Anion template effect

Scheme 15 A new anion template effect for the syntheses of ether rotaxanes.

Because earlier reviews deal explicitly with these template effects [4, 7, 12], the following discussion is restricted to a recently discovered anion template effect (Scheme 6.5.3). fn non-competitive solvents, the tetralactam macrocyde strongly binds anions such as chloride, bromide, or phenolate. Phenolate stoppers bound to the macrocycle can act as wheeled nudeophiles and react through the wheel with a semi-axle generated in situ to yield ether rotaxanes in yields of 57% to 95% [13]. Consequently, the macrocyde not only represents an anion receptor, but as a concave template simultaneously provides the correct orientation of the guest for threading the axle into the wheel. 

Scheme 6.5.3. Anion template effect for efficient rotaxane synthesis. 

Campos-Femandez C, Schottel BL, Chifotides HT et al (2005) Anion template effect on the self-assembly and interconversion of metallacyclophanes. J Am Chem Soc 127 12909-12923... 

While the synthesis of macrocycles 9.24-9.27 can be considered as being conceptually straightforward in terms of experiment, one fact proved unexpected and interesting. This was the finding that nitric acid proved to be the most efficient catalyst for macrocycle formation. Indeed, attempts to prepare system 9.26, for instance, using other acid catalysts (e.g., HCl, HBr, H2SO4) led to considerably lower yields. This led Sessler and coworkers to suggest that the formation of this macrocycle was mediated via an anion template effect. This was a rather new conception at the time it was put forward. ... 

A tetraphenyl substituted derivative of [12]mercuracarborand-4 (40), which binds one iodide ion in its cavity due to steric hinderance, has been reported. The stereochemistry of the phenyl groups was found to depend on the mercury counteranion used during the synthesis (106). This observation provided yet further evidence for a direct anion templating effect in mercuracarborane syntheses. Most recently, the C—Hg—C link in this type of host has been replaced with a B—Hg—B link, which alters the electron demands of the mercury centers (reducing their electron deficiency) and apparently switches off anion complexation (107). 

In the formation of 113, an anion template effect was not visible because of the diminished ability of the 1,2,4-triazole nucleus to form H-bonds, but the enhanced... 

Zheng, Z. Knobler. C.B. Hawthorne. M.F. Stereoselective anion template effects Syntheses and molecular structures of tetraphenyl [12]mercuracarborand-4 complexes of halide ions. J. Am. Chem. Soc. 1995. 117 (18), 5105-5113. 

Anion-template effect of Bp4 ion on self-assembly of a r-symmetric C04L6 ortho-... 

Although we have stressed the aqueous chemistry aspect in this chapter, all condensation processes are influenced by. some countercations and by the solvent. The few examples discussed here show that these factors are likely to open the way for new synthesis routes for many applications such as the control of the formation of oligomers by the cationic (or anionic) template effect. 

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