Chiral molecules aromatic moieties

Small chiral molecules. These CSPs were introduced by Pirkle about two decades ago [31, 32]. The original brush -phases included selectors that contained a chiral amino acid moiety carrying aromatic 7t-electron acceptor or tt-electron donor functionality attached to porous silica beads. In addition to the amino acids, a large variety of other chiral scaffolds such as 1,2-disubstituted cyclohexanes [33] and cinchona alkaloids [34] have also been used for the preparation of various brush CSPs. 

The cage-type peptide cyclophanes (7 and 8) exhibit discrimination toward steroid hormones, as effected by hydrophobic and n-n interactions. In addition, the chirality-based discrimination between a- and -estradiol as well as between D- and L-amino acids bearing an aromatic moiety is performed on the basis of their capacity of forming efficient hydrogen bonding with the host molecules in aqueous media [41, 43]. 

Table 2 summarizes the racemization barriers in unsubstituted chiral alkenes 23 with different bridging moieties in their upper and lower halves. As is evident from these data, the tetrahydrophenanthrene-type upper part is large enough to prevent fast racemization by movement of the aromatic moieties of upper and lower halves through the mean plane of the molecule. On the other hand, there is enough conformational flexibility in the molecules to prevent excessive distortion of the central olefmic bond (leading to ground state destabilization), which would lower the racemization barrier. 

The chiral SOs bear either a strongly electron-deficient aromatic group (rt-acid), e.g. 3,5-dinitrophenyl, or an electron-rich aromatic moiety (rt-base), e.g. naphthyl, placed for face-to-face and/or face-to-edge 7i-it-interaction with complementary sites within the SA molecule. If these molecular features are not available in the SA, they have to be introduced by achiral derivatization. This concept includes also rt-amphiphilic SOs. 

Although CTA-I has been used to resolve a large number of compounds, the solutes appear to be limited to molecules that contain a phenyl group. This limitation may be due to the fact that it is the phenyl moiety that enters the chiral cavity to form the solute/CSP complex, as postulated by Hesse and Hagel (40), Francotte and Wolf (41) have recently reported the results of a study on the effect on enantioselectivity of pflra-substituents in the aromatic moieties of a series of compounds. They concluded that there are (41)... 

To achieve chiral separations on CD CSPs, a part or all of the solute molecules must enter the cyclodextrin cavity. In most cases, the solutes that are successfully resolved contain an aromatic moiety at or adjacent to the stereogenic center, and it is the aromatic portion of the molecule that inserts itself into the chiral cavity of the cyclodextrin molecule to form the inclusion complex (66,67). The size of the aromatic moiety and cyclodextrin cavity determine which CD CSP will form the best inclusion complex, and single aromatic rings fit best in the ot-CD, naphthylrings in the p-CD and aromatic system larger than naphthyl in the -y-CD (68), In addition to the... 

The formation of the soIute-CSP diastereomeric complexes in these CSPs usually requires the insertion of an aromatic moiety on the solute into the chirality of the optically active polymer. Thus, the solutes should contain an aromatic moiety near or at the stereogenic center. Enantiomeric molecules containing the necessary aromatic moiety and one of the following functionalities have been resolved on these CSPs alcohol, amide, ester, ether, and ketone (9-11). 

Double bridging of porphyrins with a dissymmetric difunctional molecule gave achiral (78) and chiral (63) porphyrins as shown in Scheme 1. These porphyrins have three interaction sites zinc as a Lewis acid, amide NH as a hydrogen-bonding site and the aromatic moiety as a steric-repulsion / attraction site they work as hosts for chiral... 

The various ribbons presented above consist of amphiphilic molecules arranged in bilayers. The long axis of the molecules is perpendicular to the ribbon plane or slightly tilted from this direction. But ribbons or tapes can also be formed from the assembly of molecular rods oriented with their long axis parallel to the width of the ribbon. This is the case for some peptides that form extended /3-sheet tapes which stack to form chiral superstructures (Fig. 3) [73]. It is also the case for numerous gelators consisting of a central aromatic core and chiral cholesteryl saccharidic moieties on the sides, such as the porphyrin derivative shown in Fig. 4 [74]. Chirality effects in these... 

Alkylated HBCs form regular monolayers on the HOPG surface in solution and asymmetric 7/V characteristics were measured by STS [34b]. This diode-like effect has been attributed to positional asymmetry of either the vertical placement of the disc in the gap or of the frontier molecular orbitals relative to the electrodes. The alkylphenyl substituted HBCs self-assemble into 2D crystals of the discs with variable vertical displacements from the substrate [93]. When the alkylphenyl chains contain chiral centers, a regular staircase superstructure results. A submolecular visualization of a covalently linked HBC dimer on the HOPG surface revealed a contrast, which reflects the structure of the aromatic parts of the molecule, with the aromatic moieties being oriented like graphene layers in graphite (Fig. 3.25) [94]. 

It has previously been stated that attack by nucleophiles on cationic arene rings occurs stereospecifically in an exo fashion. In fact, coordination by a transition metal confers a third dimension on the molecule which has several stereochemical consequences. For example, electrophilic or nucleophilic attack at the reactive center of an alicyclic ring ortho-condensed to an aromatic moiety always occurs stereospecifically in an exo fashion. Arene derivatives containing different ortho or meta substitutents are chiral, and numerous examples exist of the resolution of benchrotrene derivatives... 

For hydrogenation of styrene and its derivatives over several cationic Rh complexes, in addition to the hyperpolarized multiplets of ethylbenzene the H NMR spectra contained similar polarized multiplets but shifted to a higher field [41,42]. These signals were attributed to the product molecules that have not yet detached from the metal center after the hydrogen-transfer stage was over (e.g., with the aromatic moiety r -coordinated to the Rh(I) center). The results demonstrate that the detachment process can be fairly slow on the NMR timescale. The use of chiral catalysts and/or asymmetrically substituted styrenes led to more complicated spectral patterns. Kinetic studies can be used to measure the rates of formation and decay of such catalyst-product complexes [43]. The fact that the observed product remains coordinated to the catalyst was confirmed [44] in experiments with polarization transfer from the product to the hydrogens of other ligands of the catalyst induced by cross relaxation. 

Two main classes of T -dienyliron complexes are known, namely the cationic tricarbonyliron complexes and neutral cyclopentadienyliron compounds. The cyclopentadienyl (Cp) ligand is relatively inert to a broad variety of reaction conditions. It is often introduced as a ligand to tune the properties of the iron complex, as a chiral auxiliary,or in material science in organoiron pol5miers. However, it is only rarely transformed itself into a more elaborate organic product. For this reason, the chemistry of the cyclopentadienyl ligand will not be discussed in more detail in this chapter. Tricarbonyl( n -dienylium)iron complexes, on the other hand, represent versatile electrophilic building blocks for the attachment of 1,3-butadiene, cyclohexadiene, or aromatic moieties to nucleophilic molecules. 

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