Macrocycles host-guest chemistry

Table 1 An illustrative timeline charting the development of supramolecular chemistry from its roots in solid-state inclusion compounds, through the birth of macrocyclic host-guest chemistry in the 1960s to its modem incarnation in self-assembled materials and nanoscale chemistry.

Chemical transformations at the macroeyclic chromophorc of expanded porphyrins are still not known. The complexation behavior of expanded porphyrins is very different from that of nonexpanded porphinoid macrocycles. The coordination hole of the expanded porphyrins is often too big for the complexation of a single metal ion, so in fact two metal ions can be chelated. With some expanded porphyrins, anion binding is observable, a striking difference to the nonexpanded porphyrins. The complexation behavior and the host-guest chemistry of expanded porphyrins is a rapidly growing field of research. The work in this field has been reviewed. Ie f... 

The shape-persistent, structurally well-defined nature of PAMs and PDMs make them attractive models for binding guest molecules within their cavities. In 1995, Hoger and Enkelmann reported the construction of the first meta/para-PAM/PDM hybrid designed to possess hydrophobic and hydrophilic substituents for subsequent use in host guest chemistry [71]. Macrocyclic amphiphile 116 was assembled via the straightforward manner depicted in Scheme 26. 

Host-guest chemistry macrocyclic hosts and non-metallic guests... 

The synthesis of the macrocycles 43 (Scheme 9) is an example of repetitive, highly stereoselective Diels-Alder reaction between bis-dienes 41 and bis-dienophiles 42, containing all oxo or methano bridges syn to one another. The consecutive inter- and intramolecular Diels-Alder reactions only succeed at high pressure. Obviously, both reactions are accelerated by pressure. The macrocycles are of interest in supramolecular chemistry (host-guest chemistry) because of their well-defined cavities with different sizes depending on the arene spacer-units. 

Macrocycles containing isoxazoline or isoxazole ring systems, potential receptors in host—guest chemistry, have been prepared by multiple (double, triple or quadruple) 1,3-dipolar cycloadditions of nitrile oxides, (prepared in situ from hydroxamoyl chlorides) to bifunctional calixarenes, ethylene glycols, or silanes containing unsaturated ester or alkene moieties (453). This one-pot synthetic method has been readily extended to the preparation of different types of macrocycles such as cyclophanes, bis-calix[4]arenes and sila-macrocycles. The ring size of macrocycles can be controlled by appropriate choices of the nitrile oxide precursors and the bifunctional dipolarophiles. Multiple cycloadditive macrocy-clization is a potentially useful method for the synthesis of macrocycles. 

It should be stressed that there is not alwaysjustice in reseach evaluation. The selective formation of inclusion complexes by cyclodextrins (such as 11) was established by Cramer [6] at least 15 years earlier than that by crown ethers. However, cyclodextrin studies forming an independent branch of host-guest chemistry seem underestimated in spite of their considerably greater practical importance at present than that of other host macrocycles (crown ethers 17, calixarenes 18, etc.). Sometimes they are even totally neglected by discussing inclusion phenomena [7]. 

Cyclodextrins, CDs, 278 and their complexes clearly fall into the domain of host-guest chemistry and the importance of these macrocycles is enhanced by their numerous practical applications [lb, lc]. However, they form an... 

Yang, X. G., Knobler, C. B., Zheng, Z. P., Hawthorne, M. F., Host-guest chemistry of a new class of macrocyclic multidentate lewis-acids comprised of carborane-supported electrophilic mercury centers. J. Am. Chem. Soc. 1994,116, 7142-7159. 

Cucurbit [6] uril and increasingly its higher homologues have been used extensively in a variety of host-guest chemistry and nanostructure assemblies. Of particular appeal are rotaxanes and molecular necklaces, mechanically interlocked assemblies of molecules based on CB [6]. Efficient synthesis of ID, 2D and 3D polyrotaxanes and molecular necklaces (cucurbituril beads linked by a macrocyclic molecule string ) has been achieved by a combination of self-assembly and coordination chemistry. We discuss rotaxanes and molecular necklaces in Section 10.7, and cucurbil-based systems are summarised in a recent review.23... 

A simple possibility for the synthesis of esters, the reaction of an acid chloride with an alcohol, was used by Schrage and Vogtle [58] for a two-step synthesis of the macrocycle 63 from the alcohol 61 and the acid chloride 62. Compound 63, an example from the field of host/guest chemistry, forms a cavity, as studied with CPK-models, which could include planar, aromatic guests. Crystals obtained from benzene/ -heptane point to a 1 2 stoichiometry of 63 and benzene according to NMR-spectroscopic data. However, whether this is a molecular inclusion complex or just a clathrate is not yet known. 

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