Coplanarity by hydrogen bonding in well-defined oligoheterocycles

(Hetero)aryl-aryl coupling for covalent bonding and intramol. hydrogen bond formation for establishing the secondary structure have been united to design and construct well-defined, functionalized macromols. The suitability of azaheterocyclic units to realize this concept is exemplified by star shaped discotic liq. cryst. compds. and ladder-type conjugated copolymers. A review with >40 ref

Deracemisations under kinetic and thermodynamic control

Deracemisation reactions occur when a racemic mixture is converted into a nonracemic mixture by increasing the quantity of one enantiomer at the expense of the other. This process can take place under thermodynamic control, but when combined with crystallisation processes kinetic factors also play a role. This review summarises the different approaches that have been taken to achieve efficient deracemisations. Starting from examples in which spontaneous symmetry breaking was found to occur, attrition enhanced deracemisation will be discussed in which solid-solution equilibria drive the deracemisation process. The combination of detailed experimental studies and mathematical models resulted in a profound understanding of this complex process, which is applicable to all congomerate forming compounds with a racemisable stereocenter. Then, we focus on deracemisations that occur under full thermodynamic control. Especially the combination of supramolecular interactions with a racemisation process gives interesting results, albeit that they are less predictable. The review will end with the possibilities supramolecular helical structures that show dynamic helicity can offer in conjunction with asymmetric catalysis. Herein, the helical preference induced by a minute amount of chiral compound is relayed to high enantiomeric excesses in a variety of reactions

Pushing the limits of amplification of chirality in benzene-1,3,5-tricarboxamides

N,N’,N’’-trialkylated-benzene-1,3,5-tricarboxamides (BTAs) self-assemble via strong, threefold a–helix type intermolecular hydrogen bonding into well-defined, helical, one dimensional columnar aggregates. The introduction of a stereogenic centre into the alkyl side chains of BTAs gives rise to strong Cotton effects in dilute apolar solutions indicating the preference for one helical conformation over the other. Here, we summarise our research on the influence of the position of the stereogenic centre on the aggregate stability and the degree of amplification of chirality in BTAs. In addition, we disclose our results on creating a preferred helical sense in BTAbased supramolecular polymers by introducing H/D isotope chirality into the alkyl side chains of BTAs at the a-position. We determine the relative stabilities of the leftand right-handed helical conformers formed by these deuterated molecules by performing a conformational analysis in different alkane solvents. Our findings show that the subtle difference between the stabilities of the two conformers leads to a cooperative self-assembly process, which is highly sensitive to the applied solvent