Introducing Phenalenyl-Based Organic Lewis Acid as a Photocatalyst to Facilitate Oxidative Azolation of Unactivated Arenes

Visible-Light Photocatalytic C-H Amination of Arenes Utilizing Acridine-Lewis Acid Complexes

This report describes the development of a visible-light photocatalytic system for C(sp2)-H amination that leverages in situ-generated photocatalysts. We demonstrate that the combination of acridine derivatives and Lewis acids forms potent photooxidants that promote the C-H amination of electronically diverse arenes upon irradiation with visible-light (440 nm). A first-generation photocatalyst composed of Sc(OTf)3 and acridine effects the C-H amination of substrates with oxidation potentials ≤ +2.5 V vs SCE with pyrazole, triazole, and pyridine nucleophiles. Furthermore, the simplicity and modularity of this system enable variation of both Lewis acid and acridine to tune reactivity. This enabled the rapid identification of two second-generation photocatalysts (derived from (i) Al(OTf)3 and acridine or (ii) Sc(OTf)3 and a pyridinium-substituted acridine) that catalyze a particularly challenging transformation: C(sp2)-H amination with benzene as the limiting reagent.

Al(OTf)3-Catalyzed Regioselective N2-Arylation of Tetrazoles with Diazo Compounds

Regioselective methods to access alkylated tetrazoles still remain a challenging goal. Herein, we describe a novel regioselective protocol for N2-arylation of tetrazoles with diazo compounds using inexpensive Al(OTf)3. This reaction could be conducted under mild conditions to access a diverse array of alkylated tetrazoles with 2-substituted tetrazoles as the major products, demonstrating a comprehensive range of substrate compatibility and excellent functional group compatibility. Mechanistic studies revealed a carbene-free process in this reaction procedure. Furthermore, the scale-up reaction and transformations of the N2-arylation of tetrazole products demonstrated the potential of this strategy.

Unveiling N-Fused Nitreniums as Potent Catalytic Photooxidants

The first example of a hitherto-unknown facet of catalytic photooxidant capability of nitrenium cations is reported herein. The fundamental limitation of inability of the traditional and reported nitreniums to achieve the excited-state redox potential beyond +2.0 V (vs Ag/AgCl), the primary requirement for a powerful photooxidant, is addressed in this work by developing a structurally unique class of N-fused nitrenium cations, with the required structural engineering involving extensive π-conjugation through ring fusion at the nitrenium site, which enabled significant lowering of the LUMO energy and easy reduction at the excited state (excited-state redox potential up to +2.5 V vs Ag/AgCl), facilitated by effective delocalization/stabilization of the generated radical. This finding opens a new way to discover novel and useful (photo)catalytic properties of nitrenium cations beyond just Lewis acidity.

Tethering Three Radical Cascades for Controlled Termination of Radical Alkyne peri-Annulations: Making Phenalenyl Ketones without Oxidants

Although Bu3Sn-mediated radical alkyne peri-annulations allow access to phenalenyl ring systems, the oxidative termination of these cascades provides only a limited selection of the possible isomeric phenalenone products with product selectivity controlled by the intrinsic properties of the new cyclic systems. In this work, we report an oxidant-free termination strategy that can overcome this limitation and enable selective access to the full set of isomerically functionalized phenalenones. The key to preferential termination is the preinstallation of a "weak link" that undergoes C-O fragmentation in the final cascade step. Breaking a C-O bond is assisted by entropy, gain of conjugation in the product, and release of stabilized radical fragments. This strategy is expanded to radical exo-dig cyclization cascades of oligoalkynes, which provide access to isomeric π-extended phenalenones. Conveniently, these cascades introduce functionalities (i.e., Bu3Sn and iodide moieties) amenable to further cross-coupling reactions. Consequently, a variety of polyaromatic diones, which could serve as phenalenyl-based open-shell precursors, can be synthesized.

Organic photoredox-catalyzed oxidative azolation of unactivated fluoroarenes

Inexpensive and commercially available 2,4,6-triphenylpyrrolium tetrafluoroborate (TPT) is used as an organic photocatalyst for the nucleophilic aromatic substitution of unactivated fluoroarenes with pyrazole derivatives (SNAr) to form azole arenes. The use of organic photoredox catalysis enables the easy operation of this method under mild conditions. Various fluorinated aromatic compounds are suitable electrophiles for this transformation.

A fruitful century for the scalable synthesis and reactions of biphenyl derivatives: applications and biological aspects

This review provides recent developments in the current status and latest synthetic methodologies of biphenyl derivatives. Furthermore, this review investigates detailed discussions of several metalated chemical reactions related to biphenyl scaffolds such as Wurtz-Fittig, Ullmann, Bennett-Turner, Negishi, Kumada, Stille, Suzuki-Miyaura, Friedel-Crafts, cyanation, amination, and various electrophilic substitution reactions supported by their mechanistic pathways. Furthermore, the preconditions required for the existence of axial chirality in biaryl compounds are discussed. Furthermore, atropisomerism as a type of axial chirality in biphenyl molecules is discussed. Additionally, this review covers a wide range of biological and medicinal applications of the synthesized compounds involving patented approaches in the last decade corresponding to investigating the crucial role of the biphenyl structures in APIs.