Interrogating the configurational stability of atropisomers

Porphyrin Atropisomerism as a Molecular Engineering Tool in Medicinal Chemistry, Molecular Recognition, Supramolecular Assembly, and Catalysis

Porphyrin atropisomerism, which arises from restricted σ-bond rotation between the macrocycle and a sufficiently bulky substituent, was identified in 1969 by Gottwald and Ullman in 5,10,15,20-tetrakis(o-hydroxyphenyl)porphyrins. Henceforth, an entirely new field has emerged utilizing this transformative tool. This review strives to explain the consequences of atropisomerism in porphyrins, the methods which have been developed for their separation and analysis and present the diverse array of applications. Porphyrins alone possess intriguing properties and a structure which can be easily decorated and molded for a specific function. Therefore, atropisomerism serves as a transformative tool, making it possible to obtain even a specific molecular shape. Atropisomerism has been thoroughly exploited in catalysis and molecular recognition yet presents both challenges and opportunities in medicinal chemistry.

Crystallization-Induced Chirality Transfer in Conformationally Flexible Azahelicene Au(I) Complexes with Circularly Polarized Luminescence Activation

Flexible and twisted annulated π-systems exhibit numerous unique and desirable features, owing to their ability to display chirality. However, preventing their racemization due to the dynamic nature of their chirality remains a challenge. One promising approach to stabilize homochirality in such systems is chirality transfer from a chiral auxiliary to a moiety displaying dynamic chirality. Herein, we introduce a new approach for dynamic chirality stabilization in conformationally flexible azahelicene species via crystallization-induced intermolecular chirality transfer in Au(I) complexes featuring azahelicene (dibenzo[c,g]carbazole and benzo[c]carbazole) and enantio-pure chiral N-heterocyclic carbene (NHC) ligands with a complementary tailored shape. Crystallization of these azahelicene Au(I) complexes not only suppresses the dynamic chirality of the dibenzocarbazole species but also stabilizes their homochirality through the intermolecular conjunction between the chiral NHC and dibenzocarbazole ligands. In the Au(I) benzocarbazole complexes, the intermolecular conjunction and chirality transfer in the crystals induce chirality in the initially achiral benzocarbazole ligand. Furthermore, the crystallization of the studied complexes activates their circularly polarized luminescence (CPL) properties, which were suppressed in solution. Importantly, chirality transfer leads to significant CPL enhancement; the complexes that feature chirality transfer within the crystal structure exhibit luminescence dissymmetry factors 5 to 10 times higher than those of the complexes without chirality transfer.

Investigating the diastereoselective synthesis of a macrocycle under Curtin-Hammett control

This work sheds new light on the stereoselective synthesis of chiral macrocycles containing twisted aromatic units, valuable π-conjugated materials for recognition, sensing, and optoelectronics. For the first time, we use the Curtin-Hammett principle to investigate a chiral macrocyclisation reaction, revealing the potential for supramolecular π-π interactions to direct the outcome of a dynamic kinetic resolution, favouring the opposite macrocyclic product to that expected under reversible, thermodynamically controlled conditions. Specifically, a dynamic, racemic perylene diimide dye (1 : 1 P : M) is strapped with an enantiopure (S)-1,1'-bi-2-naphthol group (P-BINOL) to form two diastereomeric macrocyclic products, the homochiral macrocycle (PP) and the heterochiral species (PM). We find there is notable selectivity for the PM macrocycle (dr = 4 : 1), which is rationalised by kinetic templation from intramolecular aromatic non-covalent interactions between the P-BINOL π-donor and the M-PDI π-acceptor during the macrocyclisation reaction.

Organocatalytic Atroposelective Construction of Pentatomic Heterobiaryl Diamines through Arylation of 5-Aminoisoxazoles with Azonaphthalenes

An efficient catalytic asymmetric Michael-type reaction of azonaphthalenes with 5-aminoisoxazoles has been developed. The reaction was based on the utilization of a chiral phosphoric acid as the catalyst, delivering a large panel of axially chiral heterobiaryl diamines in generally good yields with excellent enantioselectivities. The gram-scale reaction and postmodification of the chiral product demonstrated their potentials in the synthesis of chiral catalysts and ligands. This approach not only provides a useful method for the construction of pentatomic heterobiaryl scaffolds but also offers new members to the axially chiral diamine family with promising applications in synthetic and medicinal chemistry.

Synthesis, structure and stereodynamics of atropisomeric N-chloroamides

Atropisomeric N-chloroamides were efficiently accessed by electrophilic halogenation of ortho-substituted secondary anilides. The stereodynamics of atropisomerism in these novel scaffolds was interrogated by detailed experimental and computational studies, revealing that racemization is correlated with amide isomerization. The stereoelectronic nature of the amide was shown to significantly influence racemization rates, with potentially important implications for other C-N atropisomeric scaffolds.

A Chirally Locked Bis-perylene Diimide Macrocycle: Consequences for Chiral Self-Assembly and Circularly Polarized Luminescence

Macrocycles containing chiral organic dyes are highly valuable for the development of supramolecular circularly polarized luminescent (CPL) materials, where a preorganized chiral framework is conducive to directing π-π self-assembly and delivering a strong and persistent CPL signal. Here, perylene diimides (PDIs) are an excellent choice for the organic dye component because, alongside their tunable photophysical and self-assembly properties, functionalization of the PDI's core yields a twisted, chiral π-system, capable of CPL. However, configurationally stable PDI-based macrocycles are rare, and those that are also capable of π-π self-assembly beyond dimers are unprecedented, both of which are advantageous for robust self-assembled chiroptical materials. In this work, we report the first bay-connected bis-PDI macrocycle that is configurationally stable (ΔG⧧ > 155 kJ mol-1). We use this chirally locked macrocycle to uncover new knowledge of chiral PDI self-assembly and to perform new quantitative CPL imaging of the resulting single-crystal materials. As such, we discover that the chirality of a 1,7-disubstituted PDI provides a rational route to designing H-, J- and concomitant H- and J-type self-assembled materials, important arrangements for optimizing (chir)optical and charge/energy transport properties. Indeed, we reveal that CPL is amplified in the single crystals of our chiral macrocycle by quantifying the degree of emitted light circular polarization from such materials for the first time using CPL-Laser Scanning Confocal Microscopy.

Studies on the Configurational Stability of Tropolone-Ketone-, Ester-, and Aldehyde-Based Chiral Axes

Recent studies have revealed that tropolone-amide aryl C-C(O) rotational barriers are dramatically higher than those of analogous benzamide-based systems, and as a result, they have an increased likelihood of displaying high configurational stability. Studies on other tropolone-based chiral axes are important to assess the generality of this phenomenon. Herein, we describe a series of studies on the rotational barriers of tropolone-ketone, tropolone-ester, and tropolone-aldehyde chiral axes. These studies are complemented with computational modeling of the dynamics of these and analogous benzenoid variants to illuminate the impact that tropolone may have on aryl-C(O) configurational stability.