Triphenylamines consisting of bulky 3,5-di–tert–butyl–4-anisyl group:Synthesis,redox properties and their radical cation species

Triphenylamine(TPA) derivatives have been widely used as useful building blocks for diverse functional materials because of their excellent redox activity. Most of the molecular structures of TPA-based organic functional materials contain 4-anisyl groups, which on one hand could reduce their oxidation potential and on the other hand significantly delocalize the spin density of the resultant TPA radical cation species and enhance their stability. However, molecular-level investigation of the redo...     

Reduction of a "cation pool": a new approach to radical mediated C--C bond formation.

Carbocations, carbon radicals, and carbanions are important reactive carbon intermediates in organic chemistry, and their interconversions can be carried out by redox processes. Although, such relationships have been well recognized, experimental work has been limited to analytical studies on highly stabilized intermediates. In this study such interconversions were examined using electrochemical reduction of "cation pools". Acyliminium cations, which were generated by low-temperature electrolysis of carbamates, were reduced electrochemically in the absence of radical acceptors. The homo coupling products formed effectively, suggesting that the one-electron reduction of the acyliminium cation produced the corresponding carbon-centered radical. Next, the electrochemical reduction of the acyliminium cations in the presence of electron-deficient olefins was examined. The cross coupling products were obtained in good-to-moderate yields. A mechanism involving radical addition to the double bond followed by the reduction of the resulting radical to the carbanion was suggested. The overall transformation serves as redox-mediated formal addition of C-H to C=C. The present strategy opens new opportunities to manipulate reactive carbon species using redox processes in organic synthesis.     

Isolable Radical Ions of Main‐Group Elements: Structures,Bonding and Properties

Synthesis of stable main‐group element‐based radicals represents one of the most interesting topics in contemporary organometallic chemistry, because of their vital roles in organic, inorganic and biological chemistry as well as materials science. However, the access of stable main‐group element‐based radicals is highly challenging owing to the lack of energetically accessible orbitals in the main‐group elements. During the last decades, several synthetic strategies have been developed in obtaining these reactive species. Among them, utilizing the sterically demanding substituents and π‐conjugated ligands has proven to be an effective approach. Weakly coordinating ions (WCAs) have also been found to be exceptionally practical in synthesizing radical cations of main‐group elements. By introducing these stabilization methods, we have successfully prepared a variety of radical ions of p‐block elements in the crystalline forms, and investigated their properties by different experimental and quantum chemical calculation methods. According to the investigations, magnetic stability was observed, resulting from the intramolecular electron‐exchange interaction. Furthermore, we also found that the singlet‐triplet energy gaps of the bis(triarylamine) diradical dications can be tunable by varying the temperature. These investigations open new avenues of the main‐group element‐based radicals for a large variety of applications.     

Cyclotetrabenzil Derivatives for Electrochemical Lithium-Ion Storage

Organic electrode materials could revolutionize batteries because of their high energy densities, the use of Earth-abundant elements, and structural diversity which allows fine-tuning of electrochemical properties. However, small organic molecules and intermediates formed during their redox cycling in lithium-ion batteries (LIBs) have high solubility in organic electrolytes, leading to rapid decay of cycling performance. We report the use of three cyclotetrabenzil octaketone macrocycles as cathode materials for LIBs. The rigid and insoluble naphthalene-based cyclotetrabenzil reversibly accepts eight electrons in a two-step process with a specific capacity of 279 mAh g⁻¹ and a stable cycling performance with ≈65 % capacity retention after 135 cycles. DFT calculations indicate that its reduction increases both ring strain and ring rigidity, as demonstrated by computed high distortion energies, repulsive regions in NCI plots, and close [C⋅⋅⋅C] contacts between the naphthalenes. This work highlights the importance of shape-persistency and ring strain in the design of redox-active macrocycles that maintain very low solubility in various redox states.     

Structures and reactions of radical cations of some bicycloalkanes and their related alkenes as studied by 4 k matrix esr

The σ radical cations of most typical bicycloalkanes such as norbornane and bicyclo[2,2,2]octane are radiolytically produced at 4 K in halogenocarbon matrices and are studied by ESR spectroscopy. Their electronic and geometrical structures as well as their dynamical behaviors have been elucidated from the hyperfine structures and their temperature changes. The semi occupied molecular orbital (SOMO) of the former cation is 4a₂, in which the unpaired electron delocalizes over the four exo C-H bonds giving large hyperfine coupling. The latter is a Jahn-Teller active species and exhibits static distortion from D_3h to C_2v at 4 K in CFCl₃, and the SOMO is likely to be 6b₂, in which the unpaired electron delocalizes over the four endo C-H bonds giving large proton coupling, although a dynamically averaged structure with 12 equivalent methylene protons is observed in C-C₆F₁₂ as well as in CFCl₂CF₂Cl matrices at 77 K. The unpaired electron distribution in bicycloalkane radical cations is similar to that in cycloalkane radical cations previously studied. Upon warming both the cations undergo deprotonation to give 2-yl alkyl radicals from the exo or endo C-H bond, at which the higher unpaired electron density is populated. In addition to these radical cations, the structures and reactions of the radical cations of the related bicycloalkenes such as norbornadiene, quadricyclane, and bicyclo[2,2,2]octene have also been studied. The hydride ion transfer to an olefinic radical cation to form an alkyl radical is observed for the bicyclo[2,2,2]octene radical cation as the first example observed by ESR.     

Redox polymers incorporating pendant 6-oxoverdazyl and nitronyl nitroxide radicals

Polymers comprised of redox-active organic radicals have emerged as promising materials for use in a variety of organic electronics, including fast-charging batteries. Despite these advances, relatively little attention has been focused on the diversification of the families of radicals that are commonly incorporated into polymer frameworks, with most radical polymers being comprised of nitroxide radicals. Here, we report two new examples prepared via ring-opening methathesis polymerization containing 6-oxoverdazyl and nitronyl nitroxide radicals appended to their backbones. The polymerization reaction and optoelectronic properties were explored in detail, revealing high radical content and redox activity that may be advantageous for their use as semiconducting thin films. Initial studies revealed that current–voltage curves obtained from thin films of the title polymers exhibited memory effects making them excellent candidates for use in resistive memory applications. © 2020 Wiley Periodicals, Inc. J. Polym. Sci. 2020 , 58, 309–319     

A comprehensive picture of the one-electron oxidation chemistry of enols, enolates and α-carbonyl radicals: oxidation potentials and characterization of radical intermediates

Oxidation potentials of 40 enols, enolates and some selected α-carbonyl radicals are presented along with their characterization by various techniques as applicable (X-ray, EPR, ENDOR, general TRIPLE, magnetic susceptibility measurements, UV-vis, fast scan cyclic voltammetry, isotope effects). The model compounds comprise representatives of stable simple enols linked to a multitude of substituents (alkyl, alkenyl, alkynyl, aryl, heteroaryl, propargyl alcohols) and of stable simple enols of amides. The results allow to clarify the primary reaction pathway of enol radical cations as a rapid deprotonation and—if warranted by the redox potential and the strength of the oxidant—a follow-up oxidation of the resultant α-carbonyl radical to the α-carbonyl cation. Moreover, the experimental oxidation potentials were linearly correlated with AM1 computed ionization potentials after correction for solvation. The correlation allows a reliable prediction of oxidation potentials of radicals including α-carbonyl radicals. After computing redox potentials of relevant radicals, the possibility of one-electron transfer between enolates and flavin and the involvement of various radicals of ascorbic acid in oxidation processes were assessed.     

Flow-injection methods for the determination of antioxidant activity based on free-radical processes

Flow-injection procedures are developed for the spectrophotometry determination of the antioxidant properties of low-weight biological molecules based on their interaction with free organic radicals, namely, the on-line generated radical cation from 2,2??-azino-bis-(3-ethylbenzothiazoline-6-sulfonic acid) and the longlived 2,2??-diphenyl-1-picrylhydrazyl radical. The optical and redox properties of these radicals in aqueous and water-ethanol media are studied with and without an antioxidant in the mixture. The applicability of the methods is demonstrated on a number of exogenous antioxidants: ascorbic and uric acid, glutathione, and others, trolox, a synthetic analogue of ??-tocopherol, and species of plant origin: gallic, ferulic, and caffeic acids. The comparative evaluation of the total antioxidant activity of known anti-inflammatory medicines is performed. The throughput of the methods reported is 90?C200 analyses per hour.     

Persistent hydrogen-bonded and non-hydrogen-bonded phenoxyl radicals

The production of stable phenoxyl radicals is undoubtedly a synthetic chemical challenge. Yet it is a useful way to gain information on the properties of the biological tyrosyl radicals. Recently, several persistent phenoxyl radicals have been reported, but only limited synthetic variations could be achieved. Herein, we show that the amide-o-substituted phenoxyl radical (i.e. with a salicylamide backbone) can be synthesised in a stable manner, thereby permitting easy synthetic modifications to be made through the amide bond. To study the effect of H-bonding on the properties of the phenolate/phenoxyl radical redox couple, simple H-bonded and non-H-bonded o,p-tBu-protected salicylamidate compounds have been prepared. Their redox properties were examined by cyclic voltammetry and showed a fully reversible one-electron oxidation process to the corresponding phenoxyl radical species. Remarkably, the redox potential appears to be correlated, at least partially, with H-bond strength, as relatively large differences (ca. 300 mV) in the redox potential between H-bonded and non-H-bonded phenolate salts are observed. The corresponding phenoxyl radicals produced electrochemically are persistent at room temperature for at least an hour; their UV/Vis and EPR characterisation is consistent with that of phenoxyl radicals, which makes them excellent models of biological tyrosyl radicals. The analyses of the experimental data coupled with theoretical calculations indicate that both the deviation from planarity of the amide function and intramolecular H-bonding influence the oxidation potential of the phenolate. The latter H-bonding effect appears to be predominantly exerted on the phenolate and not (or only a little) on the phenoxyl radical. Thus, in these systems the H-bonding energy involved in the phenoxyl radical appears to be relatively small.     

Oxime Ether Radical Cations Stabilized by N‐Heterocyclic Carbenes

N‐heterocyclic carbene (NHC) nitric oxide (NHCNO) radicals, which can be regarded as iminoxyl radicals stabilized by NHCs, were found to react with a series of silyl and alkyl triflates to generate the corresponding oxime ether radical cations. The structures of the resulting oxime ether radical cations were determined by X‐ray crystallography, along with EPR and computational analysis. In contrast, lutidinium triflate produced a 1:1 mixture of [NHCNO⁺][OTf^?] and [NHCNHOH⁺][OTf^?] upon the reaction with NHCNO. This study adds an important example of stable singlet carbenes for stabilizing main‐group radicals because of their π‐conjugating effect, the synthesis and structures of which have not been reported previously.     

Effects of metal ions on physicochemical properties and redox reactivity of phenolates and phenoxyl radicals: mechanistic insight into hydrogen atom abstraction by phenoxyl radical-metal complexes.

Phenolate and phenoxyl radical complexes of a series of alkaline earth metal ions as well as monovalent cations such as Na+ and K+ have been prepared by using 2,4-di-tert-butyl-6-(1,4,7,10-tetraoxa-13-aza-cyclopentadec-13-ylmethyl)phenol (L1H) and 2,4-di-tert-butyl-6-(1,4,7,10,13-pentaoxa-16-aza-cyclooctadec-16-ylmethyl)phenol (L2H) to examine the effects of the cations on the structure, physicochemical properties and redox reactivity of the phenolate and phenoxyl radical complexes. Crystal structures of the Mg2+- and Ca2+-complexes of L1- as well as the Ca2+- and Sr2+-complexes of L2- were determined by X-ray crystallographic analysis, showing that the crown ether rings in the Ca2+-complexes are significantly distorted from planarity, whereas those in the Mg2+- and Sr2+-complexes are fairly flat. The spectral features (UV-vis) as well as the redox potentials of the phenolate complexes are also influenced by the metal ions, depending on the Lewis acidity of the metal ions. The phenoxyl radical complexes are successfully generated in situ by the oxidation of the phenolate complexes with (NH4)(2)[Ce4+(NO3)6] (CAN). They exhibited strong absorption bands around 400 nm together with a broad one around 600-900 nm, the latter of which is also affected by the metal ions. The phenoxyl radical-metal complexes are characterized by resonance Raman, ESI-MS, and ESR spectra, and the metal ion effects on those spectroscopic features are also discussed. Stability and reactivity of the phenoxyl radical-metal complexes are significantly different, depending on the type of metal ions. The disproportionation of the phenoxyl radicals is significantly retarded by the electronic repulsion between the metal cation and a generated organic cation (Ln+), leading to stabilization of the radicals. On the other hand, divalent cations decelerate the rate of hydrogen atom abstraction from 10-methyl-9,10-dihydroacridine (AcrH2) and its 9-substituted derivatives (AcrHR) by the phenoxyl radicals. On the basis of primary kinetic deuterium isotope effects and energetic consideration of the electron-transfer step from AcrH2 to the phenoxyl radical-metal complexes, we propose that the hydrogen atom abstraction by the phenoxyl radical-alkaline earth metal complexes proceeds via electron transfer followed by proton transfer.     

A triphenylamine double-decker: from a delocalized radical cation to a diradical dication with an excited triplet state

The redox properties and electronic structures of polycationic species were examined using a triphenylamine double-decker species. The double-decker has a strong electron-donating ability, and the spin in the radical cation is delocalized over the whole caged skeleton, despite no direct transannular π-π interaction between two TPA decks. Moreover, the diradical dication has spin-singlet character, despite the meta-phenylene linkage.     

Post-Synthetic Modification of Metal-Organic Frameworks Bearing Phenazine Radical Cations for aza-Diels-Alder Reactions

Metal-organic frameworks (MOFs) consisting of organic radicals are of great interest because they have exhibited unique and intriguing optical, electronic, magnetic, and chemo-catalytic properties, and thus have demonstrated great potential applications in optical, electronic, and magnetic devices, and as catalysts. However, the preparation of MOFs bearing stable organic radicals is very challenging because most organic radicals are highly reactive and difficult to incorporate into the framework of MOFs. Herein we reported a post-synthetic modification strategy to prepare a novel MOF containing phenazine radical cations, which was used as heterogeneous catalyst for aza-Diels-Alder reaction. The zinc-based metal-organic framework Zn₂(PHZ)₂(dabco) ( N ) was successfully synthesized from 5,10-di(4-benzoic acid)-5,10-dihydrophenazine (PHZ), triethylene diamine (dabco) with Zn(NO₃)₂ ⋅ 6H₂O by solvothermal method. The as-synthesized MOF N was partially oxidized by AgSbF₆ to form MOF R containing ∼10% phenazine radical cation species. The resultant MOF R was found to keep the original crystal type of N and very persistent under ambient conditions. Consequently, MOF R was successfully employed in radical cation-catalyzed aza-Diels-Alder reactions with various imine substrates at room temperature with high reaction conversion. Moreover, heterogeneous catalyst MOF R was reusable up to five times without much loss of catalytic activity, demonstrating its excellent stability and recyclability. Therefore, the post-synthetic modification developed in this work is expected to become a versatile strategy to prepare radical-based MOFs for the application of heterogeneous catalysts in organic synthesis.     

Persistent,highly localized,and tunable [4]helicene radicals

Persistent organic radicals have gained considerable attention in the fields of catalysis and materials science. In particular, helical molecules are of great interest for the development and application of novel organic radicals in optoelectronic and spintronic materials. Here we report the syntheses of easily tunable and stable neutral quinolinoacridine radicals under anaerobic conditions by chemical reduction of their quinolinoacridinium cation analogs. The structures of these [4]helicene radicals were determined by X-ray crystallography. Density functional theory (DFT) calculations, supported by electron paramagnetic resonance (EPR) measurements, indicate that over 40% of spin density is located at the central carbon of our [4]helicene radicals regardless of their structural modifications. The localization of the charge promotes a reversible oxidation to the cation upon exposure to air. This unusual reactivity toward molecular oxygen was monitored via UV-Vis spectroscopy.

We report a series of tunable and persistent [4]-helicene neutral radicals by chemical reduction of the [4]-helicenium cation analogue. EPR spectroscopy and DFT calculations indicate that the unpaired electron is localized at the central carbon atom.     

Elektrochemische untersuchungen von mono-, di- und tricarboniumionen, carbanionen und radikalen der triphenylmethanreihe

Electrochemical investigations of mono-, di- and tri-carboniumions, carbanions and radicals of the triphenyl-methane series have been carried out in dry benzonitrile, using DC voltammetry, cyclic voltammetry and potential controlled coulometry. Half wave potentials and assignments of the redox equilibria are given. Radical cations and radical anions have been established as stable intermediates in the redox equilibria of bi- and trifunctional compounds.     

Tuneable Redox Chemistry and Electrochromism of Persistent Symmetric and Asymmetric Azine Radical Cations

Molecular organic radicals have been intensively studied in the last decades, due to their interesting optical, magnetic and redox properties. Here we report the synthesis and characterisation of persistent organic radicals from one-electron oxidation of redox-active azines (RAAs), composed of two guanidinyl or related groups. By connecting two different groups together, asymmetric compounds result. In this way a series of compounds with varying redox potential is obtained that could be oxidised reversibly to the mono- and the dicationic charge states. The accessible redox states were fully determined by chemical redox reactions. The standard Gibbs free energy change for disproportionation of the radical monocation into the dication and the neutral molecule in solution, estimated from cyclovoltammetric measurements, varies between 43 and 71 kJ mol⁻¹. While the neutral RAAs absorb predominately UV light, the radical monocations display strong absorptions covering almost the entire visible region and extending for some compounds into the NIR region. A detailed analysis of this highly reversible electrochromism is presented, and the fast switching characteristics are demonstrated in an electrochromic test device.     

Structural diversity of metal–organic frameworks via employment of azamacrocycles as a building block

Research on incorporating macrocycles into metal–organic frameworks (MOFs) has been performed intensively due to the opportunities afforded by merging a merit of macrocycles with MOF chemistry, which lead to novel hybrid materials for potential application. Among the numerous kinds of macrocycles, azamacrocycles are used as traditional and popular chelating agents in supramolecular coordination chemistry, because they are very easily functionalized by joining pendant arms and possess a strong propensity to complex metal cations, accounting for the amine functionalities. With this as background, many types of azamacrocyclic MOFs have been synthesized, granting compositionally and topologically new MOFs. The macrocyclic rings can serve as additional adsorption sites or catalytic sites, and the pendant arms on the macrocycles can also play versatile roles such as structure-directing agents, pore-decorating moieties, or rotatable molecular gates for opening/closing pores. In this review, we comprehensively discuss the syntheses, structures, and features of azamacrocyclic MOFs reported to date. Based on representative studies, advantages of these compounds are described, such as how the azamacrocycles increase the structural diversity and complexity of the MOFs and induce novel structural properties within the architectures.     

Molecular Machines: Stimulation of Cation Motion in Molecular Switches

The theoretical aspects of the mechanism of the motion of cations and ligands in molecular machines referred to as redox switches are presented. The interrelated properties of cations—the energetic, electrochemical, spectral, and magnetic properties; their propensity to form either covalent or ionic bonds; and the relative softness and hardness of cations and ligands—stimulate molecular motion. These properties determine the thermal stability and stability to destruction caused by electrochemical processes and, eventually, the maximal number of transformation cycles. The maximal efficiency of redox switches is attained when the redox reaction involves a cation with a half-filled (d ⁵, f ⁷) or complete (d ¹⁰, f ¹⁴) electronic shell. The role of the Jahn-Teller effect is considered: it is responsible for geometry distortion, which stimulates cation motion. The properties of nd and 4f cations are compared from the standpoint of their use for designing redox switches. In switches constructed on the basis of supramolecular compounds containing hard and soft moieties, softer cations (Fe²⁺, Co²⁺, Cu⁺, etc.) prefer to coordinate to soft ligands and harder cations (Fe³⁺, Co³⁺, Cu²⁺, etc.) prefer to coordinate to hard ligands. A cation moves due to the soft-hard change of its coordination sphere in the course of the redox reaction. Design of redox switches based on solid compounds with a cation in mixed oxidation state is shown to be promising. Cations can change their oxidation state with a change in temperature or pressure. The possibility of designing “magnetic switches” is considered.     

Crowned Macrocycles Containing Two Pyrrolo[1,2-a] Indoles Created By Intramolecular Fusion

Heterocyclic fused-ring systems are of utmost importance because of their presence in many natural products with biological activity. Pyrroloindoles are tricyclic heterocycles that are present in various bioactive and medicinally valuable compounds. Herein, we report the synthesis of phenylene-bridged bis-pyrrolo[1,2-a]indole crowned macrocycles 1 – 3 in which the pyrrolo[1,2-a]indole moieties were formed via intramolecular fusion. The macrocycles were thoroughly characterized by 1D and 2D NMR, HRMS and X-ray crystallographic studies. The X-ray structure revealed that the two pyrrolo[1,2-a]indole moieties were parallel to each other, and one pyrrolo[1,2-a]indole unit was deviated by an angle of 9.54° while the other pyrrolo[1,2-a]indole unit was deviated by an angle of 12.0° from the mean plane defined by 28 core atoms. The macrocycles 1 – 3 absorb in the visible region and readily undergo oxidations because of their electron rich nature. The macrocycles 1 – 3 upon treatment with trifluoroacetic acid (TFA) generates the corresponding cation radicals 1 – 3 ^.+ which were stable in the open air for a week. The cation radicals 1 – 3 ^.+ absorb strongly in the NIR region and the experimental observations on crowned macrocycles 1 – 3 were corroborated by DFT and TD-DFT studies.     

Growing Utilization of Radical Chemistry in the Synthesis of Pharmaceuticals

Our current unhealthy lifestyle and the exponential surge in the population getting affected by a variety of diseases have made pharmaceuticals or drugs an imperative part of life, making the development of innovative strategies for drug discovery or the introduction of refined, cost-effective and modern technologies for the synthesis of clinically used drugs, a need of the hour. Ever since their discovery, free radicals and radical cations or anions as reactive intermediates have captivated the chemists, resulting in an exceptional utilization of these moieties throughout the field of chemical synthesis, owing to their unprecedented and widespread reactivity. Sticking with the idea of not judging the book by its cover, despite the conventional thought process of radicals being unstable and difficult to control entities, scientists and academicians around the globe have done an appreciable amount of work utilizing both persistent as well as transient radicals for a variety of organic transformations, exemplifying them with the synthesis of significant biologically active pharmaceutical ingredients. This review truly accounts for the organic radical transformations including radical addition, radical cascade cyclization, radical/radical cross-coupling, coupling with metal-complexes and radical cations coupling with nucleophiles, that offers fascinating and unconventional approaches towards the construction of intricate structural frameworks of marketed APIs with high atom- and step-economy; complementing the otherwise employed traditional methods. This tutorial review presents a comprehensive package of diverse methods utilized for radical generation, featuring their reactivity to form critical bonds in pharmaceutical total synthesis or in building key starting materials or intermediates of their synthetic journey, acknowledging their excellence, downsides and underlying mechanisms, which are otherwise poorly highlighted in the literature. Despite great achievements over the past few decades in this area, many challenges and obstacles are yet to be unraveled to shorten the distance between the academics and the industry, which are all discussed in summary and outlook.