Sabine Laschat

Thermotropic Ionic Liquid Crystals

The last five years’ achievements in the synthesis and investigation of thermotropic ionic liquid crystals are reviewed. The present review describes the mesomorphic properties displayed by organic, as well as metal-containing ionic mesogens. In addition, a short overview on the ionic polymer and self-assembled liquid crystals is given. Potential and actual applications of ionic mesogens are also discussed.

Selective Catalytic Oxidation of CH Bonds with Molecular Oxygen

Although catalytic reductions, cross‐couplings, metathesis, and oxidation of CC double bonds are well established, the corresponding catalytic hydroxylations of CH bonds in alkanes, arenes, or benzylic (allylic) positions, particularly with O2, the cheapest, “greenest”, and most abundant oxidant, are severely lacking. Certainly, some promising examples in homogenous and heterogenous catalysis exist, as well as enzymes that can perform catalytic aerobic oxidations on various substrates, but these have never achieved an industrial‐scale, owing to a low space‐time‐yield and poor stability. This review illustrates recent advances in aerobic oxidation catalysis by discussing selected examples, and aims to stimulate further exciting work in this area. Theoretical work on catalyst precursors, resting states, and elementary steps, as well as model reactions complemented by spectroscopic studies provide detailed insight into the molecular mechanisms of oxidation catalyses and pave the way for preparative applications. However, O2 also poses a safety hazard, especially when used for large scale reactions, therefore sophisticated methodologies have been developed to minimize these risks and to allow convenient transfer onto industrial scale.

Rational design of a minimal and highly enriched CYP102A1 mutant library with improved regio-, stereo- and chemoselectivity.

Monooxygenase mutants: A minimal and highly enriched CYP102A1 mutant library was constructed by combining five hydrophobic amino acids in two positions. The library was screened with four different terpene substrates. Eleven variants demonstrated either a strong shift or improved regio‐ or stereoselectivity during oxidation of at least one substrate as compared to CYP102A1 wild type.

Counterion Effects on the Columnar Mesophases of Triphenylene‐Substituted [18]Crown‐6 Ethers: Is Flatter Better?

The twisted lateral tetraalkyloxy ortho-terphenyl units in dibenzo[18]crown-6 ethers 1a-f were readily converted into the flat tetraalkyloxytriphenylene systems 2a-f by oxidative cyclization with FeCl(3) in nitromethane. Reactions of the latter with potassium salts gave complexes KX.2, which displayed mesomorphic properties. The aromatization increased both the clearing and melting points; the mesophase stabilities, however, were mainly influenced by the respective anions upon complexation with various potassium salts. In contrast, the alkyl chain lengths played only a secondary role. Among the potassium complexes of triphenylene-substituted crown ethers KX.2, only those with the soft anions I(-) and SCN(-) displayed mesophases with expanded phase temperature ranges of 93 degrees C and 132 degrees C (for KX.2e), respectively, as compared to the corresponding o-terphenyl-substituted crown ether complexes KI.1e (DeltaT=51 degrees C) and KSCN.1e (plastic crystal phase). Anions such as Br(-), Cl(-), and F(-) decreased the mesophase stability, and PF(6)(-) led to complete loss of the mesomorphic properties of KPF(6).2 although not for KPF(6).1. For crown ether complexes KX.2 (X=F, Cl, Br, I, BF(4), and SCN), columnar rectangular mesophases of different symmetries (c2mm, p2mg, and p2gg) were detected. In contrast to findings for the twisted o-terphenyl crown ether complexes KX.1, the complexation of the flat triphenylene crown ethers 2 with KX resulted in the formation of organogels. Characterization of the organogel of KI.2e in CH(2)Cl(2) revealed a network of fibers.

Convergent Synthesis of Columnar Twins and Tetramers from Triphenylene Building Blocks – The First Example of a Columnar Spiro-Twin

The novel spiro-twins 3a–f were prepared in three steps by one-pot oxidative coupling of guaiacol (9) and 1,2-dialkoxybenzenes 10a–f, followed by demethylation of the triphenylene 11 and subsequent etherification with the tetrabromide 6. A related strategy gave the tetramers 7a–f in two steps. Whereas derivatives 3a,b and 7a,b with pentyloxy and hexyloxy chains showed isotropic melting behavior, compounds 3c–f and 7c–f with longer alkyl chains displayed columnar mesophases. DSC, polarizing microscopy, X-ray diffraction and molecular modeling studies were used to further characterize the type of mesophase. For the spiro-twin 3 either a rectangular or a hexagonal columnar phase were conceivable. In case of 7f a hexagonal columnar mesophase was assigned.

Novel Columnar Tetraphenylethenes via McMurry Coupling

Several tetraphenylethenes 2a, 4a–i with lipophilic side chains were synthesized and their mesomorphic properties were investigated. The most promising candidates turned out to be tetrakis[4-(trisalkyloxybenzoyloxy)phenyl]ethenes 4e–i with chain lengths between C7 and C12. Compounds 4 were prepared by a convergent strategy employing a McMurry coupling of 4,4′-dimethoxybenzophenone 1c, followed by demethylation and subsequent esterification of 2c with either 4-decyloxybenzoic acid 3a, 3,4-didecyloxybenzoic acid 3b, or O-alkylated gallic acids 3c–i. Despite the twisting of the central tetraphenylethene moiety compounds 4c–i display hexagonal columnar mesophases according to differential scanning calorimetry (DSC), polarizing microscopy, and X-ray diffraction studies.

Columnar Mesophases Controlled by Counterions in Potassium Complexes of Dibenzo[18]crown-6 Derivatives

Dibenzo[18]crown-6 derivatives 1 with two lateral tetraalkyloxy o-terphenyl units were prepared and converted to the corresponding complexes KX1 (X = halide, BF(4), PF(6), SCN) and NH(4)PF(6)1. Complexation was probed by MALDI-TOF spectrometry and NMR spectroscopy. Downfield shifts of (1)H NMR signals for complexes with soft anions Br, I, SCN, and PF(6) indicated the presence of tight ion pairs, whereas complexes with hard anions F, Cl, or BF(4) showed no or little shifts. In (13)C NMR spectra, upfield shifts were detected for soft anions. The character of the anion also influenced the mesomorphic properties of complexes MX1 (M = K, NH(4)), which were investigated by differential scanning calorimetry (DSC), polarizing optical microscopy (POM), and XRD in comparison to neat 1. Hard anions slightly stabilize or even destabilize the mesophase. Soft anions, however, improve the mesomorphic properties yielding mesophases with up to 70 degrees C phase widths in the case of KI1, KPF(6)1, and NH(4)PF(6)1. For complexes KSCN1 with a soft and bridging anion, the balance between mesophase stabilization and high order is shifted in favor of the plastic crystal phase.

Substituted crown ethers as central units in discotic liquid crystals: effects of crown size and cation uptake

In this paper, a series of crown ether-based liquid crystals was synthesised. The effect of different crown ether ring sizes, terminal groups and complexed salts on the mesomorphic properties was investigated. The first example of [12]crown-4 as the central core of a discotic liquid crystal is described. Columnar hexagonal phases were found for the [12]crown-4-ethers carrying o-terphenyl (1) and triphenylene (2) groups. The [24]crown-8 ethers showed columnar hexagonal phases when substituted with o-terphenyl groups (7) while no mesophases could be detected in the case of the triphenylene substituted derivatives 8. It was found that an increase in the size of the macrocycle leads to a decrease in the clearing points and phase stabilities. Depending on the size of the crown ether, uptake of suitable cations can lead to drastic increases in the clearing temperatures and phase stabilities due to structural rearrangements or ion pair formation, as deduced from 1H and 13C NMR spectroscopy.

Intramolecular hetero-Diels-Alder reaction of N-arylimines. Applications to the synthesis of octahydroacridine derivatives

A new intramolecular Lewis acid catalyzed hetero-Diels-Alder reaction of N-arylimines 5 with nonactivated olefins tethered to the 2-azadiene system was developed in order to prepare 1,2,3,4,4a,9,9a,lO-octahydroacridine derivatives 6. Both reactivity and cis/ trans selectivity of 6 were mainly dependent on the substitution pattern in the 3-position of the cyclization precursor 5. The type of Lewis acid plays only a minor role in determination of the cis/trans ratio. The synthetic utility of this new cyclization was increased by performing it as a one-pot reaction (3 + 4 6). Both absolute configuration and preferred conformation were assigned by detailed NMR studies.

Application of intramolecular Heck reactions to the preparation of steroid and terpene intermediates having cis A-B ring fusions. Model studies for the total synthesis of complex cardenolides.

The cis-fused tricyclic dienone 13 is the major product formed from intramolecular Heck cyclization of the dienyl triflate 12 (Scheme I). Similarly, the cis-hexahydrophenanthridine 22 is formed in good yield from Heck cyclization of the aryl triflate 21. This latter conversion demonstrates that allylic ether substitution is compatible with intramolecular Heck chemistry and suggests applications of this chemistry in the synthesis of highly oxidized cardenolides.