Synthetic organic guests

Much attention has recently been focused on organoboronic acids and their esters because of their practical usefulness for synthetic organic reactions including asymmetric synthesis, combinatorial synthesis, and polymer synthesis [1, 3, 7-9], molecular recognition such as host-guest compounds [10], and neutron capture therapy in treatment of malignant melanoma and brain tumor ]11]. New synthetic procedures reviewed in this article wiU serve to find further appHcations of organoboron compounds. 

A number of capped cyclodextrins which are able to bind a metal ion at one end of their cavity, together with an organic guest in the cavity, have been synthesized. Such species parallel in several respects the family of completely synthetic vaulted transition-metal complexes prepared by Busch and coworkers and already discussed in Chapter 3 (section 3.5). 

Cyclophanes consist of a class of artificial hosts featured with well-defined hydrophobic cavities constructed by aromatic rings incorporated in their macrocy-clic structures, and also with high design versatility because they are totally synthetic.The first direct evidence of the formation of an inclusion complex with an organic guest was obtained for tetraazacyclophane 62, the cavity of which is constructed with diphenylmethane units bridged by tetramethylene chains. 

D.J. Cram, J.M. Cram, Design of Complexes between Synthetic Hosts and Organic Guest , Acc. Chem. Res., 11,8 (1978)... 

The above discussion indicates that guest molecules or ions play different roles under different synthetic conditions. However, in most cases, it is very difficult to tell exactly the roles of inorganic or organic guest molecules in the synthesis. A large number of experiments still cannot be clearly interpreted. 

The first chapter by Bates and Gale provides an overview of the coordination of anions by synthetic organic hosts. The different organic functional groups used to bind anions are presented and this provides an introduction to the structural and electronic properties that hosts must have to recognise anionic guests. On the other hand, Bayly and Beer give a detailed account of the use of metal complexes as anion receptors. Besides the important structural features that metals can confer to receptors, their optical and redox properties make them attractive for the development of anion sensors. 

In this chapter, we will concentrate on the subjects of synthetic strategies for (1) porphyrins and the capability of the products for molecular recognition of small organic substrates via intermolecular and noncovalent interactions (2) effective electron-transfer reaction regulated by intermolecular interaction " and (3) self-assembled porphyrin to construct supramolecular system for the guest. The Structures of porphyrins and their metalloporphyrin derivatives have attracted chemists due to their suitability as a host framework for organic guest molecules. 

Since these hosts are totally synthetic, design for selective and efficient artificial systems via host-guest complex formation is open as one of the most challenging and exciting fields in synthetic organic chemistry. 

The most common, and well-studied, clathrate systems are those in which water forms the host network, the clathrate hydrates. These are naturally occurring materials and are also of industrial importance and will be discussed separately in Section 4.4. First, we will look at a few representative examples of synthetic, organic clathrates that are created from a wide variety of molecules, each producing a unique network into which guests are included. 

The incorporation of hydrophobic subunits such as aryl, alkyl, napthyl, binaphthyl, etc., into a macrocyclic structural framework leads to synthetic host molecules containing large endolipophilic cavities which have the potential of accommodating neutral organic guest species. 

Cram DJ, Cram JM. Design of complexes between synthetic hosts and organic guests. Acc Chem Res 1978 11 8-14. 

A synthetic advance associated with the synthesis of a//-homocalixarenes is that almost any binding site or functional group can be readily incorporated. These multiple possibilities for structural modification enable the host structure to be optimised for metal cations, anions, and both neutral and charged organic guests. Many applications as analytical sensors can then be envisaged. 

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