Stereoselective host/guest interaction

Fig. 4.70 Dendrocleft (according to Diederich et al.) for stereoselective host/guest interaction depending upon the size of the dendrimer scaffold surrounding the chiral core (marked with a red asterisk)...

A regio- and stereoselective Beckmann rearrangement utilized diastereose-lective host guest interactions of the inclusion complexes 225 and 228 in a solid state reaction. Initially, a 1 1 mixture of the chiral host 223 and the racemic oximes 224 and 227, respectively, was treated with ultra sound in the solid state to induce the optical resolution. Then H2SO4 was added to start the Beckmann rearrangement, the corresponding c-caprolactams 226 and 229 were isolated in 68 % and 64 % yields and ee of about 80 % and 69 % (determined by HPLC analysis on chiracel OC) (Scheme 43) [46]. 

Bach, T., Bergmann, H., Grosch, B., and Harms, K. (2002) Highly enantioselective intra- and intermolecular [2 + 2] photocycloaddition reactions of 2-quinolones mediated by a chiral lactam host host-guest interactions, product configuration, and the origin of the stereoselectivity in solution. Journal of the American Chemical Society, 124, 7982-7990. 

We have four goals for this chapter 1) present an overview of the steps commonly employed to study organometallic catalysis, 2) show how the principles underlying molecular mechanics methods are applied to three specific examples (stereoselectivity in asymmetric hydrogenation, olefin polymerization, and host/guest interactions in zeolites), 3) briefly illustrate the practical applications of molecular modeling to catalysts used in industry, and 4) present a limited survey of the literature to illustrate how different workers have applied molecular mechanics to the study of properties of catalysts of importance to organometallic chemists. 

Besides, information on intermolecular interactions has been derived in these studies from complexation-induced shifts (CIS). The chemical shift is an indicator for the shielding of a nucleus and thus for the electronic state of a specific proton. Since the electronic environment may change on complexation, CIS can be used to monitor where host-guest contacts may take place. If these interactions occur stereoselectively, the CIS will be different for the two guest enantiomers (AS distinct from 0) giving possibly some insight into the chiral recognition mechanism. 

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. 

Stereoselective catalysis using biocatalysts (e.g. enzymes) and also of rationally designed small chiral molecules, deals essentially with the same principle the spatial and selective docking of guest molecules to a chiral host molecule to form complementary interactions to form reversible transient molecule associates (see the specific sections in this volume). The enantiomeric excess of a certain reaction and hence the result will be determined by the degree of chiral discrimination. Along the same theoretical lines the concepts of protein (enzyme, antibody, etc.) mimicks via imprinted" synthetic polymers should be mentioned and will be discussed further. 

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