Supramolecular effects

The next important phenomena that the result of supramolecular effect are the concentration and proximity effects concerning the components of analytical reaction, even through they are considerably different in hydrophobicity, charge of the species, complexing or collisional type of interaction. The concentration and proximity effects determine the equilibrium of analytical reaction, the efficiencies of intramolecular or intermolecular electronic energy or electron transfer and as a result the sensitivity of analytical reactions. 

Fig. 20. Thermodynamic (A) vs. supramolecular effect (B) in the formation of spirocyclic and ansa products.

Finally, one may note that the variations in the beautiful colours of flowers due to anthocyanins result from supramolecular effects, namely copigmentation, intramolecular sandwich-type stacking and intermolecular association [8.61]. 

Crystallization amounts by nature to the self-assembly of very large, boundaryless supramolecular species. Its control is a goal of major importance in order to be able to generate solid-state materials of specific structural and physical properties (see also Sections 7.1, 7.2, 9.4.4 [7.39-7.42, 9.105, 9.245]). Supramolecular effects play a crucial role. Directional growth of materials may be induced by a template and involve molecular recognition [9.246], occurring by epitaxy [9.247] or on oriented thin films [9.248]. 

Whilst such a classification [1, 9] is useful because it indicates the different types of chemical changes which can occur at the metal centres during phase transfer, it oversimplifies the situation in many cases and fails to indicate the importance of the outer sphere coordination chemistry and of solvation effects in general on the free energies of extraction. Alternative classifications of extraction processes which take better account of these have been presented recently by the Moyer Group. [22] The importance of such supramolecular effects in the design of reagents will be stressed in the examples below. 

The redox chemistry of quinones has been reviewed in the context of hydrogen bonding, protonation, and supramolecular effects that can modify their reactivity.315... 

The final contribution turns back to the more simple aspects of PET in homogeneous media. As was shown for ion pairs in one of the golden ages of physical organic chemistry, the controlled formation of various types of radical ion pairs by photochemical methods can be utilized to control the course of chemical reactions. At this point, the medium effects which govern the formation and the fate of radical ion pairs resemble the supramolecular effects of the arranged systems discussed in the first articles. 

The rhodium containing hybrid materials prepared with the BINOL bicarbamate moieties are less active and enantioselective than the previous hybrid catalyst. The observed enantioselectivity was attributed to supramolecular effect of the chiral tridimensional network owing to the weakness of the interaction of the transition metal and the chiral ligand. The control of the texture and morphology of these solids by templating methods firstly reported by Macquarrie39 and Mann40 with suitable surfactants would improve the catalytic performance of this new class of chiral materials. 

H. Durr, A. Thome, H. Kilburg, S. Bossmann, E. Blasius, K. Janzen, and C. Kranz, Supramolecular effects on photochromism—Properties of crown ether-modified dihydroindolizines, J. Phys. Org. Chem. 5, 689-698 (1992). 

D. Lexa and J. M. Saveant, Supramolecular Effects in the Redox and Coordination Chemistry of Superstructured Iron Porphyrins, in Redox Chemistry and Interfacial Behaviour of Biological Molecules , eds.G. DryhurstandK. Niki, Plenum, New York, 1988, p. 1. 

Y. Murakami, O. Hayashida, Supramolecular Effects and Molecular Discrimination by Macrocyclic Hosts Embedded in Synthetic BUayer Membranes , Proc. Natl. Acad. Sci. USA, 90, 1140 (1993)... 

Depending on the particular structure, the following supramolecular effects can be distinguished (i) encapsulation of a metal-polypyridine unit, (ii) interaction between linked metal-polypyridine units, (iii) interactions between metal-polypyridine units and chemically different redox centers, and, (iv) host-guest interactions where a metal-polypyridine unit is the part of the host. 

Several kinds of supramolecular effects on the redox behavior of metal-polypyridine units were mentioned in Section 5.3.6. Besides influencing photophysical properties, incorporation of metal-bipyridine chromophores into supramolecular structures enables new electron-transfer reactions. Since these processes are dealt with in detail in other chapters, only basic principles and links between the behavior of isolated and supramolecular metal-polypyridine units will be mentioned here. 

More than thirty years ago,62 the first intentional preparation of a catenane provided significant imaginative impetus for further adventures in the realm of chemical topology. While catenanes themselves are not branched species, as is implied by the terms dendrimers or cascade molecules, they are materials which incorporate some of the most interesting of supramolecular effects.63... 

For an alternative analysis of supramolecular effects in bimolecular reactions using the effective molarity approach, see S. Di Stefano, L. Mandolini, Effective molarities in supramolecular catalysis of two-substrate reactions, Acc. Chem. Res., 2004, 37, 113. 

The scope of the area is so wide that all we can attempt to do here is discuss key examples and encourage the interested reader to investigate the other interesting results eited therein. We concentrate strictly on supramolecular effects in chiral induction. There is a separate article in this Encyclopedia that deals with the related field of chiral guest recognition. 

The diastereoselectivity of covalent bond-forming reactions can also be influenced by supramolecular effects. The polymerization of isocyanides shows this effect. 

Xirst, S.C. Hamilton. A.D. Complexation control of pericyclic reactions Supramolecular effects on the Diels-Alder reactions. J. Am. Chem. Soc. 1991, 113. 882. 

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