Host-guest assemblies

Asymmetric Induction in the Photochemistry of Crystalline Host-Guest Assemblies... 

Dynamic combinatorial chemistry (DCC) is marvelously effective at discovering receptors for a broad array of analytes. The nature of the internal competition experiment ensures (normally) that the most effective binder for the analyte of interest is amplified for subsequent identification and characterization. In the context of a host-guest assembly, the issue of stereochemistry can be manifested in a number of scenarios. These include various permutations of chiral or achiral guests, along with achiral, enan-tiopure, or racemic dynamic library components. 

Pluth, M.D. and Raymond, K.N. (2007) Reversible guest exchange mechanisms in supramolecular host-guest assemblies. Chem. Soc. Rev, 36 (2), 161-171. 

Use of chiral single crystals to convert achiral reactants to chiral products in high optical yield application to die di-Jt-methane and Norrish type II photorearrangements, J. Am. Chem. Soc., 108, 5648-5649. (b) Chen, J., Pokkuluri, P. R., Scheffer, J. R., and Trotter J. (1990) Absolute asymmetric induction differences in dual pathway photoreactions, Tetrahedron Lett., 31, 6803-6806. (c) Fu, T. Y., Liu, Z., Scheffer, J. R., and Trotter, J. (1993) Supramolecular photochemistry of crystalline host-guest assemblies absolute asymmetric photorearrangement of the host component, J. Am. Chem. Soc., 115, 12202-12203. (d) Leibovitch, M.,... 

The chiral sources employed include circularly polarized light, chiral sensitizers, chiral solvents, chiral substituents, chiral host-guest assemblies, and chiral crystalline environments. 

The above are examples of the types of reactions carried out in isotropic media. The stereoselectivities attained in these types of reactions are usually low, except when the chiral source is covalently attached to the substrate. If the substrate and/or the chiral source are forced to be in the same neighborhood it might increase the probability of the substrate and the chiral source interacting within the lifetime of the excited state of the substrate. This idea is realized in the reactions carried out in host-guest assemblies and in solid-state reactions. 

The inclusion complex 26, shown in Scheme 11, has been used as a host by Toda and coworkers to carry out a number of enantioselective reactions [231]. For example, irradiation of a 1 1 host-guest assembly of a-tropolone methyl ether 27 and (S,S)-( — )-26, in the solid state gave (lS,5R)-( — )-28 of 100% ee. The authors state that the high stereoselectivity is a result of the steric hindrance to disrotatory ring closure from one direction due to the structure of the host. This leads to the formation of only one enantiomer of the product. More details on this topic are available in Chap. 13. 

Fig. 1. Schematic representation of some host systems used for host-guest assemblies from left calixarene, cucurbituril, and p-cyclodextrine.

The successful syntheses of the above host-guest assemblies pointed the way to the formation of molecular threads - namely, compounds formed by threading a cyclic species, such as the cyclobis(paraquat-/ -phenylene) tetracation, on to a separate extended linear component the product is a pseudorotaxane. Pseudorotaxanes are a derivative category of the rotaxanes in which one of the assembled components is a longer (often oligomeric) linear fragment. They are thus similar to rotaxanes, with the exception that the linear component is not terminated by bulky stoppers . 

S. Ishihara, S. Takeoka, Host-guest assembly of pyridiniirm-conjugated calix[4]arene via cation- r interaction. Tetrahedron Lett., 2006, 47, 181-184. 

Ramamurthy. V, Eaton. D.F. Perspectives on solid-state host-guest assemblies. Chem. Mater. 1994. 6. 1128-1136. 

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