Enantiomers self-assembly

Bikadi Z, Zsila F, Deli J, Mady G, and Simonyi M. 2002. The supramolecular structure of self-assembly formed by capsanthin derivatives. Enantiomer 7 67-76. 

Chromatography. Under certain conditions, even homochiral and het-erochiral self-assemblies can be separated by achiral methods. Thus, chromatography of partially resolved enantiomers can cause depletion or enrichment of enantiomers on achiral stationary phases with an achiral mobile phase. 14C-Labeled nicotine was first resolved into its enantiomers by high-performance liquid chromatography (HPLC) on an achiral stationary phase (Partisil-ODS or -SCX) through coinjection with optically active nicotine (59). This observation was followed by resolution of a number of chiral compounds by chromatography (<50-62) (Scheme 34). When a chiral diamide in 74% ee was separated on a Kieselgel 60... 

An interesting question is whether or not component selection occurs in the course of the self-assembly if a mixture is used, in particular in the case of chiral compounds. Thus, the assembly of a bis-L-tryptophan TAP derivative by di-w-butyl-B A was studied both as the optically pure LL compound and as a racemic mixture with the DD enantiomer [9.169]. The crystal structure of the latter showed... 

MIP films, applied to a QCM transducer, have been employed for chiral recognition of the R- and 5-propranolol enantiomers [107]. MIP films were prepared for that purpose by surface grafted photo-radical polymerization. First, a monolayer of 11-mercaptoundecanoic acid was self-assembled on a gold electrode of the quartz resonator. Then, a 2,2 -azobis(2-amidinopropane) hydrochloride initiator (AAPH), was attached to this monolayer. Subsequently, this surface-modified resonator was immersed in an ACN solution containing the MAA functional monomer, enantiomer template and trimethylolpropane trimethacrylate (TRIM) cross-linker. Next, the solution was irradiated with UV light for photopolymerization. The resulting MIP-coated resonator was used for enantioselective determination of the propranolol enantiomers under the batch [107] conditions and the FIA [107] conditions with an aqueous-ACN mixed solvent solution as the carrier. The MIP-QCM chemosensor was enantioselective to 5-propranolol at concentrations exceeding 0.38 mM [107]. 

As described in the Introduction, Pasteur showed beautifully that racemic molecules resolve spontaneously into chiral forms when they crystallize. We call them conglomerates, in which molecules form condensates comprised of only one enantiomer. The condensation into conglomerates can now be observed not only in crystals but in monolayers, fibers, and supramolecules self-assembled in solution [35]. The researches became possible because of the development of microscopic observation techniques at the nanometer scale. However, in crystals we still do not know what kinds of molecules show spontaneous resolution. Hence, observation of chiral resolution in soft matter may provide important information on the general question. 

We present here a few examples of enantiomeric supramolecular absolute structures. In fact, many supramolecular diastereomers with chiral ligands have been isolated stereoselectively. For example, enantiomers of the chiral self-assembled triple helicates trinuclear [Ni3(L)3](C104)6 (L = tris(bpy)) and dinuclear [Co2(L)3]4 + (L = 5-bismpmb) were partially resolved by SP Sephadex chromatography.63,64 The ligands and structures are given in Figure 5.17a and b. 

Helicenes have an intrinsic, rigid helical shape and can be separated into enantiomers. Katz et al. found that the nonracemic helicene 6 with quinone residues self-assembles into one-handed helical columns in apolar solvents, where the molecules are stacked along their helix axes as shown in Fig. 4 [40-42], In this case, the 7r-donor-acceptor interactions appear to induce aggregations and stabilize the columnar stacks. Interestingly, unlike the nonracemic 6, the corresponding racemic 6 produced no such aggregation. 

Finally, we mention a remarkable example of lateral resolution reported for supramolecular nanostructures on hopg [96]. Held together by 72 hydrogen bonds, the molecular nanostructure is formed from three melamine-substituted calix[4]arene units and twelve 5,5-diethylbarbiturate molecules (Fig. 31a). The nanostructure, basically a stack of four rosettes, has chiral symmetry. With its components all being achiral, both enantiomers are formed upon self-assembly in solution. Deposition of the tetrarosettes on hopg leaves this nanostructure intact and allows surface self-assembly. AFM studies revealed close-packed 2D lattices formed by the tetrarosettes on hopg... 

Fig. 15 a, b GIXD patterns measured from self-assembled 2-D crystallites of enantiomer-ically pure and racemic Ci8-TE-Lys on water at 4 °C c 2D packing arrangement of the racemic crystallites viewed perpendicular to the water surface. For clarity only part of the hydrocarbon chains is shown... 

Racemic and enantiomorphous 2-D and 3-D crystals display different physical and chemical properties. This difference has been utilized to enhance chirality in non-racemic systems that self-assemble in racemic and enantiomorphous crystallites. Morowetz [196] has elaborated a mathematical model that considers an evaporation/crystallization process where the racemate is less soluble than the pure enantiomorphous crystal and the enantiomer (in excess) is concentrated in the solution. A similar enrichment of chirality has... 

In 1994, Hanessian and co-workers [50] reported the first examples of metal-free three-dimensional triple-stranded helicates through spontaneous self-assembly of chiral C2-symmetrical diols and chiral C2-symmetrical diamines. The initial observation resulted from the utilization of enantiopure C2-symmetrical vicinal trans-1,2-diaminocyclohexane [51,52] as ligands in the asymmetric dihydroxylations of olefins [53] and as reagents for asymmetric synthesis [54], When equimolar amounts of (5,5)-frfl x-l,2-diaminocyclohexane (28) and its (i ,i )-enantiomer (29) were individually mixed with (5,5)-frfl x-l,2-cyclohexanediol and heated in refluxing benzene, crystals of the respective supraminol complexes 28 30 and 29 30 were formed quantitatively (Scheme 12). This was the physical basis for the separation of racemic diols with tr[Pg.104]

Some chiral self-assembling systems are able to incorporate enantiomers of opposite chirality into the supramolecular structure because the chiral centre is relatively remote from the moiety that is involved in self-assembly [18,19]. By contrast, NDI... 

The 13C CP/MAS-NMR spectrum of rac-[Li4 C6H4[CH(Me)NMe2]-2 4] shows that it is a self-assembled tetranuclear aggregate comprising 2 dimeric units, derived from (R)- and (,S)-enantiomers respectively.6... 

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