Modular Synthesis of Phenanthro[9,10-c]thiophenes by a Sequence of CH Activation, Suzuki Cross-Coupling and Photocyclization Reactions

Parameters Influencing the Photochemical Cyclodehydrochlorination (CDHC) Reaction

The photochemical cyclodehydrochlorination (CDHC) reaction has recently been used to prepare a wide variety of polycyclic aromatic hydrocarbons and graphene nanoribbons (GNRs). However, the parameters affecting the efficiency of this reaction have been scarcely studied. In this work, we investigated how the reaction conditions influence the outcome of the reaction. The effect of functional groups on the different phenyl rings of the o-terphenyl scaffold was also studied. The reaction kinetics follow the same trend as Hammett's constant when electron-donating and electron-withdrawing groups are present on the ring bearing the chlorine. The CDHC reaction can be successfully performed using less energetic 365 nm light in the presence of a triplet sensitizer. Computational results provide insight on the reaction mechanism, notably by identifying its three intermediate structures as well as its limiting step.

Photoredox Annulation of Polycyclic Aromatic Hydrocarbons

The rise of interest in using polycyclic aromatic hydrocarbons (PAHs) and molecular graphenoids in optoelectronics has recently stimulated the growth of modern synthetic methodologies giving access to intramolecular aryl-aryl couplings. Here, we show that a radical-based annulation protocol allows expansion of the planarization approaches to prepare functionalized molecular graphenoids. The enabler of this reaction is peri-xanthenoxanthene, the photocatalyst which undergoes photoinduced single electron transfer with an ortho-oligoarylenyl precursor bearing electron-withdrawing and nucleofuge groups. Dissociative electron transfer enables the formation of persistent aryl radical intermediates, the latter undergoing intramolecular C-C bond formation, allowing the planarization reaction to occur. The reaction conditions are mild and compatible with various electron-withdrawing and -donating substituents on the aryl rings as well as heterocycles and PAHs. The method could be applied to induce double annulation reactions, allowing the synthesis of π-extended scaffolds with different edge peripheries.

Photostability and Antiproliferative Activity of Furan Analogues of Combretastatin A-4

Cancer is one of the most serious health problems that usually require heavy medical treatment. It is important to ensure that no additional burden is placed on patients due to the modes of administration and/or poor quality of pharmaceuticals. In this regard, understanding, quantifying, and improving the photostability (resistance to UV light or sunlight) of drugs is among the important elements that can improve the patient's quality of life. In this work, the photochemical properties of a wide range of furanone analogues of combretastatin A-4 and their antiproliferative activity against A-431 epidermoid carcinoma cells were studied in a search for compounds with improved photostability and antiproliferative activity. It was found that the incorporation of an arylidene moiety led to a significant improvement in photostability, while the antiproliferative activity strongly depends on the nature of the aryl residue in the arylidene moiety. The high photostability of arylidenes was achieved due to the delocalization of the central double bond of the 1,3,5-hexatriene system, which limited the 6π-electrocyclization. The best results in terms of antiproliferative activity were obtained for thiophene arylidene (IC₅₀ = 0.6 μM) and 3,4-diarylfuran (IC₅₀ = 0.047 μM). The obtained results address the lack of data available now in scientific literature on the photodegradation of combretastatin A-4 analogues and should be taken into account in studies of the side effects of pharmaceuticals based on them.

Strategic Advances in Sequential C-Arylations of Heteroarenes

Sequence-specific C-arylation strategies have important applications in medicinal and material research. These strategies allow C-C bond formations in a regioselective manner to synthesize large molecular libraries for studying structure-activity profiles. The past decade has seen the development of single C-C bond forming reactions using various transition-metal catalysts, cryogenic metalation strategies, and metal-free methods. Sequential arylations of heterocycles allow for the formation of multiaryl derivatives and are a preferred choice over de novo synthetic routes. This perspective sheds light on recent strategic advances to develop various sequential synthetic routes for the multiarylation of heteroarenes. This perspective addresses many challenges in optimizing sequential routes with respect to catalysts, reaction parameters, and various strategies adopted to obtain diversely arylated products.

Synthesis of Redox-Active Phenanthrene-Fused Heteroarenes by Palladium-Catalyzed C-H Annulation

Pd-catalyzed C-H annulation reactions of halo- and aryl-heteroarenes were developed using readily available o-bromobiaryls and o-dibromoaryls, respectively. A variety of five-membered heteroarenes rapidly provided the corresponding phenanthrene-fused heteroarenes, which led to the identification of phenanthro-pyrazole and thiazole as new, stable -2 V redox couples. The flexible syntheses and tunability of the redox potentials of these azole-fused phenanthrenes over a wide range are expected to facilitate their application as redox-active organic functional materials.

Synthesis of Phenanthro[9,10-c]thiophenes by 2,3-Dichloro-5,6-dicyano-1,4-benzoquinone (DDQ)-Promoted Cyclo-Oxidation

A new method to prepare phenanthro[9,10-c]thiophenes has been developed. In the presence of triflic acid, 3,4-diaryl thiophenes undergo 2,3-dichloro-5,6-dicyano-1,4-benzoquinone (DDQ)-promoted cyclo-oxidation. NMR and computational studies indicate that protonation of the thiophene plays a key role in this reaction. The reaction can be used to prepare phenanthro[9,10-c]thiophene, as well as derivatives with alkyl, bromo, and methoxy substituents. However, the yields and selectivity of the reaction depend on both the nature and location of the substituents. Bis(3-methoxyphenyl)thiophene reacts under these conditions to give the desired product in 57 % yield, while bis(4-methoxyphenyl)thiophene gives no product. Bis(3-bromophenyl)thiophene did not react, but cyclo-oxidation of bis(4-bromophenyl)thiophene provides the desired product in 34 % yield.

Design and Synthesis of Novel Breast Cancer Therapeutic Drug Candidates Based upon the Hydrophobic Feedback Approach of Antiestrogens

Based upon hydrophobic feedback approaches, we designed and synthesized novel sulfur-containing ERα modulators (4 and 5) as breast cancer therapeutic drug candidates. The tetrahydrothiepine derivative 5a showed the highest binding affinity toward ERα because of its high hydrophobicity, and it acted as an agonist toward MCF-7 cell proliferation. The corresponding alkylamino derivative 5d maintained high binding affinity to ERα and potently inhibited MCF-7 cell proliferation (IC50: 0.09 μM). Docking simulation studies of compound 5d with the ERα BD revealed that the large hydrophobic moiety of compound 5d fit well into the hydrophobic pocket of the ERα LBD and that the sulfur atom of compound 5d formed a sulfur-π interaction with the amino acid residue His524 of the ERα LBD. These interactions play important roles for the binding affinity of compound 5d to the ERα LBD.

Photochemical Reaction Cascade from O-Pent-4-enyl-Substituted Salicylates to Complex Multifunctional Scaffolds

The arene ring of the title compounds is cleaved by a reaction cascade which is initiated by an intramolecular ortho photocycloaddition reaction. Tricyclic products were obtained in a highly regio- and diastereoselective fashion via a cyclooctatriene intermediate. The facial diastereoselectivity exerted by a stereogenic center in the tether is moderate to good (dr = 65/35 to 82/18). Yields were acceptable (44-87%) except for a single substrate which had a geminal dimethyl substitution in the tether and which gave the respective product in only 14% yield. The reaction is stereoconvergent with regard to the olefin configuration ( E or Z) in agreement with a triplet mechanism of the ortho photocycloaddition step.

Aromatic metamorphosis: conversion of an aromatic skeleton into a different ring system

Aromatic skeletons are generally regarded as being uncleavable because of their aromatic stabilization energy. Compared to exocyclic functionalizations of aromatic compounds, much less attention has been paid to substitutions of endocyclic atoms in aromatic cores through partial disassembly of the cyclic skeletons and subsequent ring reconstruction. In this Feature Article, we describe our endeavours to establish "aromatic metamorphosis", where general aromatic compounds such as dibenzothiophenes, dibenzofurans, and benzofurans are transformed into different ring systems using a multi-step strategy or ideally in one step.

Cooperation and competition between halogen bonding and van der Waals forces in supramolecular engineering at the aliphatic hydrocarbon/graphite interface: position and number of bromine group effects

Herein, the photophysical properties of two π-conjugated thienophenanthrene derivatives (6,9- and 5,10-DBTD) are reported. Their self-assembled monolayers in aliphatic hydrocarbon solvents under different concentrations were investigated by scanning tunneling microscopy on a graphite surface. The STM results revealed that the self-assembled structures of the two geometrical isomers exhibited absolutely different behaviors. At the aliphatic solvent/graphite interface, 6,9-DBTD produced almost a single stable coassembled linear structure, except for that with n-tridecane as the solvent. However, the self-assembly of 5,10-DBTD showed structural diversity, and it presented a gradient variety through increasing the chain length of the aliphatic solvents as well as the solution concentration. All ordered self-assembled adlayers critically depend on not only the interchain van der Waals (vdW) interactions, but also on multiple intermolecular interactions, including BrO[double bond, length as m-dash]C and BrS hetero-halogen bonds, homo-BrBr interactions, and HBr and HO hydrogen bonds. We proposed that the cooperation and competition of the intermolecular interactions involving a Br atom and interchain vdW forces induce this structural variety. Density functional theory calculations support to unravel the different elementary structural units based on halogen bonds and hydrogen bonds and were useful tools to dissect and explain the formation mechanism.

Controllable Orientation of Ester-Group-Induced Intermolecular Halogen Bonding in a 2D Self-Assembly

Halogen bonding with high specificity and directionality in the geometry has proven to be an important type of noncovalent interaction to fabricate and control 2D molecular architectures on surfaces. Herein, we first report how the orientation of the ester substituent for thienophenanthrene derivatives (5,10-DBTD and 5,10-DITD) affects positive charge distribution of halogens by density functional theory, thus determining the formation of an intermolecular halogen bond and different self-assembled patterns by scanning tunneling microscopy. The system presented here mainly includes heterohalogen X···O═C and X···S halogen bonds, H···Br and H···O hydrogen bonds, and I···I interaction, where the directionality and strength of such weak bonds determine the molecular arrangement by varying the halogen substituent. This study provides a detailed understanding of the role of ester orientation, concentration, and solvent effects on the formation of halogen bonds and proves relevant for identification of multiple halogen bonding in supramolecular chemistry.

Regioselectivity in palladium-catalysed direct arylation of 5-membered ring heteroaromatics

In the past few years, several results using modified catalysts and new reaction conditions have been reported permitting better control of the regioselectivity of Pd-catalysed arylations.

Regioselective oxidative Pd-catalysed coupling of alkylboronic acids with pyridin-2-yl-substituted heterocycles

A total of 19 alkylated heterocycles (thiophenes, benzo-thiophenes, pyrroles, furans) were prepared (36–99% yield) from the respective pyridin-2-yl-substituted precursors employing alkylboronic acids as the alkylating reagents in an oxidative (Ag₂CO₃ and 2,6-dimethyl-1,4-benzoquinone as oxidants) Pd-catalysed coupling reaction.