Reduced-Phenalenyl-Based Molecule as a Super Electron Donor for Radical-Mediated C-N Coupling Catalysis at Room Temperature

Para-selective nitrobenzene amination lead by C(sp2)-H/N-H oxidative cross-coupling through aminyl radical

Arylamines, serving as crucial building blocks in natural products and finding applications in multifunctional materials, are synthesized on a large scale via an electrophilic nitration/reduction sequence. However, the current methods for aromatic C-H amination have not yet attained the same level of versatility as electrophilic nitration. Here we show an extensively investigated transition metal-free and regioselective strategy for the amination of nitrobenzenes, enabling the synthesis of 4-nitro-N-arylamines through C(sp2)-H/N-H cross-coupling between electron-deficient nitroarenes and amines. Mechanistic studies have elucidated that the crucial aspects of these reactions encompass the generation of nitrogen radicals and recombination of nitrobenzene complex radicals. The C(sp2)-N bond formation is demonstrated to be highly effective for primary and secondary arylamines as well as aliphatic amines under mild conditions, exhibiting exceptional tolerance towards diverse functional groups in both nitroarenes and amines (>100 examples with yields up to 96%). Notably, this C(sp2)-H/N-H cross-coupling exhibits exclusive para-selectivity.

Cross-Coupling Between Aryl Halides and Aryl Alkynes Catalyzed by an Odd Alternant Hydrocarbon

Catalytic cross-coupling between aryl halides and alkynes is considered an extremely important organic transformation (popularly known as the Sonogashira coupling) and it requires a transition metal-based catalyst. Accomplishing such transformation without any transition metal-based catalyst in the absence of any external stimuli such as heat, photoexcitation or cathodic current is highly challenging. This work reports transition-metal-free cross-coupling between aryl halides and alkynes synthesizing a rich library of internal alkynes without any external stimuli. A chemically double-reduced phenalenyl (PLY)-based molecule with the super-reducing property was employed for single electron transfer to activate aryl halides generating reactive aryl radicals, which subsequently react with alkyne. This protocol covers not only various types of aryl, heteroaryl and polyaryl halides but also applies to a large variety of aromatic alkynes at room temperature. With a versatile substrate scope successfully tested on more than 75 entries, this radical-mediated pathway has been explained by several control experiments. All the key reactive intermediates have been characterized with spectroscopic evidence. Detailed DFT calculations have been instrumental in portraying the mechanistic pathway. Furthermore, we have successfully extended this transition-metal-free catalytic strategy for the first time towards solvent-free cross-coupling between solid aryl halide and alkyne substrates.

Sequential C-F Bond Transformation of the Difluoromethylene Unit in Perfluoroalkyl Groups: A Combination of Fine-Tuned Phenothiazine Photoredox Catalyst and Lewis Acid

A sequential process via photoredox catalysis and Lewis acid mediation for C-F bond transformation of the CF2 unit in perfluoroalkyl groups has been achieved to transform perfluoroalkylarenes into complex fluoroalkylated compounds. A phenothiazine-based photocatalyst promotes the defluoroaminoxylation of perfluoroalkylarenes with (2,2,6,6-tetramethylpiperidin-1-yl)oxyl (TEMPO) under visible light irradiation, affording the corresponding aminoxylated products. These products undergo a further defluorinative transformation with various organosilicon reagents mediated by AlCl3 to provide highly functionalized perfluoroalkyl alcohols. Our novel phenothiazine catalyst works efficiently in the defluoroaminoxylation. Transient absorption spectroscopy revealed that the catalyst regeneration step is crucial for the photocatalytic aminoxylation.

Unveiling Three Interconvertible Redox States of Boraphenalene

Neutral 1-boraphenalene displays the isoelectronic structure of the phenalenyl carbocation and is expected to behave as an attractive organoboron multi-redox system. However, the isolation of new redox states have remained elusive even though the preparation of neutral boron(III)-containing phenalene compounds have been extensively studied. Herein, we have adopted an N-heterocyclic carbene ligand stabilization approach to achieve the first isolation of the stable and ambipolar 1-boraphenalenyl radical 1•. The 1-boraphenalenyl cation 1+ and anion 1- have also been electrochemically observed and chemically isolated, representing new redox forms of boraphenalene for the study of non-Kekulé polynuclear benzenoid molecules. Experimental and theoretical investigations suggest that the interconvertible three-redox-state species undergo reversible electronic structure modifications, which primarily take place on the polycyclic framework of the molecules, exhibiting atypical behavior compared to known donor-stabilized organoboron compounds. Initial reactivity studies, aromaticity evaluations, and photophysical studies show redox-state-dependent trends. While 1+ is luminescent in both the solution and solid states, 1• exhibits boron-centered reactivity and 1- undergoes substitution chemistry on the boraphenalenyl skeleton and serves as a single-electron transfer reductant.

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.

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

By revealing the robust photooxidant properties of phenalenyl-based organic Lewis acid, we have introduced this moiety as an effective organophotocatalyst for the oxidative azolation of unactivated and feedstock arenes. In addition to its tolerance for various functional groups and scalability, this photocatalyst was shown to be promising for the defluorinative azolation of fluoroarenes.

Mechanochemical solid state single electron transfer from reduced organic hydrocarbon for catalytic aryl-halide bond activation

Solid-state radical generation is an attractive but underutilized methodology in the catalytic strong bond activation process, such as the aryl-halide bond. Traditionally, such a process of strong bond activation relied upon the use of transition metal complexes or strongly reducing photocatalysts in organic solvents. The generation of the aryl radical from aryl halides in the absence of transition-metal or external stimuli, such as light or cathodic current, remains an elusive process. In this study, we describe a reduced organic hydrocarbon, which can act as a super reductant in the solid state to activate strong bonds by solid-state single electron transfer (SSSET) under the influence of mechanical energy leading to a catalytic strategy based on the mechano-SSSET or mechanoredox process. Here, we investigate the solid-state synthesis of the super electron donor phenalenyl anion in a ball mill and its application as an active catalyst in strong bond (aryl halide) activation. Aryl radicals generated from aryl halides by employing this strategy are competent for various carbon-carbon bond-forming reactions under solvent-free and transition metal-free conditions. We illustrate this approach for partially soluble or insoluble polyaromatic arenes in accomplishing solid-solid C-C cross-coupling catalysis, which is otherwise difficult to achieve by traditional methods using solvents.