Single‐Crystal‐to‐Single‐Crystal Transition in an Enantiopure [7]Helquat Salt: The First Observation of a Reversible Phase Transition in a Helicene‐Like Compound

Here it is reported that crystals of an enantiopure [7]helquat salt undergo reversible thermal solid–solid phase transition at 404 K. Differential scanning calorimetry (DSC), capillary electrophoresis (CE), and X‐ray diffraction analysis were used to unravel the mechanistic details of this process. The single‐crystal‐to‐single‐crystal course enabled direct monitoring of the structural changes by in situ variable‐temperature X‐ray diffraction, thus providing the first direct evidence of a solid phase transition in a helicene‐like compound.     

Gold(I) and gold(III) corroles

Corrole complexes with gold(I) and gold(III) were synthesized and their structural, photophysical, and electrochemical properties investigated. This work includes the X-ray crystallography characterization of gold(I) and gold(III) complexes, both chelated by a corrole with fully brominated β-pyrrole carbon atoms. The mononuclear and chiral gold(I) corrole appears to be the first of its kind within the porphyrinoid family, while the most unique property of the gold(III) corrole is that it displays phosphorescence at ambient temperatures.     

Preferred 3D‐Structure of Peptides Rich in a Severely Conformationally Restricted Cyclopropane Analogue of Phenylalanine

Terminally blocked, homo‐peptide amides of (R,R)‐1‐amino‐2,3‐diphenylcyclopropane‐1‐carboxylic acid (c₃diPhe), a chiral member of the family of C^α‐tetrasubstituted α‐amino acids, from the dimer to the tetramer, and diastereomeric co‐oligopeptides of (R,R)‐ or (S,S)‐c₃diPhe with (S)‐alanine residues to the trimer level were prepared in solution and fully characterized. The synthetic effort was extended to terminally protected co‐oligopeptide esters to the hexamer, where c₃diPhe residues are combined with achiral α‐aminoisobutyric acid residues. The preferred conformations of the peptides were assessed in solution by FT‐IR absorption, NMR, and CD techniques, and for seven oligomers in the crystal state (by X‐ray diffraction) as well. This study clearly indicates that c₃diPhe, a sterically demanding cyclopropane analogue of phenylalanine, tends to fold peptides into β‐turn and 3₁₀‐helix conformations. However, when c₃diPhe is in combination with other chiral residues, the conformation preferred by the resulting peptides is also dictated by the chiral sequence of the amino acid building blocks. The (S,S)‐enantiomer of this α‐amino acid, unusually lacking asymmetry in the main chain, strongly favors the left‐handedness of the turn/helical peptides formed.     

Characterization of Supramolecular Hidden Chirality of Hydrogen‐Bonded Networks by Advanced Graph Set Analysis

Supramolecular hidden chirality of hydrogen‐bonded (HB) networks of primary ammonium carboxylates was exposed by advanced graph set analysis from a symmetric viewpoint in topology. The ring‐type HB (R‐HB) networks are topologically regarded as faces, and therefore exhibit prochirality and positional isomerism due to substituents attached on the faces. To describe the symmetric properties of the faces, additional symbols, Re (right‐handed or clockwise), Si (left‐handed or anticlockwise), and m (mirror), were proposed. According to the symbols, various kinds of faces were classified based on the symmetry. This symmetry consideration of the faces enables us to precisely evaluate supramolecular chirality, especially its handedness, of 0D‐cubic, 1D‐ladder and 2D‐sheet HB networks that are composed of the faces. The 1D‐ladder and 2D‐sheet HB networks generate chirality by accumulating the chiral faces in 1D and 2D manners, respectively, whereas 0D‐cubic HB networks generate chirality based on combinations of eight kinds of faces, similar to the chirality of dice.     

A Route to Stereochemically Pure Benzyllithium Reagents by Stereocontrolled Epimerisation

Alkyllithium compounds are not generally stable at room temperature, therefore strategies were established to overcome this limitation. We present a systematic approach to obtain a stereochemically enriched benzyllithium compound, starting with the simple addition of a chiral auxiliary and ending by incorporation of the auxiliary in the substrate. Thereby, a very unusual dimer of a lithiated benzylsilane was obtained, which could be split into stereochemically enriched monomers by the addition of Lewis bases. Furthermore, we were able to understand the factors responsible for this stereochemical enrichment by using quantum chemical calculations and clarify the configuration of the lithiated compound and the corresponding trapping product by crystallisation. This enabled us to determine the stereochemical course of the deprotonation and the subsequent metathesis reaction.     

Growth of Textured Adenine Thin Films to Exhibit only Chiral Faces

The growth of adenine on Au(111) from the submonolayer up to several microns film thickness is studied by combining STM and surface X‐ray diffraction techniques. The study shows that adenine thin films are composed of highly textured crystallites with the reported α‐adenine crystal structure in such a way that they only exhibit (001)‐faces. Such faces, with chiral p2 symmetry planes, have an average size of 15 nm. Thus, the formed adenine films reported herein, which expose only chiral and biocompatible faces, might find applications such as enantioselective heterogeneous catalysts for the chemical and pharmaceutical industries.