Shape-selective recognition

Very recently, interesting macrocyclic diureido derivatives 39a,b lower rim bridged by a spacer unit were reported [50]. Their oxidation with (CF3COO)3Tl led to the corresponding quinones 39c,d, potentially useful as electrochemical anion receptors. A similar upper rim bridged system 40 was shown to exhibit good shape-selective recognition abilities towards various aromatic dicarbox-ylates in DMSO [51]. The molecular mechanics force field calculations indicate... 

Sitlani A, Long EC, Pyle AM, Barton JK. DNA Photocleavage by phenanthrenequinone diimine complexes of rhodium(III) shape-selective recognition and reaction. / Am Chem Soc 1992 114 2303-12. 

This affinity for metals results not only from the structural organization of the new diacids but from stereoelectronic effects at carboxyl oxygen as well. The in-plane lone pairs of a carboxylate 18 differ in basicity by several orders of magnitude 16). Conventional chelating agents17> derived from carboxylic acids such as EDTA, 19a are constrained by their shape to involve the less basic anti lone pairs [Eq. (4)]. The new diacids are permitted the use of the more basic syn lone pairs in contact with the metal 19b. These systems represent a new type of chelate for highly selective recognition of divalent ions. 

FIGURE 5.4 Comparison of shape selectivity and planar recognition test procedures SRM 869a test (solutes PhPh, TEN, BaP eluent acetonitrile/water 85/15 (v/v), flow 2 ml/min, detection UV 254 nm) and Tanaka test (solutes o-TER, TRI eluent methanol/water 80/20 (v/v), flow 1 ml/min, detection UV 254 nm. (Reproduced from Engelhardt, H., et al., Chromatographia, A%, 183, 1998. With permission.)... 

Reversed-phase liquid chromatography shape-recognition processes are distinctly limited to describe the enhanced separation of geometric isomers or structurally related compounds that result primarily from the differences between molecular shapes rather than from additional interactions within the stationary-phase and/or silica support. For example, residual silanol activity of the base silica on nonend-capped polymeric Cis phases was found to enhance the separation of the polar carotenoids lutein and zeaxanthin [29]. In contrast, the separations of both the nonpolar carotenoid probes (a- and P-carotene and lycopene) and the SRM 869 column test mixture on endcapped and nonendcapped polymeric Cig phases exhibited no appreciable difference in retention. The nonpolar probes are subject to shape-selective interactions with the alkyl component of the stationary-phase (irrespective of endcapping), whereas the polar carotenoids containing hydroxyl moieties are subject to an additional level of retentive interactions via H-bonding with the surface silanols. Therefore, a direct comparison between the retention behavior of nonpolar and polar carotenoid solutes of similar shape and size that vary by the addition of polar substituents (e.g., dl-trans P-carotene vs. dll-trans P-cryptoxanthin) may not always be appropriate in the context of shape selectivity. 

In addition to imprinted acid-base catalysts [49-55], attempts to imprint metal complexes have been reported and constitute the current state of the art [46, 47]. In most cases of metal-complex imprinting, ligands of the complexes are used as template molecules, which aims to create a cavity near the metal site. Molecular imprinting of metal complexes exhibits several notable features (i) attachment of metal complex on robust supports (ii) surrounding of the metal complex by polymer matrix and (iii) production of a shape selective cavity on the metal site. Metal complexes thus imprinted have been appHed to molecular recognition [56, 57], reactive complex stabilization [58, 59], Hgand exchange reaction [60] and catalysis [61-70]. 

Zimmermann, Shapes, Selectivity, and Complementarity in Molecular Recognition, in Frontiers in Supramolecular Chemistry, H.-J. Schneider. H. Durr, Eds., VCH. Weinheim. 1991, p. 29. 

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