Chiral macrocycles cryptands

Fig. 18 High-pressure synthesis (a) of cryptands and (b) of chiral macrocyclic tetraamides.

All chiral crown ethers incorporating one carbohydrate subunit possess two diastereo-topic faces of the macrocyclic ring. They are able to form diastereoisomeric complexes with primary alkylammonium cations. Since nonbonding interactions are responsible of the chiral recognition of optically active species, it would be desirable to form monofacial ligands in which the inclusion of a chiral molecule (or chiral ion) could proceed from the sterically hindered side only. This special molecular architecture may be followed by the fusion of the cryptand framework and the chiral unit. 

Cryptands are macro-bi- or -poly-cycles able to encapsulate an ion by providing it higher protection because of their cagelike structures, as in (147) and (148). For these ligands the correspondence between cavity size and complex stability is more pronounced than for simple crown ethers. Recent approaches to improve the metal-ion selectivity of cryptands, as for example by replacement of ethylene units between each donor atoms with propylene units, or by insertion of several substituents into the macrocycles, have been reviewed.245 A new, interesting family of cryptands is constituted by borocryptands (149), which are useful receptors for chiral substrates, where enantiomeric differentiation can be achieved by using NMR spectroscopy.246... 

Chiral Guest Recognition, p. 236 Crown Ethers, p. 326 Cryptands, p. 334 Enzyme Mimics, p. 546 Ion-Selective Electrodes, p. 747 lonophores, p. 760 Kinetics of Complexation, p. 116 Lariat Ethers, p. 782 Macrocycle Synthesis, p. 830 Podands, p. 1106... 

An established area of application of macrocyclic polyethers is the stereoselective complexation of chiral guest primary alkylammonium salts by optically active host macrocycles. Full details of the resolution, optical stability, and inclusion into chiral hosts of the binaphthol (84), and also of the chiral recognition properties of crowns [e.g. (85)] based on simple carbohydrate precursors, have been reported this year. An extension of the latter work" utilizes derivatives of the more complex carbohydrates D-glucose and D-galactose. The macrobicyclic polyethers (86) derived from D-glycerol or pentaerythritol have been suggested as potential chiral (at the bridgeheads) cryptands. 

Applications of ferrocenes are numerous,in particular with the use of ferrocenyl phosphines, including chiral ones (see for insance the aminophosphine below), as ligands for catalysis. l,l -bis(diphenylphosphino)ferrocene (dppf, below) is the best known ferrocenyl-based ligand in the catalysis of most classic reactions, and its chemistry and uses are prolific. The redox property of ferrocene has been used to attach it to macrocycles (below), cryptands, calixarenes and other endo-receptors for sensing, a chemistry beautifully developed by Paul Beer at Oxford,... 

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