Synthesis, Chiral Resolution, and Absolute Configuration of Functionalized Tröger’s Base Derivatives: Part II

Tunable Excimer Circularly Polarized Luminescence in Isohexide Derivatives from Renewable Resources

Organic compounds showing circularly polarized luminescence (CPL) are at the forefront of novel applications and technologies. Here we show the synthesis and chiroptical properties of pyrene and perylene derivatives of inexpensive chiral scaffolds: isomannide and isosorbide. Low‐intensity ECD spectra were obtained, suggesting the absence of chromophore interaction in the ground state, except in the case of isomannide bis‐perylenecarboxylate, whose ECD spectrum showed a positive exciton couplet. All isomannide derivatives, with the only exception of the one containing a pyrenecarboxylate and a perylenecarboxylate, exhibited excimer CPL spectra, whereas isosorbide derivatives did not show any CPL. Isomannide derivatives bearing two pyrenecarboxylate or two pyrenylacetate groups showed positive CPL emission with dissymmetry factors up to 10⁻², which depends on the conformational freedom of the appended units. The CPL sign, Stokes shift and order of magnitude of dissymmetry factor were reproduced by excited‐state calculations on a representative compound. Interestingly, the mixed derivative containing pyrenic units with different spacing from the isomannide scaffold showed an oppositely signed excimer band with respect to the homo‐substituted derivatives.

Fluorescent 4-amino-1,8-naphthalimide Tröger's bases (TBNaps) possessing (orthogonal) 'α-amino acids', esters and di-peptides and their solvent dependent photophysical properties

The synthesis of fifteen luminescent bis-naphthalimide based Tröger's bases (TBNaps) derived from 4-amino-1,8-naphthalimide (4-Amino-Nap) precursors is described; these scaffolds possess α-amino acids, esters or di-peptides conjugated at the imide site and show minor fluorescence in aqueous solution while being highly emissive in organic solvents. The investigation shows that these TBNaps possessing ICT excited state properties are capable of generating either positive or negative solvatochromic effects in response to changes in polarity and/or the hydrogen bonding capabilities of the medium.

Sugar-Based Ionic Liquids: Multifaceted Challenges and Intriguing Potential

Carbohydrates represent a promising option in transitioning from oil-based chemical resources to renewable ones, with the goal of developing chemistries for a sustainable future. Cellulose, hemicellulose, and largely available monosaccharides already provide useful chemical building blocks, so-called platform chemicals, such as levulinic acid and hydroxymethyl furfural, as well as solvents like cyrene or gamma-valerolactone. Therefore, there is great anticipation for novel applications involving materials and chemicals derived from sugars. In the field of ionic liquids (ILs), sugar-based ILs have been overlooked for a long time, mainly on account of their multistep demanding preparation. However, exploring new strategies for accessing sugar-based ILs, their study, and their exploitation, are attracting increasing interest. This is due to the growing concerns about the negative (eco)toxicity profile of most ILs in conjunction with their non-sustainable nature. In the present review, a literature survey concerning the development of sugar-based ILs since 2011 is presented. Their preparation strategies and thermal behavior analyses, sorted by sugar type, make up the first two sections with the intention to provide the reader with a useful guide. A final overview of the potential applications of sugar-based ILs and their future perspectives complement the present analysis.

Chiral Self-Sorting Effects in the Self-Assembly of Metallosupramolecular Aggregates Comprising Ligands Derived from Tröger's Base

Five ligands with either nitrile or isonitrile metal binding motifs have been synthesized based on the 2,8- or 3,9-disubstituted Tröger's base scaffold, respectively. These ligands self-assemble into dinuclear cyclic metallosupramolecular aggregates upon coordination to [(dppp)Pd(OTf)₂ ] in a highly diastereoselective manner, by heterochiral self-sorting in a chiral self-discriminating manner as shown by ESI mass spectrometry, NMR spectroscopy, and single crystal XRD analysis. This observation is in contrast to earlier studies with ligands derived from Tröger's base that have larger metal binding motifs and bis(nitrile) and bis(isonitrile) ligands based on other rigid dissymmetric cores such as [2.2]paracyclophanes. Thus, the combination of these slim metal binding motifs with the rigid v-shaped 2,8- or 3,9-disubstituted Tröger's base scaffolds seems to be especially well preorganized to ensure high-fidelity social self-sorting behavior.

Efficient resolution of racemic crown-shaped cyclotriveratrylene derivatives and isolation and characterization of the intermediate saddle isomer

The preparative resolution of a trifunctionalized C₃-symmetrical chiral cyclotriveratrylene derivative was achieved via high-performance liquid chromatography (HPLC) on a chiral stationary phase. This approach is a promising alternative to the previously reported resolution through formation of diastereomeric esters because it involves fewer synthetic steps and is less prone to thermal (re)racemization. During these studies an intermediate saddle conformer could also be isolated and characterized by ¹H and ¹³C NMR spectroscopy. The HPLC separation method was further developed in order to allow investigations on the racemization behavior of the cyclotriveratrylene derivative.

From diiodo Tröger's bases towards halogen-bonded porous organic crystalline materials

Diiodo Tröger's base derivatives proved to be self-complementary tectons that are particularly suitable for the formation of porous supramolecular structures by halogen bonding.

Diastereoselective Self-Assembly of a Neutral Dinuclear Double-Stranded Zinc(II) Helicate via Narcissistic Self-Sorting

A new bis(salicylimine) ligand based on the Tröger's base scaffold was synthesized in racemic and enantiomerically pure form. Upon coordination to zinc(II) ions this ligand undergoes highly diastereoselective self-assembly into neutral dinuclear double-stranded helicates as proven by XRD analysis and via comparison of experimental ECD spectra with those simulated with quantum-chemical methods. When the racemic ligand was used, self-assembly occurs under narcissistic self-sorting resulting in the formation of a racemic pair of helicates as revealed by NMR spectroscopy and XRD analysis.

Resolution of Thiele’s acid

Thiele’s acid has been resolved for the first time by diastereomeric salt formation with brucine. Determination of absolute stereochemistry was accomplished by X-ray crystallography of the corresponding diester. We anticipate that access to optically resolved Thiele’s acid will stimulate its use in a diverse range of applications requiring chiral molecular clefts.

Accessing N-Stereogenicity through a Double Aza-Michael Reaction: Mechanistic Insights

Further development of the chemistry and applications of chiral compounds that possess configurationally stable stereogenic nitrogen atoms is hampered by the lack of efficient strategies to access such compounds in an enantiomerically pure form. Esters of propiolic acid and chiral alcohols were evaluated as cheap and readily available Michael acceptors in a diastereoselective synthesis of N-stereogenic compounds by means of a double aza-Michael conjugate addition. Diastereomeric ratios of up to 74:26 and high yields were achieved with (-)-menthyl propiolate as a substrate. Furthermore, a detailed mechanistic investigation was undertaken to shed some light on the course of this domino transformation. Kinetic studies revealed that the protic-solvent additive acts as a Brønsted acid and activates the ester toward the initial attack of the tetrahydrodiazocine partner. Conversely, acidic conditions proved unfavorable during the final cyclization step that provides the product.

Asymmetric synthesis of N-stereogenic molecules: diastereoselective double aza-Michael reaction

A novel approach towards the asymmetric synthesis of N-stereogenic molecules via double aza-Michael addition was developed. The diastereomeric ratio can be increased by a thermodynamically controlled isomerization mechanism. Simple separation and functionalization of the products afford N-stereogenic compounds in high enantiomeric purity.

Tröger's base quasiracemates and crystal packing tendencies

A family of 7 Tröger's base (TB) compounds that vary in configuration (quasiracemates, racemates, and enantiomers) and chemical substitutions (Me, Cl, and Br) has been investigated.

Modular Synthesis of Optically Active Tröger's Base Analogues

For the first time, enantioselective catalysis was applied for the preparation of Tröger's base derivatives affording N-stereogenic building blocks not only in excellent enantiomeric purity but also in an easily scalable fashion. Enzymatic kinetic resolution proved efficient to yield functionalized Tröger's bases, which can be subsequently modified by various chemical methods without any erosion of stereogenic information. In concert with a preparatively convenient protocol for the synthesis of the racemic substrates from commercially available anilines, a highly practicable and flexible route towards a wide range of enantiopure Tröger's base analogues is provided.