only for small bicyclic systems, for instance N(CH2CH20)3B from triethanolamine and B(OH)3 (87), N(CH2CH2CH2)3B from triallylamine and BH3 (88). However, macrobicyclic structures may be obtained in this way from a tripod type structure by making use of the template effect of a complexed transition metal cation, which remains included in the product (89-91). 

According to the opinion of the authors of Ref. 32, the negative results of the experiments above do not mean that metal-free Pc could not be theoretically obtained from these precursors additional detailed studies (combination of different techniques, such as UV irradiation, microwave treatment, use of other inert solvents, electrolysis in the systems producing free radicals, etc.) are required for successful resolution of this problem. However, under the same conditions it is possible to obtain some transition metal complexes of the Pc due to the template effects (see below). 

Asymmetric induction using a transition metal complex (67), use of a carbohydrate template (68), and use of the chiral lactone 49 (69) or ester 50 (70) have all given effective syntheses of stereospecifically labeled samples of glycine. A further synthesis by Santaniello etal. (71, 72) has used glutamate decarboxylase to prepare labeled samples of y-aminobutyric acid 51 (Scheme 16). On cyclization, protection, and oxidation, these gave the labeled enamides 55, which were degraded to the labeled samples of glycine 23 (71, 72). 

This first case is the least interesting, although it is by far the most common result in putative template reactions, even when the metal ion is apparently the right size for the desired product A good example is found in the reaction presented in Fig. 6-29. All of the dipositive first-row transition metal ions have similar sizes, but only nickel(n) is effective for the formation of complexes of 6.30 in a template condensation. The other metals either give polymers, complexes of 6.31, or in the case of cobalt(n), compounds that are halfway to the desired product containing 6.32. 

In many cases it is possible to utilise the hole size effects for the synthesis of specific types of macrocycle. Thus, a tetradentate macrocycle (6.33) is expected to be obtained from a template condensation of 2,6-diacetylpyridine with 1,5,9-triazanonane in the presence of small, first-row transition metal dications. The hole size of 6.33 closely matches the size of these metal ions. This is indeed what happens when Ni2+ (r = 0.8 A) is used as a template for the condensation and the nickel(n) complex of 6.33 is obtained in good yield (Fig. 6-32). However, when Ag+ (r = 1.0 A) is used as a template, the metal ion... 

The use of metal ions as kinetic synthetic templates is extremely widespread, and is an excellent way in which to bring about the organisation of a number of reacting components in order to direct the geometry of the product. Because some metal ions, such as the transition metals, often have preferred coordination geometries (e.g. tetrahedral, square planar, octahedral etc), changes in metal ion may have a profound effect on the nature of the templated product. Metal-ion-templated syntheses may be classified more generally as examples of self-assembly with covalent postmodification. For example, the synthesis of the artificial siderophore 10.2 is effected by the use of an octahedral Fe3+ template.8 In this case, the macrobicyclic product is obtained as the Fe3+ complex from which it is difficult to separate. 

The ideal monomer for the synthesis of a symmetrically substituted tetrabenzoporphyrin by an analogous tetramerization would be expected to be the isoindole 96. This substance has not been prepared, but the expectation of its usefulness is confirmed by the observation that 1,3,4,7-tetra-methylisoindole is an excellent precursor, in spite of the fact that it appears to be structurally unsuitable in that it has one a-methyl group too many. Thus, heating 1,3,4,7-tetramethylisoindole with nickel(II) acetate in 1,2,4-trichlorobenzene under reflux provides the nickel(II) octamethyltetra-benzoporphyrin (97) in 71% yield.123 The transition metal ion has a template effect since without it only a small amount (5%) of the metal-free tetrabenzoporphyrin is formed. Various other metal complexes (Mg, Mn, Fe, Co, Cu) have been made this way.123 124 An alternative route involves fusing the... 

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. 

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