Oxygen supramolecular chemistry

On their own polyethers have had limited use in supramolecular chemistry. Low molecular mass compounds have been shown to bind a number of metal ions, notably those in the lanthanide series that can accommodate ligands with large numbers of oxygen donor atoms. One of the main problems is the lack of diversity in the compounds functional groups which limits the range of their ligating opportunities. Where polyethers have been highly successful is as substituents to other molecules, such as calixarenes, and in their cyclic forms as the crown ethers, which will be described later. 

Particular interest has been directed towards the use of the MNK reaction in supramolecular chemistry. The possibility of modifying a substrate backbone, by substituting an oxygen atom with sulfur, can be of extreme importance for changing a molecule s recognition properties. Indeed, this transformation has been exploited in typical hosts such as crown ethers and calixarenes via the MNK rearrangement. 

In 1987, Pedersen and Lehn shared the Nobel Prize for the development of Supramolecular Chemistry with a third recipient, Donald Cram, also very well-known for his work in carbanion chemistry. Cram realised that the chemical reactivity of a carbanion could be increased by the separation of a counter-cation from its anion. This separation was achieved by fully encapsulating the cation with cyclic polyethers and represents an example of the naked anion effect. This is where the anion has been stripped away from its counter-ion, such that there is little or no association between the anion and cation. Cram and his team went about synthesising cyclic polyethers in which the oxygen atoms comprise a small cavity that is rigidly preorganised in an octahedral array. These macrocycles are termed spherands, of which the Li -selective spherand-6 (2.15) is one of the best-known examples. 

Crown ethers are very simple macrocyclic ligands constituted by a cyclic array of ether oxygen atoms connected through carbon atoms, which have been successfully used as appealing hosts in supramolecular chemistry for cations as well as neutral molecules. Recently, crown... 

The final chapter, Heterocyclic Supramolecular Chemistry of Fullerenes and Carbon Nanotubes by N. Komatsu presents an extremely unique review that focuses on the noncovalent chemistry of fullerenes and carbon nanotubes with nitrogen- and/or oxygen-containing heterocyclic molecules such as porphyrin, DNA, protein, peptide, and carbohydrate. Not only exohedral but also endohedral fimctionahzation is reviewed, because the above guest molecules can interact with both faces of the carbon nanotubes. The hurdles in structural separation, nanofabrication, and bioappHcations of carbon nanotubes will hopefully be addressed by the supramolecular strategy. 

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