Integrating Organic Lewis Acid and Redox Catalysis: The Phenalenyl Cation in Dual Role

Nickel-Catalyzed Cross-Coupling of Aryl Diazonium Salts with Aryl Bromides

Herein, we present a one-pot method for the direct cross-coupling of aryl diazonium salts and aryl bromides in an economical way that avoids the use of sensitive organometallic reagents. The reaction is accomplished with the assistance of nickel catalysts, ligands, magnesium turnings, lithium chloride, and triethylamine, avoiding the use of pre-activated organometallic reagents.

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.

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.

A closed-shell phenalenyl-based dinuclear iron(III) complex as a robust cathode for a one-compartment H2O2 fuel cell

Closed-shell phenalenyl (PLY) systems are increasingly becoming more attractive as building blocks for developing promising catalysts and electroactive cathode materials, as they have tremendous potential to accept electrons and participate in redox reactions. Herein, we report a PLY-based dinuclear [FeIII2(hmbh-PLY)3] complex, 1, and its utility as a cathode material in a H2O2 fuel cell. Complex 1 was synthesized from a new Schiff base ligand, (E)-9-(2-(2-hydroxy-3-methoxybenzylidene)hydrazineyl)-1H-phenalen-1-one, hmbh-PLYH2, designed using a PLY precursor, Hz-PLY. The newly derived ligand and complex 1 were characterized by various analytical techniques, including single-crystal X-ray diffraction (SCXRD). The cyclic voltammetry (CV) study revealed that complex 1 undergoes five electron reductions under an applied electric potential. When the electroactive complex 1 was employed as a cathode in a membrane-less one-compartment H2O2 fuel cell, with Ni foam as the corresponding anode, the designed fuel cell exhibited an exceptionally high peak power density (PPD) of 2.41 mW cm-2, in comparison with those of all the previously reported Fe-based molecular complexes. DFT studies were performed to gain reasonable insights into the two-electron catalytic reduction (pathway I) of H2O2 by the Fe-center of complex 1 and to explore the geometries, energetics of the electrocatalyst, reactive intermediates and transition states.

Catalytic Photoredox C-H Arylation of 4-Oxo-4H-pyrido[1,2-a]pyrimidine-3-diazonium Tetrafluoroborates and Related Heteroaryl Diazonium Salts

Irradiation of mixtures of title diazonium salts and heteroarenes with green light (510 nm) in the presence of eosin Y disodium salt (EY-Na2) as a photocatalyst furnished the corresponding arylation products in 8-63% yields. The proposed photocatalytic cycle is analogous to that proposed previously for closely related photoredox C-H arylations with aryl diazonium salts as aryl radical sources. This method has a broad substrate scope and represents a metal-free alternative for the synthesis of 3-heteroaryl-substituted 4H-quinolizin-4-ones and azino- and azolo-fused pyrimidones with a bridgehead nitrogen atom.

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.

Palladium-Catalyzed Cross-Electrophile Coupling between Aryl Diazonium Salt and Aryl Iodide/Diaryliodonium Salt in H2O-EtOH

We report herein a mild highly chemoselective palladium-catalyzed cross-electrophile coupling between readily accessible aromatic diazonium salt and aryl iodide or diaryliodonium salt in water-ethanol (2:1) medium. Mechanistic studies revealed that ethanol is crucial to generate an active Pd(0) catalyst, and the counterion of the diazonium salt renders a cationic Pd(II) species that facilitates subsequent oxidative addition to aryl iodide/diaryliodonium salt. Silver(I) salt was crucial to retain the catalytic activity of palladium, removing the iodide ion as a precipitate.

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

We demonstrate that an in situ generated di-reduced phenalenyl (PLY) species accumulates sufficiently high energy and acts as a super electron donor to generate aryl radicals from aryl halides to accomplish Buchwald-Hartwig-type C-N cross-coupling reactions at room temperature. This catalytic protocol does not require any external stimuli such as heat, light, or cathodic current. This protocol shows a wide variety of substrate scope covering different genres of aryl and heteroaryl halides with various aromatic as well as aliphatic amines and late-stage functionalization of the well-known natural products. The control experiments, along with extensive density functional theory (DFT) calculations, unveil that the aryl radical is generated by a single electron transfer from the di-reduced PLY to the aryl halide substrate. The aryl radical acts as an electrophile and binds with amine, leading to the chemically driven radical-mediated C-N cross-coupling under transition-metal-free conditions.

Phenalenyl Radical: Smallest Polycyclic Odd Alternant Hydrocarbon Present in the Graphene Sheet

Phenalenyl, a zigzag-edged odd alternant hydrocarbon unit can be found in the graphene nanosheet. Hückel molecular orbital calculations indicate the presence of a nonbonding molecular orbital (NBMO), which originates from the linear combination of atomic orbitals (LCAO) arising from 13 carbon atoms of the phenalenyl molecule. Three redox states (cationic, neutral radical, and anionic) of the phenalenyl-based molecules were attributed to the presence of this NBMO. The cationic state can undergo two consecutive reductions to result in neutral radical and anionic states, stepwise, respectively. The phenalenyl-based radicals were found as crucial building blocks and attracted the attention of various research fields such as organic synthesis, material science, computation, and device physics. From 2012 onward, a strategy was devised using the cationic state of phenalenyl-based molecules and in situ generated phenalenyl radicals, which created a new domain of catalysis. The in situ generated phenalenyl radicals were utilized for the single electron transfer (SET) process resulting in redox catalysis. This emerging range of applications rejuvenates the more than six decades-old phenalenyl chemistry. This review captures such developments ranging from fundamental understanding to multidirectional applications of phenalenyl-based radicals.

Biaryl Coupling of Aryldiazonium Salts and Arylboronic Acids Catalysed by Gold

A gold-catalysed coupling of aryldiazonium salts with arylboronic acids is described. The reactions proceed in satisfactory yields under irradiation with blue LEDs in the presence of KF and a catalytic amount of ascorbic acid. Notably, 4-nitrobenzendiazonium tetrafluoro­borate is sufficiently reactive to undergo the coupling with a variety of arylboronic acids in the absence of aryl radical initiators. The coupling is applicable for electron-donating and electron-withdrawing groups present at the para, ortho, and meta positions of both substrates.

Synergism between few-layer black phosphorus and graphitic carbon nitride enhances the photoredox C–H arylation under visible light irradiation

A volcano-shaped relation between the amount of FLBP in the FLBP/g-CN heterojunctions in the photoredox C–H arylation was exhibited. To understand the activity of 35 wt% FLBP/g-CN, band alignments of heterojunction was studied in detailed.

Aromatization as the driving force for single electron transfer towards C–C cross-coupling reactions

There is a strong current interest in C–H functionalization reactions under metal-free conditions. We report herein that the deprotonated form of dihydrophenazine (DPh) as a potent initiator under photochemical conditions...

Pyrazole-Mediated C-H Functionalization of Arene and Heteroarenes for Aryl-(Hetero)aryl Cross-Coupling Reactions

Herein we introduce a transition-metal-free protocol that involves a commercially available, inexpensive pyrazole molecule to conduct C-C cross-coupling reactions at room temperature via a radical pathway. Using this method, an aryldiazonium salt has been coupled to a wide range of arenes and heteroarenes including benzene, mesitylene, thiophene, furan, benzoxazole to result the corresponding biaryl products. The full reaction mechanism is elucidated along with the crystallographic probation of an active initiator species. A potassium-stabilized deprotonated pyrazole steers single-electron transfer to the substrate and behaves as an initiator for the reaction.

Real Metal-Free C–H Arylation of (Hetero)arenes: The Radical Way

Synthetic methodologies involving the formation of carbon–carbon bonds from carbon–hydrogen bonds are of significant synthetic interest, both for efficiency in terms of atom economy and for their undeniable usefulness in late-stage functionalization approaches. Combining these aspects with being metal-free, the radical C–H intermolecular arylation procedures covered by this review represent both powerful and green methods for the synthesis of (hetero)biaryl systems.

1 Introduction

2 Arylation with Arenediazonium Salts and Related Derivatives

2.1 Ascorbic Acid as the Reductant

2.2 Hydrazines as Reductants

2.3 Gallic Acid as the Reductant

2.4. Polyanilines as Reductants

2.5 Chlorpromazine Hydrochloride as the Reductant

2.6 Phenalenyl-Based Radicals as Reductants

2.7 Electrolytic Reduction of Diazonium Salts

2.8 Visible-Light-Mediated Arylation

3 Arylation with Arylhydrazines: Generation of Aryl Radicals Using an Oxidant

4 Arylation with Diaryliodonium Salts

5 Arylation with Aryl Halides

6 Conclusions

Switching between mono and doubly reduced odd alternant hydrocarbon: designing a redox catalyst

Since the early Hückel molecular orbital (HMO) calculations in 1950, it has been well known that the odd alternant hydrocarbon (OAH), the phenalenyl (PLY) system, can exist in three redox states: closed shell cation (12π e⁻), mono-reduced open shell neutral radical (13π e⁻) and doubly reduced closed shell anion (14π e⁻). Switching from one redox state of PLY to another leads to a slight structural change owing to its low energy of disproportionation making the electron addition or removal process facile. To date, mono-reduced PLY based radicals have been extensively studied. However, the reactivity and application of doubly reduced PLY species have not been explored so far. In this work, we report the synthesis of the doubly reduced PLY species (14π e⁻) and its application towards the development of redox catalysis via switching with the mono-reduced form (13π e⁻) for aryl halide activation and functionalization under transition metal free conditions without any external stimuli such as heat, light or cathodic current supply.

A doubly reduced redox non-innocent phenalenyl based transition metal free catalyst has been designed and utilized in the development of the C–C cross coupling reaction through the activation of aryl halides at room temperature.

Carbazole based Electron Donor Acceptor (EDA) catalysis for the synthesis of biaryl and aryl-heteroaryl compounds

A highly regioselective, carbazole based Electron Donor Acceptor (EDA) catalyzed synthesis of biaryl and aryl-heteroaryl compounds is described. Various indole and carbazole derivatives were screened for the Homolytic Aromatic Substitution (HAS) reaction. Tetrahydrocarbazole (THC) was very efficient for the HAS transformation and proceeded via a complex formation between diazonium salt and electron rich tetrahydrocarbazole. The UV-Vis spectroscopy technique has been used to confirm the complex formation. The in situ generated EDA complex even in a catalytic amount is found to be efficient for the Single Electron Transfer (SET) process without any photoactivation. Biaryl compounds, 2-phenylfuran, 2-phenylthiophene, and 2-phenylpyrrole and bioactive compounds such as dantrolene and canagliflozin have been synthesized in moderate to excellent yields.

Transition-Metal-Free Catalytic Carboalkoxylation of Styrenes at Room Temperature

Herein, we describe the first transition-metal-free catalytic carboalkoxylation of styrenes with aryl diazonium salts by Meerwein addition in the presence of a phenalenyl ligand at room temperature without requiring any light stimulation. This three-component reaction allows facile difunctionalization of styrene derivatives with various alcohols (such as 1, 2, and 3°) as the source of alkoxy group during this transformation. The key intermediates and the transition states involved in this reaction path were unraveled by a series of control experiments coupled with density functional theory calculations. The full mechanistic investigation provides an understanding of the selectivity toward carboalkoxylation (Meerwein arylation addition elimination) in the presence of various alcohols over the simple arylation to multiple bond (Meerwein arylation-elimination) reaction.

Storing redox equivalent in the phenalenyl backbone towards catalytic multi-electron reduction

Storing and transferring electrons for multi-electron reduction processes are considered to be the key steps in various important chemical and biological transformations. In this work, we accomplished multi-electron reduction of a carboxylic acid via a hydrosilylation pathway where a redox-active phenalenyl backbone in Co(PLY-O,O)2(THF)2, stores electrons and plays a preponderant role in the entire process. This reduction proceeds by single electron transfer (SET) from the mono-reduced ligand backbone leading to the cleavage of the Si-H bond. Several important intermediates along the catalytic reduction reaction have been isolated and well characterized to prove that the redox equivalent is stored in the form of a C-H bond in the PLY backbone via a ligand dearomatization process. The ligand's extensive participation in storing a hydride equivalent has been conclusively elucidated via a deuterium labelling experiment. This is a rare example where the ligand orchestrates the multielectron reduction process leaving only the metal to maintain the conformational requirements and fine tunes the electronics of the catalyst.

Redox Cascades and Making of a C-C Bond: 1,2-Benzodiazinyl Radicals and a Copper Complex of a Benzodiazine

Two 1,2-benzodiazinyl radicals, cinnolinyl radicals by name, were successfully isolated by cascade routes using 1,4-naphthoquinone as a precursor. Reaction of 1,4-naphthoquinone with hydrazine hydrate promotes a (5e + 5H⁺) redox cascade affording benzo[ g]naphtho[1,2- c]cinnolinyl-7,12,14-trione (Cn^•) in 69% yields, while the similar reaction with 2-hydrazinopyridine is a (7e + 7H⁺) oxidative cascade and furnishes N-pyridinecinnolinyl radical (^PyCn^•). The cascades are composed of C-N and C-C bond making reactions. The neutral even alternate arenes are always diamagnetic; thus, the isolation of Cn^• and ^PyCn^• is a breakthrough. The Cn^•/Cn^- and ^PyCn^•/^PyCn^- redox couples are reversible, and the reaction of Cn^• with [Cu^I(PPh₃)₃Cl] in the presence of hydrazine hydrate and Et₃N affords a Cn^- complex of copper(I), [(Cn^-)Cu^I(PPh₃)₂] (1). Similar to phenalenyl radical, ^PyCn^• exists in three redox states, ^PyCn⁺, ^PyCn^•, and ^PyCn^-, in a smaller potential range (-0.30 V to -0.60 V vs Fc⁺/Fc couple) and can be used as an oxidant as well as a reductant. ^PyCn^• acts as a catalyst for the oxidative cleavages of benzil to benzoic and 2,2'-pyridil to picolinic acids in methanol in the presence of air. The molecular and electronic structures of Cn^•, ^PyCn^•, and 1·1/2MeOH were confirmed by single crystal X-ray crystallography, EPR spectroscopy, and DFT calculations.

A phenalenyl-based nickel catalyst for the hydroboration of olefins under ambient conditions

In this report, nickel-catalyzed hydroboration of vinylarenes and aliphatic alkenes is investigated. The non-innocent phenalenyl ligand moiety in the nickel complex Ni(PLY)2(THF)2 (1) was utilized as an electron reservoir for the selective hydroboration reaction in the presence of pinacolborane under ambient conditions. The mechanistic investigations revealed that the alkene hydroboration reaction takes place through a single electron transfer (SET) from the phenalenyl ligand backbone leading to the cleavage of the B-H bond.

Phenalenyl Based Aluminum Compound for Catalytic C-H Arylation of Arene and Heteroarenes at Room Temperature

Main group metal based catalysis has been considered to be a cost-effective alternative way to the transition metal based catalysis, due to the high abundance of main group metals in the Earth's crust. Among the main group metals, aluminum is the most abundant (7-8%) in the Earth's crust, making the development of aluminum based catalysts very attractive. So far, aluminum based compounds have been popularly used as Lewis acids in a variety of organic reactions, but chemical transformation demanding a redox based process has never utilized an Al(III) complex as a catalyst. Herein, we tuned the redox noninnocence behavior of a phenalenyl ligand by coupling with Al(III) ion, which subsequently can store the electron upon reduction with K to carry out direct C-H arylation of heteroarenes/mesitylene at ambient temperature. A mechanistic investigation revealed that a three-electron reduced phenalenyl based triradical aluminum(III) complex plays the key role in such catalysis. The electronic structure of the catalytically active triradical species has been probed using EPR spectroscopy, magnetic susceptibility measurements, and electronic structure calculations using a DFT method.

Graphene oxide-phenalenyl composite: transition metal-free recyclable and catalytic C-H functionalization

An efficient route towards a heterogeneous transition metal-free catalytic C-H functionalization using a covalently linked graphene oxide-phenalenyl conjugate is described herein (28 examples, which include a core of some biologically relevant biaryl and hetero-biaryls). It is an environmentally benign, economical and heterogeneous platform, whose catalytic activity can easily be regenerated through a simple washing-drying technique and the catalytic activity can be retained even after 10 cycles.

Synthesis of Indoles via Electron-Catalyzed Intramolecular C-N Bond Formation

A new protocol for the preparation of N-substituted indole-3-carboxylates has been developed. The key C-N bond formation occurs under transition-metal-free conditions employing a t-BuOK/DMF system without special initiators or additives. Across a number of substrates, indoles were afforded in yields higher or comparable to those obtained under transition-metal-catalyzed conditions. While demonstrating high functional group tolerance, new conditions are particularly attractive for manufacturing halogenated indoles that cannot be made in a pure form using other metal-based catalytic methods.

An Iron-Based Long-Lived Catalyst for Direct C-H Arylation of Arenes and Heteroarenes

Direct C-H arylation of arenes and heteroarenes to biaryls at ambient temperature has been accomplished using a phenalenyl-supported iron(III) catalyst. The present catalyst requires a chemical reductant such as potassium and functions without any light stimulation. C-H arylation of various heteroarenes including pyridine as well as unactivated arene such as benzene delivered good to excellent yield (28 examples, up to 92 %) at room temperature. A combined effort based on experiments and theoretical calculations established that a phenalenyl-based radical species (generated by chemical reduction of the iron(III) coordinated phenalenyl complex) plays key role during the catalysis. Furthermore, this catalyst displayed remarkable stability during the catalysis, as evident from the fact that it was still usable over ten consecutive catalytic runs without losing its catalytic efficiency.