Metal Cations as Templates

Early work involved the use of metal cations as templates in macrocyclization reactions. Seidel [6] reacted 2-aminobenzaldehyde 1 with anhydrous zinc chloride (Scheme 1-1), but did not recognize that he had prepared the macrocycle 2, and it was not until 40 years later that Busch [7] and co-workers elucidated the structure of this type of macrocycle and investigated how metal cations template its formation. 

The use of transition metal cations as templates in self-assembly pre-dates their utilization in the synthesis of catenanes. They were employed by Curtis and Busch in their pioneering work on macrocycle synthesis, while Sargeson... 

This observation led to the notion that particular alkali metal cations appear to have specific affinities for different crown ethers. Greene s insight was to use the appropriate metal cation to template the formation of the crown in much the same way as transition metals had been used as templates for other, nitrogen containing, ligands and to experiment with different solvents. In every example of crown ether synthesis reported by Pedersen, the only metal cation present was Na" " (except in one case where K+ was used) as no hnk had been made between the size of the metal cation used and the target crown at that time. The purpose of a template in the synthesis of 18-crown-6 can be seen in Figure 3. The relationship between metal... 

The potassium cation as template provides the formation of calix[4]tubus LI854 from tetratosylate L1852 and p-iert-butylcalix[4]arene (L1853) in 51% yield (Eq. 9.19) [37]. Direct complexation reactions showed the high selectivity and strong binding ability of LI854 towards potassium ion in contrast to other alkali metal ions. 

Shinkai [15] concluded that p-zert-butyl calix[n]ar-ene tetra esters form stable monolayers at the air-water interface and the metal responds, therein, quite differently from that in solution. They reported that examination of the metal template effect on the conformer distribution established that when the metal cation present in the base used serves as a template, the cone conformer results are predominant [16]. Hence, Na in... 

Chiral-at-metal cations can themselves serve as chirality inducers. For example, optically pure Ru[(bipy)3] proved to be an excellent chiral auxihary for the stereoselective preparation of optically active 3D anionic networks [M(II)Cr(III)(oxalate)3]- n (with M = Mn, Ni), which display interesting magnetic properties. In these networks all of the metalhc centers have the same configuration, z or yl, as the template cation, as shown by CD spectroscopy and X-ray crystallography [43]. 

In the reaction of unprotected uridine with tri(imidazolyl-l)phosphine (ratio 1 1.5) under mild conditions (THF, — 78° to 0 °C, 10 min) and subsequent oxidation with iodide, two types of polymers [(Up)n (n = 2-6) and (Up)nU (n = 2—5)] are formed. Additives such as metal cations or polynucleotides (poly U and poly A) acting as templates in the oxidation process showed a significant effect on the ratio of the 3, 5 -linked to 2, 5 -linked oligomer 17 177 ... 

Aza macrocycles have also been used as templating agents in the formation of zinc and other metal cation-containing aluminophosphates via hydrothermal synthesis.705 The zeolite-like structures have been formed in the presence of cyclam- and hexaaza-based macrocycles. 

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 use of heterogeneous catalysts in the synthesis of urethanes from aliphatic and aromatic amines, C02 and alkyl halides has been explored only recently. Titanosilicate molecular sieves [60a], metal phthalocyanine complexes encapsulated in zeolite-Y [60a], beta-zeolites and mesoporous silica (MCM-41) containing ammonium cations as the templates [60b, c], and adenine-modified Ti-SBA-15 [60d, e] each function as effective catalysts, even without any additional base. 

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