Helicate macrocyclic

The linear helicate forms first as a kinetic product, upon heating for a few minutes at 170°C in ethylene glycol. Longer reaction times resulted in disassembly of the initial structure and the formation of the thermodynamic product consisting of a circular pentanuclear helicate. This structure is apparently templated by the chloride anion, which is bound strongly inside the cavity of the helical macrocycle. 

Figure 6.27 Structure of the double-helical, macrocyclic complex, [PbLp (L = 61) ...

Zhao, C., Ren, J Gregolinski, J. et al. (2012) Contrasting enantioselective DNA preference chiral helical macrocyclic lanthanide complex binding to DNA. Nucleic Acids Ties., 40 (16), 8186-96. 

On the basis of the chiral building block 105, a series of enantiopure helical macrocycles (110,112, and 114) containing tetraphenylene motifs were obtained. Platinum(II) complexes 110 and 112 were assembled in tetrahydrofuran (THF)... 

Furthermore, a rigid helical macrocycle 114 was prepared in 56% yield through the self-assembly of (S,S) or R,R)-105 and dimetallic complex 113 under similar conditions (Scheme 5.26) [51]. Circular dichroism (CD) spectra of these three molecules indicated their enantiomeric properties. The heterometallic platinum(II)-ruthenium(II) macrocycle 114 was selected for photophysical study. It exhibited an interesting luminescence behavior. Emission... 

Utilizing the same achiral building block 5, Haley and coworkers have also synthesized the less-symmetrical [22]annulene (Scheme 6.2) [11]. Triyne 5 was converted to oligomer 11 via a one-pot desilylation/alkynylation reaction with bis(2-iodophenyl)ethyne. Compound 11 was subsequently desilylated and subjected to intramolecular oxidative homocoupling conditions to provide the chiral annulene 12 in 59% yield. X-ray crystallographic analysis of 12 showed that the helical macrocycle adopts a distorted saddle-shaped conformation in the solid-state that is relatively free from strain. 

In addition to these pyridine-containing helical macrocycles, there are numerous reports of polyaza-macrocyclic species that are chiral due to the adaptation of a non-planar conformation. Some current examples included the ruffled benzimidazole-based ligands and the distorted phthalocyanines recently prepared by the groups of Chan [40] and Kobayshi [41], respectively. These systems do not fall within the context of this Chapter due to their resemblance to porphyrinic systems but they nevertheless represent an interesting class of chiral macrocycles based upon the incorporation of heterocyclic rings. 

FIG. 14 Construction of periodic framework and geometric objects from DNA. (a) Construction of two-dimensional DNA lattices from tetravalent four-arm DNA junctions (14) [8]. (b) Synthesis of a macrocyclic molecule from bivalent three-arm DNA junctions (13a) containing two cohesive ends [83]. For simplification, linear double-helical stretches are represented by parallel lines. 

The cyclooctapyrroles shown in Figure 55 appear predestined to form binuclear metal complexes since the loop-shaped conformation of these macrocycles exhibits two structurally identical, helical N4 cavities. Enantiomers of such complexes, which are presumably generally very stable towards racemization owing to the rigidity of the molecule imposed by the incorporation of the metal, are of interest as possible models for binuclear metalloenzymes and as potential catalysts in asymmetric synthesis. The first two ligands as well as their recently obtained palladium complexes601 were... 

Figure 14 Molecular structure showing the helical nature of the zinc complex with a 19-methoxyoctaethyl-bilinone ligand formed on ring opening of the oxaporphyrin macrocycle.828...

In the following sections, examples of hydrogen-bonding templates for the synthesis of macrocycles, cages, interlocked species, helicates and for the photochemical reaction of olefins will be discussed. The use of hydrogen-bonding templates in dynamic combinatorial libraries will also be presented. 

The energetics of peptide-porphyrin interactions and peptide ligand-metal binding have also been observed in another self-assembly system constructed by Huffman et al. (125). Using monomeric helices binding to iron(III) coproporphyrin I, a fourfold symmetric tetracarboxylate porphyrin, these authors demonstrate a correlation between the hydropho-bicity of the peptide and the affinity for heme as well as the reduction potential of the encapsulated ferric ion, as shown in Fig. 12. These data clearly demonstrate that heme macrocycle-peptide hydrophobic interactions are important for both the stability of ferric heme proteins and the resultant electrochemistry. 

Macrocyclic ligands of biological importance as thiophenolate-containing Schiff-base macrocycles and their amine analogs (see review [138]) and new helical complexes with bis(bidentate) SchifF-base ligand [139] were also described. 

Metal-directed Self-assembly of Complex Supramolecular Architecture Chains, Racks, Ladders, Grids, Macrocycles, Cages, Nanotubes and Self-intertwining Strands (Helicates)... 

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