Rafael Martín-Rapún

Asymmetrically Substituted Benzene‐1,3,5‐tricarboxamides: Self‐Assembly and Odd–Even Effects in the Solid State and in Dilute Solution

Molecular organization: Chiral benzene tricarboxamides with methyl substituents at defined positions self-assemble into supramolecular polymers of preferred helicity by three-fold alpha-helical-type hydrogen bonding. The odd-even effect is operative and all derivatives are liquid crystalline showing a Col(ho) phase (see figure).Asymmetric benzene-1,3,5-tricarboxamides (aBTAs) comprising two n-octyl and one chiral methyl-alkyl side chain were synthesised and characterised. The influence of the position and the configuration of the chiral methyl group (methyl at the alpha, beta or gamma position) in the aliphatic side chains on the liquid-crystalline properties and the aggregation behaviour of the aBTAs was systematically studied and compared to symmetrical benzene-1,3,5-tricarboxamides (sBTAs). Solid-state characterisation (polarised optical microscopy, IR spectroscopy, X-ray diffraction and differential scanning calorimetry) revealed that all aBTAs show threefold, alpha-helical-type intermolecular hydrogen bonding between neighbouring molecules and exhibit a columnar hexagonal organisation from room temperature to well above 200 degrees C. Moving the chiral methyl group closer to the amide group stabilises the liquid-crystalline state, as evidenced by a higher clearing temperature and corresponding enthalpy. The self-assembly of dilute solutions of the aBTAs in methylcyclohexane ( approximately 10(-5) mol L(-1)) was investigated with circular dichroism (CD) spectroscopy. The sign of the Cotton effect demonstrated a pronounced odd-even effect, whereas the value of the molar ellipticity, Deltaepsilon, in the aBTAs was independent of the position of the methyl group. Subsequent temperature-dependent CD measurements showed that the aggregation of all aBTAs can quantitatively be described by the nucleation-growth model and that the stability of the aggregates increases when the chiral methyl group is closer to the amide moiety. The results presented herein illustrate that even small changes in the molecular structure of substituted benzene-1,3,5-tri-carboxamides affect their solid-state properties and their self-assembly behaviour in dilute solutions.

Dynamic Supramolecular Polymers Based on Benzene-1,3,5-tricarboxamides: The Influence of Amide Connectivity on Aggregate Stability and Amplification of Chirality

N-Centred benzene-1,3,5-tricarboxamides (N-BTAs) composed of chiral and achiral alkyl substituents were synthesised and their solid-state behaviour and self-assembly in dilute alkane solutions were investigated. A combination of differential scanning calorimetry (DSC), polarisation optical microscopy (POM) and X-ray diffraction revealed that the chiral N-BTA derivatives with branched 3,7-dimethyloctanoyl chains were liquid crystalline and the mesophase was assigned as Col(ho). In contrast, N-BTA derivatives with linear tetradecanoyl or octanoyl chains lacked a mesophase and were obtained as crystalline compounds. Variable-temperature infrared spectroscopy showed the presence of threefold, intermolecular hydrogen bonding between neighbouring molecules in the mesophase of the chiral N-BTAs. In the crystalline state at room temperature a more complicated packing between the molecules was observed. Ultraviolet and circular dichroism spectroscopy on dilute solutions of N-BTAs revealed a cooperative self-assembly behaviour of the N-BTA molecules into supramolecular polymers with preferred helicity when chiral alkyl chains were present. Both the sergeants-and-soldiers as well as the majority-rules principles were operative in stacks of N-BTAs. In fact, the self-assembly of N-BTAs resembles closely that of their carbonyl (C=O)-centred counterparts, with the exception that aggregation is weaker and amplification of chirality is less pronounced. The differences in the self-assembly of N- and C=O-BTAs were analysed by density functional theory (DFT) calculations. These reveal a substantially lower interaction energy between the monomeric units in the supramolecular polymers of N-BTAs. The lower interaction energy is due to the higher energy penalty for rotation around the Ph--NH bond compared to the Ph--CO bond and the diminished magnitude of dipole-dipole interactions. Finally, we observed that mixed stacks are formed in dilute solution when mixing N-BTAs and C=O BTAs.

Continuous-flow enantioselective α-aminoxylation of aldehydes catalyzed by a polystyrene-immobilized hydroxyproline

Summary The application of polystyrene-immobilized proline-based catalysts in packed-bed reactors for the continuous-flow, direct, enantioselective α-aminoxylation of aldehydes is described. The system allows the easy preparation of a series of β-aminoxy alcohols (after a reductive workup) with excellent optical purity and with an effective catalyst loading of ca. 2.5% (four-fold reduction compared to the batch process) working at residence times of ca. 5 min.

Asymmetric anti-Mannich reactions in continuous flow

A polystyrene-supported, pyrrolidine-based catalyst depicting very high activity and excellent stereoselectivity in the anti-Mannich reaction of aldehydes and ketones has been developed. The very robust, immobilized catalyst has been successfully used in the implementation of a continuous flow process with short residence times (down to 6 min) for the production of highly enantioenriched anti-type Mannich adducts at the preparative scale.

Highly Active Organocatalysts for Asymmetric anti-Mannich Reactions

Lighten the load! A family of enantiopure 4-oxy-substituted 3-aminopyrrolidines arising from the enantioselective ring-opening of meso-3-pyrroline oxide have been developed as catalysts for the asymmetric, anti-selective Mannich reaction (see scheme; PMP=p-methoxyphenyl; PG=protecting group). Very high catalytic activity (down to 0.01 mol % loading) and stereoselectivity have been recorded.

Unravelling the fine structure of stacked bipyridine diamine-derived C-3-discotics as determined by X-ray diffraction, quantum-chemical calculations, Fast-MAS NMR and CD spectroscopy

An in depth investigation of the fine structure adopted by the helical stacks of C3-discotics 1 incorporating three 3,3′-diamino-2,2′-bipyridine units is described. In the bulk the molecules display liquid crystalline behaviour in a temperature window of >300 K and an ordered rectangular columnar mesophase (Colro) with an inter-disc distance of 3.4 A is assigned. X-Ray diffraction on aligned samples has also revealed a helical superstructure in the liquid crystalline state, and a rotation angle of 13–16° between consecutive discs. The proposed superstructure in the bulk phase has been further substantiated by a combination of quantum-chemical calculations and solid-state NMR spectroscopy. Dilute solution NMR spectroscopy and elaborate CD spectroscopy on aggregated samples have revealed an isodesmic growth pattern of the C3-discotics. From the combined results it has become evident that the fine tuning interaction responsible for the highly ordered helical architectures is not weak intermolecular hydrogen bonding, but rather rigidification, due to propeller formation after preorganisation by π–π interactions. Although all the techniques used underpin the structural features proposed, none of them individually is able to point to a unique structure. However, together the techniques give very strong evidence for a confined ship-screw arrangement in which all amidic carbonyl oxygens point in one direction.

The influence of oligo(ethylene glycol) side chains on the self-assembly of benzene-1,3,5-tricarboxamides in the solid state and in solution

Substituted benzene-1,3,5-tricarboxamides (BTAs) 1–4 comprising polar tetraethyleneglycol (tetraEG) and/or apolar (R)-3,7-dimethyloctyl side chains were synthesised and their self-assembly in the solid state and in solution was investigated. While BTA 1 (comprising 3 apolar side chains) shows helical columnar packing via threefold α-helical type intermolecular hydrogen bonding in the solid state and up to high dilutions in alkane solution (10−5 M), helical columnar order is only preserved for asymmetric BTA 2 (comprising 1 polar and 2 apolar side chains) in the solid state and in a concentrated alkane solution (10−2 M). The association constant Kass is reduced by a factor of 107 by introducing one polar tetraEG chain into the BTA. A further increase in the number of polar tetraEG chains attached to BTA core results in the complete loss of intermolecular hydrogen bond formation in the solid state and in solution. Moreover, for the polar BTAs 3–4, comprising 2 or 3 polar tetraEG chains, no self-assembly in water occurs because of the lack of hydrophobic shielding. We propose that tetraEG side chains interfere with the intermolecular hydrogen bonds, weakening the stacking behaviour of these asymmetric derivatives and drastically lowering the association constant due to competing intramolecular hydrogen bonding interactions. In contrast, one methoxyethyl unit does not affect the stability of the aggregation of BTAs (Kass = 3 × 107 M−1) showing that more than one EG unit is required to disrupt the self-assembly of BTAs.

“Click chemistry” as a versatile route to synthesize and modulate bent-core liquid crystalline materials

Three series of bent-shaped compounds containing the 1,2,3-triazole ring in the central core of the molecule have been prepared by the most extended “click chemistry” reaction, the copper-catalyzed azide-alkyne cycloaddition (CuAAC). This research demonstrates the versatility of this synthetic approach with the aim of achieving innovative compact supramolecular organizations. The appropriate combination of the 1,2,3-triazole synthon linked either to a methylene unit (series M) or to a methylenoxycarbonyl block (series MC) has allowed the induction of a variety of non-classical bent-core liquid crystal phases versus the classic mesophases promoted by 1,4-diphenyl-1,2,3-triazole derivatives (series T). Through a suitable selection of common lateral structures connected by “click chemistry” both the transition temperatures and mesomorphism, ranging from lamellar to columnar or B4-like supramolecular liquid crystalline organizations, can be tuned.