Accessing Heterobiaryls through Transition-Metal-Free C-H Functionalization

Recent Progress on the Synthesis of Bipyridine Derivatives

Bipyridine and related compounds are starting materials or precursors for a variety of valuable substances such as biologically active molecules, ligands for catalysts, photosensitizers, viologens, and supramolecular architectures. Thus, it is important to classify their synthesis methods and understand their characteristics. Representative examples include methods using homo and heterocoupling of pyridine derivatives in the presence of a catalyst. Because bipyridine compounds strongly coordinate with metal centers, a decrease in catalytic activity and yield is often observed in the reaction system. To address this issue, this review provides insights into advances over the last ~30 years in bipyridine synthesis using metal complexes under both homogeneous and heterogeneous conditions. Moreover, strategies for bipyridine synthesis involving sulfur and phosphorous compounds are examined. These alternative pathways offer promising avenues for overcoming the challenges associated with traditional catalysis methods, providing a more comprehensive understanding of the synthesis landscape.

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.

Nitrilium ion trapping as a strategy to access structurally diverse heterobiaryl-containing peptide macrocycles

Biaryl and heterobiaryl-containing cyclic peptides represent promising scaffolds for the development of bioactive molecules. The incorporation of heterobiaryl motifs continues to pose synthetic challenges, which is partially due to the difficulties in effecting late-stage metal-catalyzed cross-couplings. We report a new strategy to form heterobiaryls that is based on trapping nitrilium ions. The sequence is exemplified using oxadiazole- and oxazole-containing biaryl linkages. NMR analysis and molecular dynamics simulations reveal structural control elements common to each member of the heterobiaryl containing peptide family in this study. Strategic substitutions on the C-terminal aminobenzoic acid moiety paired with installation of oxadiazole or oxazole heterobiaryl backbone linkages allow for the modulation of peptide backbone conformation, which should assist efforts to optimize the biophysical properties of peptide macrocycles.

Redox Switching Behavior in Resistive Memory Device Designed Using a Solution-Processable Phenalenyl-Based Co(II) Complex: Experimental and DFT Studies

We herein report a novel square-planar complex [Co^IIL], which was synthesized using the electronically interesting phenalenyl-derived ligand LH₂ = 9,9'-(ethane-1,2-diylbis(azanediyl))bis(1H-phenalen-1-one). The molecular structure of the complex is confirmed with the help of the single-crystal X-ray diffraction technique. [Co^IIL] is a mononuclear complex where the Co(II) ion is present in the square-planar geometry coordinated by the chelating bis-phenalenone ligand. The solid-state packing of [Co^IIL] complex in a crystal structure has been explained with the help of supramolecular studies, which revealed that the π···π stacking present in the [Co^IIL] complex is analogous to the one present in tetrathiafulvalene/tetracyanoquinodimethane charge transfer salt, well-known materials for their unique charge carrier interfaces. The [Co^IIL] complex was employed as the active material to fabricate a resistive switching memory device, indium tin oxide/Co^IIL/Al, and characterized using the write-read-erase-read cycle. The device has interestingly shown a stable and reproducible switching between two different resistance states for more than 2000 s. Observed bistable resistive states of the device have been explained by corroborating the electrochemical characterizations and density functional theory studies, where the role of the Co^II metal center and π-conjugated phenalenyl backbone in the redox-resistive switching mechanism is proposed.

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.

Designing a Redox Noninnocent Phenalenyl-Based Copper(II) Complex: An Autotandem Catalyst for the Selective Oxidation of Polycyclic Aromatic Hydrocarbons (PAHs)

A square-planar [CuIIL] complex 1, based on the redox-active phenalenyl unit LH2 = 9,9'-(ethane-1,2-diylbis(azanediyl))bis(1H-phenalen-1-one), is prepared and structurally characterized by single-crystal X-ray diffraction analysis. Complex 1 crystallizes at room temperature with the P1 space group. The molecular structure of 1 reveals the presence of intriguing C-H···Cu intermolecular anagostic interactions of the order ∼2.7715 Å. Utilizing the presence of anagostic interactions and the free nonbonding molecular orbitals (NBMOs) of the closed-shell phenalenyl unit in 1, the oxidation reactions of some industrially important polycyclic aromatic hydrocarbons (PAHs) in the presence of the [CuIIL] complex under very mild conditions have been reported. The direct conversion of anthracene-9-carbaldehyde to 9,10-anthraquinone in one step concludes that the catalyst shows dual activity in the chemical transformations. This also includes the first report of a "single-step" catalytic transformation of pyrene-1-carbaldehyde to the synthetically difficult pyren-4-ol, a precursor for the synthesis of several novel fluorescent probes for cell imaging.

Organocatalytic synthesis of (Het)biaryl scaffolds via photoinduced intra/intermolecular C(sp2)-H arylation by 2-pyridone derivatives

A unique N,O-bidentate ligand 6-oxo-1,6-dihydro-pyridone-2-carboxylic acid dimethylamide (L1) catalyzed direct C(sp²)-H (intra/intermolecular) arylation of unactivated arenes has been developed to expedite access to (Het)biaryl scaffolds under UV-irradiation at room temperature. The protocol tolerated diverse functional groups and substitution patterns, affording the target products in moderate to excellent yields. Mechanistic investigations were also carried out to better understand the reaction pathway. Furthermore, the synthetic applicability of this unified approach has been showcased via the construction of biologically relevant 4-quinolone, tricyclic lactam and sultam derivatives.

Reduced Phenalenyl in Catalytic Dehalogenative Deuteration and Hydrodehalogenation of Aryl Halides

Dehalogenative deuteration reactions are generally performed through metal-mediated processes. This report demonstrates a mild protocol for hydrodehalogenation and dehalogenative deuteration of aryl/heteroaryl halides (39 examples) using a reduced odd alternant hydrocarbon phenalenyl under transition metal-free conditions and has been employed successfully for the incorporation of deuterium in various biologically active compounds. The combined approach of experimental and theoretical studies revealed a single electron transfer-based mechanism.

Mimicking transition metals in borrowing hydrogen from alcohols

Borrowing hydrogen from alcohols, storing it on a catalyst and subsequent transfer of the hydrogen from the catalyst to an in situ generated imine is the hallmark of a transition metal mediated catalytic N-alkylation of amines. However, such a borrowing hydrogen mechanism with a transition metal free catalytic system which stores hydrogen molecules in the catalyst backbone is yet to be established. Herein, we demonstrate that a phenalenyl ligand can imitate the role of transition metals in storing and transferring hydrogen molecules leading to borrowing hydrogen mediated alkylation of anilines by alcohols including a wide range of substrate scope. A close inspection of the mechanistic pathway by characterizing several intermediates through various spectroscopic techniques, deuterium labelling experiments, and DFT study concluded that the phenalenyl radical based backbone sequentially adds H+, H˙ and an electron through a dearomatization process which are subsequently used as reducing equivalents to the C-N double bond in a catalytic fashion.

Organocatalyst in Direct C(sp2)-H Arylation of Unactivated Arenes: [1-(2-Hydroxyethyl)-piperazine]-Catalyzed Inter-/Intra-molecular C-H Bond Activation

This article describes the identification of 1-(2-hydroxyethyl)-piperazine as a new, cost-effective, highly efficient organocatalyst, which promotes both inter- and intra-molecular direct C(_sp²)-H arylations of unactivated arenes in the presence of potassium tert-butoxide. While the inter-molecular C-H arylation of unactivated benzenes with aryl halides (Ar-X; X = I, Br, Cl) toward biaryl syntheses underwent smoothly in the presence of only 10 mol % organocatalyst, the intra-molecular C-H arylation catalytic system composed of 40 mol % each of the catalyst and the additive (4-dimethylaminopyridine (DMAP)). The novel catalyst was also able to perform both inter- and intra-molecular direct arylations simultaneously in a single pot. The mechanistic studies confirmed the involvement of aryl radical anions and proceeded via a single-electron-transfer (SET) mechanism. The large substrate scope, high functional group tolerance, competition experiments, gram-scale synthesis, and kinetic studies further highlight the importance and versatile nature of the methodology as well as the compatibility of the new catalyst. To the best of our knowledge, this is the first report on any organocatalyst that reported detailed investigations of both inter- and intra-molecular direct C(_sp²)-H arylations of unactivated arenes in a single representation.

Small organic molecules with tailored structures: initiators in the transition-metal-free C–H arylation of unactivated arenes

A small organic molecule was tailored for the efficient synthesis of biphenyl and its derivatives from aryl iodides.

Haridasan R, Das A, Ahmed J, Mandal SK. Primary amides to amines or nitriles: a dual role by a single catalyst

We report the first earth abundant single mononuclear Mn(iii) complex which can selectively and catalytically transform primary amides to nitriles as well as reduce primary amides to amines.

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.

Potassium tert-butoxide mediated aerobic hydroxylation of arylboronic acids: an application towards the synthesis of (E)-phenoxy acrylates

The first example of potassium tert-butoxide mediated aerobic hydroxylation of arylboronic acids affording phenols is described.

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.