Construction of Benzo-Fused Polycyclic Heteroaromatic Compounds through Palladium-Catalyzed Intramolecular C-H/C-H Biaryl Coupling

Dibenzo-fused five-membered heteroaromatic compounds, including dibenzofuran, carbazole, and dibenzothiophene, are fundamental structural units in various important polycyclic heteroaromatic compounds. The intramolecular C-H/C-H biaryl coupling of diaryl (thio)ethers and amines based on palladium(II) catalysis under oxidative conditions is known to be one of the most effective, step-economic methods for their construction. Representative examples for the construction of structurally intriguing π-extended polycyclic heteroaromatics through catalytic coupling reactions are briefly summarized in this mini-review.

Base-Mediated Annulation of Electrophilic Benzothiophene with Naphthols and Phenols: Accessing Benzothiophene-Fused Heteroacenes

A base-mediated annulation of 2-nitrobenzothiophenes with naphthols was realized for the synthesis of hitherto unknown class of heteroacenes, namely benzothieno[2,3-b]naphthofurans. By using naphthols with a hydroxyl group positioned at 1st or 2nd position, we could synthesize two positional isomers, benzothieno[2,3-b]naphtho[2,1-d]furans or benzothieno[2,3-b]naphtho[2,3-d]furans. The annulation was found to be general with a range of substituted 2-nitrobenzothiophenes and naphthols. This heteroannulation of benzothiophene was extended using a range of phenols affording the corresponding benzothieno[2,3-b]benzofurans in moderate yields. The basic photophysical properties of these heteroacenes were evaluated, and we also demonstrated the applicability of this annulation on the gram scale.

Copper-mediated construction of benzothieno[3,2-b]benzofurans by intramolecular dehydrogenative C-O coupling reaction

An efficient method to synthesize benzothieno[3,2-b]benzofurans via intramolecular dehydrogenative C-H/O-H coupling has been developed. Good to excellent yields (64-91%) could be obtained no matter if the substituted group is electron-donating or electron-withdrawing. Notably, three-to-six fused ring thienofuran compounds could be constructed using this method. A reaction mechanism study showed that 1,1-diphenylethylene can completely inhibit the reaction. Therefore, it is a radical pathway initiated by single electron transfer between the hydroxyl of the substrate and the copper catalyst.

Synthesis of Benzothienobenzofurans via Annulation of Electrophilic Benzothiophenes with Phenols

We have developed a metal-free, mild, and green synthetic route toward benzothieno[3,2-b]benzofurans by the annulation of 3-nitrobenzothiophene with phenols. The reaction was found to be general with a range of substituted phenols. In addition, we could extend the methodology for the synthesis of pentacenes and could demonstrate the synthesis in gram-scale. Moreover, we extended the strategy for the synthesis of benzothieno[2,3-b]benzofurans by starting from 2-nitrobenzothiophenes.

Synthesis and circularly polarized luminescence properties of BINOL-derived bisbenzofuro[2,3-b:3',2'-e]pyridines (BBZFPys)

A series of optically active bisbenzofuro[2,3-b:3’,2’-e]pyridine (BBZFPy) derivatives was synthesized starting with the readily available (S)- and (R)-1,1’-bi-2-naphthols through a palladium-catalyzed multiple intramolecular C–H/C–H coupling as the key ring-closure step. The effect of terminal tert-butyl substituents on the BBZFPy skeleton was systematically investigated to uncover a unique aggregation-induced enhancement of CPL characteristics in the solid state. The crystal structures of the coupling products were also evaluated by single crystal X-ray analysis and the well-ordered intermolecular stacking arrangements appeared to be responsible for the enhanced CPL.

One-pot synthesis and property study on thieno[3,2-b]furan compounds

Based on the regioselective intermolecular Suzuki coupling and subsequent intramolecular Ullmann coupling reactions, one-pot synthesis of benzo[4,5]thieno[3,2-b]benzofurans (BTBFs) has been developed.

Syntheses of Diverse Donor-Substituted Bisbenzofuro[2,3- b:3',2'- e]pyridines (BBZFPys) via Pd Catalysis, and Their Photophysical Properties

A series of bisbenzofuro[2,3- b:3',2'- e]pyridines (BBZFPys) bearing a chlorine functionality have been efficiently synthesized through a Pd-catalyzed double oxidative intramolecular C-H/C-H coupling of monochlorinated 2,6-diaryloxypyridines. The subsequent Buchwald-Hartwig amination of the chlorinated BBZFPys allowed for the access to a new class of donor-acceptor (D-A) π-conjugated compounds that comprise BBZFPy as an electron acceptor (A) and diarylamines as a donor (D) unit. The investigation of the steady-state photophysical properties of the prepared D-A compounds revealed that they are emissive in both solution and solid states in the blue-to-green color region. The singlet-triplet energy splitting (Δ E_ST) was found to be much smaller than that of substituent-free BBZFPy (0.70 eV), ranging from 0.01 to 0.56 eV. The time-resolved spectroscopy revealed that the D-A compounds, comprising a bis( tert-butyl)carbazole as the D and CF₃-attached BBZFPy as the A, showed delayed fluorescence (DF) in nonpolar matrix host material (Zeonex), while in a polar matrix (DPEPO), room-temperature phosphorescence (RTP) was faintly observed. Furthermore, organic light-emitting diodes (OLEDs) fabricated with the D-A compounds as a blue emitter showed a moderate external quantum efficiencies (EQEs) up to 1.5%.

A Pd-catalyzed optional approach for the synthesis of dibenzothiophenes

A direct and practical approach for the construction of DBTs was developed via a Pd-catalyzed tandem reaction, in which commercially available o-bromo-iodobenzenes combined with benzene thiols or iodobenzenes combined with o-bromo-benzene thiols were applied. These two approaches will provide an alternative for the synthesis of DBT derivatives.

Oxidative C-H/C-H Coupling Reactions between Two (Hetero)arenes

Transition metal-mediated C-H bond activation and functionalization represent one of the most straightforward and powerful tools in modern organic synthetic chemistry. Bi(hetero)aryls are privileged π-conjugated structural cores in biologically active molecules, organic functional materials, ligands, and organic synthetic intermediates. The oxidative C-H/C-H coupling reactions between two (hetero)arenes through 2-fold C-H activation offer a valuable opportunity for rapid assembly of diverse bi(hetero)aryls and further exploitation of their applications in pharmaceutical and material sciences. This review provides a comprehensive overview of the fundamentals and applications of transition metal-mediated/catalyzed oxidative C-H/C-H coupling reactions between two (hetero)arenes. The substrate scope, limitation, reaction mechanism, regioselectivity, and chemoselectivity, as well as related control strategies of these reactions are discussed. Additionally, the applications of these established methods in the synthesis of natural products and exploitation of new organic functional materials are exemplified. In the last section, a short introduction on oxidant- or Lewis acid-mediated oxidative Ar-H/Ar-H coupling reactions is presented, considering that it is a very powerful method for the construction of biaryl units and polycylic arenes.

Rhodium-catalyzed odorless synthesis of diaryl sulfides from borylarenes and S-aryl thiosulfonates

Various diaryl sulfides, including heteroaryl- and nitrogen-containing sulfides, have been efficiently prepared by rhodium-catalyzed odorless deborylative arylthiolation of organoborons with S-aryl thiosulfonates.

Palladium(ii)-catalyzed synthesis of dibenzothiophene derivatives via the cleavage of carbon-sulfur and carbon-hydrogen bonds

A new process has been developed for the palladium(ii)-catalyzed synthesis of dibenzothiophene derivatives via the cleavage of C-H and C-S bonds. In contrast to the existing methods for the synthesis of this scaffold by C-H functionalization, this new catalytic C-H/C-S coupling method does not require the presence of an external stoichiometric oxidant or reactive functionalities such as C-X or S-H, allowing its application to the synthesis of elaborate π-systems. Notably, the product-forming step of this reaction lies in an oxidative addition step rather than a reductive elimination step, making this reaction mechanistically uncommon.