A Three‐Dimensionally π‐Conjugated Diradical Molecular Cage

π‐Conjugated molecular cages are very challenging targets in structural organic chemistry, supramolecular chemistry, and materials science. The synthesis and physical characterizations are reported of the first three‐dimensionally π‐conjugated diradical molecular cage PTM‐C, in which two polychlorotriphenylmethyl (PTM) radicals are linked by three bis(3,6‐carbazolyl) bridges. This cage compound was synthesized mainly by intermolecular Yamamoto coupling followed by deprotonation and oxidation. It is stable and its structure was confirmed by X‐ray crystallographic analysis. The two carbon‐centered PTM radicals are weakly coupled through electronic interactions with the carbazole spacers, as revealed by optical, electronic, and magnetic measurements as well as theoretical calculations.     

Quantum mechanical study of regioselectivity of radical additions to substituted olefins

Regiochemical trends in the addition of free radicals to substituted olefins are investigated by different quantum chemical approaches with special reference to oxygen centered radicals. From a methodological point of view, density functional methods provide correct general trends but they do not reach quantitative accuracy, especially for intermediate complexes. More reliable results are obtained by single point post‐Hartree–Fock computations at density functional geometries. A number of test computations show that reoptimization of the geometry and computation of vibrational frequencies by correlated methods can be safely avoided. As a consequence, the overall computational approach has very reasonable computer costs. From a more chemical point of view, a careful analysis of computational results points out the significant role of anomeric and polar effects in tuning the common filicity of carbon centered radicals. © 2000 John Wiley & Sons, Inc. J Comput Chem 21: 675–691, 2000     

The Persistent Radical Effect in Organic Synthesis

Radical–radical couplings are mostly nearly diffusion‐controlled processes. Therefore, the selective cross‐coupling of two different radicals is challenging and not a synthetically valuable transformation. However, if the radicals have different lifetimes and if they are generated at equal rates, cross‐coupling will become the dominant process. This high cross‐selectivity is based on a kinetic phenomenon called the persistent radical effect (PRE). In this Review, an explanation of the PRE supported by simulations of simple model systems is provided. Radical stabilities are discussed within the context of their lifetimes, and various examples of PRE‐mediated radical–radical couplings in synthesis are summarized. It is shown that the PRE is not restricted to the coupling of a persistent with a transient radical. If one coupling partner is longer‐lived than the other transient radical, the PRE operates and high cross‐selectivity is achieved. This important point expands the scope of PRE‐mediated radical chemistry. The Review is divided into two parts, namely 1) the coupling of persistent or longer‐lived organic radicals and 2) “radical–metal crossover reactions”; here, metal‐centered radical species and more generally longer‐lived transition‐metal complexes that are able to react with radicals are discussed—a field that has flourished recently.     

Tricoordinate Nontrigonal Pnictogen‐Centered Radical Anions: Isolation,Characterization, and Reactivity

The search for main‐group element‐based radicals is one of the main research topics in contemporary chemistry because of their fascinating chemical and physical properties. The Group 15 element‐centered radicals mainly feature a V‐shaped two coordinate structure, with a couple of radical cations featuring trigonal tricoordinated geometry. Now, nontrigonal compounds R₃E (E=P, As, Sb) were successfully synthesized by introducing a new rigid tris‐amide ligand. The selective one‐electron reduction of R₃E afforded the first stable tricoordinate pnictogen‐centered radical anion salts; the pnictogen atoms retain planar T‐shaped structures. EPR spectroscopy and calculations reveal that the spin density mainly resides at the p orbitals of the pnictogen atoms, which is perpendicular to the N₃E planes.     

The magnetochemistry of verdazyl radical-based materials

Stable organic radicals have received much attention as building blocks for the construction of molecular magnetic materials because they are readily functionalized using modern synthetic techniques. In this context, the nitroxide radical family has been the dominant class of radicals in molecular magnetochemistry. However, other stable radical systems have also been explored. One such example is the verdazyl family of radicals. Their high chemical stability and synthetic versatility make verdazyls one of the more attractive alternatives to nitroxides in molecular magnet design. This article reviews the magnetism of verdazyl-based systems, including through-bond coupling in polyradicals, coordination complexes and intermolecular interactions in the solid state.     

Tricoordinate Nontrigonal Pnictogen‐Centered Radical Anions: Isolation,Characterization, and Reactivity

The search for main‐group element‐based radicals is one of the main research topics in contemporary chemistry because of their fascinating chemical and physical properties. The Group 15 element‐centered radicals mainly feature a V‐shaped two coordinate structure, with a couple of radical cations featuring trigonal tricoordinated geometry. Now, nontrigonal compounds R₃E (E=P, As, Sb) were successfully synthesized by introducing a new rigid tris‐amide ligand. The selective one‐electron reduction of R₃E afforded the first stable tricoordinate pnictogen‐centered radical anion salts; the pnictogen atoms retain planar T‐shaped structures. EPR spectroscopy and calculations reveal that the spin density mainly resides at the p orbitals of the pnictogen atoms, which is perpendicular to the N₃E planes.     

Direct vs Indirect Grafting of Alkyl and Aryl Halides

The direct and indirect electrochemical grafting of alkyl and aryl halides (RX, ArX) on carbon, metal and polymer surfaces is examined. Their electrochemical reduction occurs at highly negative potential in organic solvents and very often produces carbanions because the reduction potentials of RX and ArX are more negative than those of their corresponding radicals. Therefore, direct electrografting of alkyl and aryl radicals generated from RX and ArX is not easy to perform. This obstacle is overcome using aryl radicals derived from the 2,6-dimethylbenzenediazonium salt (2,6-DMBD), which do not react on the electrode surface due to their steric hindrance but react in solution by abstracting an iodine or bromine atom from RX (X=I, Br) or ArI to give alkyl or aryl radicals. As a consequence, alkyl and aryl radicals are generated at very low driving force by diverting the reactivity of aryl radicals derived from an aryl diazonium salt; they attack the electrode surface and form strongly attached organic layers. This strategy applies to the chemical modification of polymers (polyethylene, polymethylmethacrylate) by alkyl halides under heating.     

High‐Power Electrochemical Energy Storage System Employing Stable Radical Pseudocapacitors

The development of electrical energy storage devices that can operate at high charge and discharge rates is fundamentally important, however although electrochemical capacitors (ECs) can charge and discharge at high rates, their electrochemical storage capacity remains an order of magnitude lower than that of conventional lithium‐ion batteries. Novel pseudocapasitors are developed, based on the stable persilyl‐susbtituted free radicals of the heavy group 14 elements, (tBu₂MeSi)₃E^. [E=Si ( 1 ), Ge ( 2 ), and Sn ( 3 )], as anode materials for energy storage system. Such systems showed a remarkable cycle stability without significant loss of power density, in comparison with similar characteristics of the known organic radical batteries, the dual carbon cell, and the electrochemical capacitor. Particularly important is that these novel electrochemical energy storage systems employing stable heavy group 14 element radicals are lithium‐free. The electrochemical properties and structures of the reduced and oxidized species were studied by the cyclic voltammetry (CV), electron paramagnetic resonance (EPR) spectroscopy, and X‐ray diffraction (XRD).     

EPR study of nitroxides formed from the reaction of nitric oxide with photolyzed amides

Free radicals generated from UV irradiation of simple aliphatic amides in anaerobic and nitric oxide (NO)‐saturated liquid mixtures or solutions gave EPR spectra of nitroxides. The application of isotopic effects to EPR spectra and the generation of radicals by transient radical attack on substrate molecules or by photolysing amine or acetoin were used to help identify photochemically produced radicals from the amides. The aliphatic amides used were formamide, acetamide and their N‐methyl‐ or deuterium‐substituted derivatives. Transient radicals used to attack the amides via hydrogen‐atom abstraction were generated from the initiator AIBN or AAPH. The observation of various nitroxides indicates the reactivity of NO for trapping acyl, carbamoyl and other carbon‐centered radicals. Possibly mechanistic pathways diagnosed with this trap are proposed. Copyright © 2003 John Wiley & Sons, Ltd.     

Free radical rearrangements of organocobaloximes: alkynyl to cycloalkylidene and hexenyl to cyclopentylmethyl

Under irradiation by tungsten light in pyridine solution, several substituted alkylcobaloximes undergo rearrangement to more stable substituted alkyl- or alkenyl-cobaloximes. When the same reactions are carried out in the presence of carbon tetrachloride or chloroform, no rearranged organocobaloximes are obtained, but a variety of organic products are obtained derived from the interception of transient organic radicals by the halogenated solvent. The rearrangements are rationalised in terms of a reversible homolysis of the carbon-cobalt bond, rearrangement of the organic radical and recapture by the cobalt(II) fragment to give complexes that are more stable to irradiation than their precursors.     

A theoretical study of the addition of silyl radicals to olefinic monomers

In this paper, the high reactivity of silyl macroradicals toward double bonds of olefinic compounds has been explained by means of quantum‐mechanical calculations through their frontier orbital characteristics. In this way, the main orbital interaction corresponds to the overlapping between the SOMO of the disilyl radical and the LUMO of the olefin. In order to obtain more accurate results of differential reactivity, an orbitalic SOMO‐HOMO interaction should be included in addition to the main SOMO‐LUMO one. Also, we theoretically studied the regioselectivity of the addition of silyl radicals to double bonds obtaining similar results as for carbon centered radicals where the reaction takes place on the less hindered carbon of the olefin. Regarding to the geometrical and electronic parameters, it has been shown that carbon radicals have a sp² geometry and a negative charge on the radical center whilst silyl radicals have a sp³ goemetry and a positive charge. Both factors contribute to the enhanced reactivity of silyl radicals with respect to carbon ones.     

Sulfamate Esters Guide Selective Radical‐Mediated Chlorination of Aliphatic C−H Bonds

Masked alcohols are particularly appealing as directing groups because of the ubiquity of hydroxy groups in organic small molecules. Herein, we disclose a general strategy for aliphatic γ‐C(sp³)?H functionalization guided by a masked alcohol. Specifically, we determine that sulfamate ester derived nitrogen‐centered radicals mediate 1,6‐hydrogen‐atom transfer (HAT) processes to guide γ‐C(sp³)?H chlorination. This reaction proceeds through a light‐initiated radical chain‐propagation process and is capable of installing chlorine atoms at primary, secondary, and tertiary centers.     

An EPR study of the radical addition to 3‐nitropentan‐2‐one as an archetype of α‐carbonylnitroalkanes

Carbon, silicon, germanium, tin and lead‐centered radicals were reacted with 3‐nitropentan‐2‐one and 3‐nitropentan‐2‐ol inside the cavity of an electron paramagnetic resonance spectrometer. In all cases, selective addition to the nitrogroup was observed with detection of the corresponding oxynitroxide radicals. In the case of the carbonyl substrate, alkyl acyl nitroxides were also detected because of α‐photocleavage. The oxynitroxides decayed with a first order kinetics via fragmentation of the carbon–nitrogen bond (denitration). Unexpectedly, the activation parameters were fairly similar to those previously reported for the corresponding tert‐butyl oxynitroxides and almost independent from the presence of a carbonyl or a hydroxyl group on the carbon adjacent to the one bearing the nitrogroup. Copyright © 2014 John Wiley & Sons, Ltd.     

Reaction between Azidyl Radicals and Alkynes: A Straightforward Approach to NH‐1,2,3‐Triazoles

Reaction between nitrogen‐centered radicals and unsaturated C?C bonds is an effective synthetic strategy for the construction of nitrogen‐containing molecules. Although the reactions between nitrogen‐centered radicals and alkenes have been studied extensively, their counterpart reactions with alkynes are extremely rare. Herein, the first example of reactions between azidyl radicals and alkynes is described. This reaction initiated an efficient cascade reaction involving inter‐/intramolecular radical homolytic addition toward a C?C triple bond and a hydrogen‐atom transfer step to offer a straightforward approach to NH‐1,2,3‐triazoles under mild reaction conditions. Both the internal and terminal alkynes work well for this transformation and some heterocyclic substituents on alkynes are compatible. This mechanistically distinct strategy overcomes the inherent limitations associated with azide anion chemistry and represents a rare example of reactions between a nitrogen‐centered radicals and alkynes.     

Interaction between alkyl radicals and single wall carbon nanotubes

The addition of primary, secondary, and tertiary alkyl radicals to single wall carbon nanotubes (SWCNTs) was studied by means of dispersion corrected density functional theory. The PBE, B97‐D, M06‐L, and M06‐2X functionals were used. Consideration of Van der Waals interactions is essential to obtain accurate addition energies. In effect, the enthalpy changes at 298 K, for the addition of methyl, ethyl, isopropyl, and tert‐butyl radicals onto a (5,5) SWCNT are: ?25.7, ?25.1, ?22.4, and ?16.6 kcal/mol, at the M06‐2X level, respectively, whereas at PBE/6‐31G* level they are significantly lower: ?25.0, ?19.0, ?16.7, and ?5.0 kcal/mol respectively. Although the binding energies are small, the attached alkyl radicals are expected to be stable because of the large desorption barriers. The importance of nonbonded interactions was more noticeable as we moved from primary to tertiary alkyl radicals. Indeed, for the tert‐butyl radical, physisorption onto the (11,0) SWCNT is preferred rather than chemisorption. The bond dissociation energies determined for alkyl radicals and SWCNT follow the trend suggested by the consideration of radical stabilization energies. However, they are in disagreement with some degrees of functionalization observed in recent experiments. This discrepancy would stem from the fact that for some HiPco nanotubes, nonbonded interactions with alkyl radicals are stronger than covalent bonds. © 2012 Wiley Periodicals, Inc.     

A General Approach to Deboronative Radical Chain Reactions with Pinacol Alkylboronic Esters

The generation of carbon‐centered radicals from air‐sensitive organoboron compounds through nucleohomolytic substitution at boron is a general method to generate non‐functionalized and functionalized radicals. Due to their reduced Lewis acidity, alkylboronic pinacol esters are not suitable substrates. We report their in situ conversion into alkylboronic catechol esters by boron‐transesterification with a substoichiometric amount of catechol methyl borate combined with an array of radical chain processes. This simple one‐pot radical‐chain deboronative method enables the conversion of pinacol boronic esters into iodides, bromides, chlorides, and thioethers. The process is also suitable the formation of nitriles and allylated compounds through C?C bond formation using sulfonyl radical traps. The power of combining radical and classical boron chemistry is illustrated with a modular 5‐membered ring formation using a combination of three‐component coupling and protodeboronative cyclization.     

Triphenylamine (TPA) radical cations and related macrocycles

Triphenylamine (TPA) derivatives and their radical cation counterparts have successfully demonstrated a great potential for applications in a wide range of fields including organic redox catalysis, organic semiconductors, magnetic materials, etc., mainly because of their excellent redox activity. The stability of TPA radical cation has significant effect on the properties of the TPA-based functional materials, especially in relation to their electronic properties. Considering the instability of parent TPA radical cation, many efforts have been devoted to the development of stable TPA radical cations and related materials. Among them, TPA radical cation-based macrocycles have attracted particular attention because their large delocalized structures can stabilize the TPA radicals, thus endow them with outstanding redox behaviors, multiple resonance structures, and wide application in various optoelectronic devices. In this review, we give a brief introduction of organic radicals and the documented stable TPA radicals. Subsequently, a number of TPA radical cation-based macrocycles are comprehensively surveyed. It is expected that this minireview will not only summarize the recent development of TPA radical cations and their macrocycles, but also shed new light on the prospect of the design of more sophisticated radical cation-based architectures and related materials.     

Isolation and Characterization of Crystalline,Neutral Diborane(4) Radicals

Diaryldihalodiboranes(4) were reacted with bis(amidinato)‐ and bis(guanidinato)silylenes to generate the first neutral diborane‐centered radicals. These formally non‐aromatic 5π electron systems are stable in the solid state as well as in solution and were characterized by solid‐state structure determination, high‐resolution mass spectrometry, and EPR spectroscopy. The reactivity of one of these radicals with the oxidant 1,4‐benzoquinone led to ring‐opening and B?O bond formation.     

Electron paramagnetic resonance study of nitroxides generated from nitric oxide by reaction with transient radicals

Photochemical or thermal decomposition of azo‐compounds (such as 2,2^′‐azobisisobutyronitrile, 2,2^′‐azobis(2‐methylpropionamidine) dihydrochloride, dialkyl peroxides (such as tert‐butyl peroxide and diacyl peroxides (such as benzoyl peroxide) in anaerobic nitric oxide (NO)‐saturated dimethylsulfoxide (DMSO) or aqueous solutions yielded nitroxides. Well‐characterized electron paramagnetic resonance spectra of nitroxides revealed that NO was favorable for reacting with carbon‐centered and less stereo‐inhibited transient alkyl radicals, giving kinds of nitrosoalkane, typically nitrosomethane, which act sequentially as C‐nitroso compounds to trap transient radicals present in solution, yielding spin‐trapping adducts, i.e. nitroxides. Radicals, including sulfinyl radicals from UV‐irradiated DMSO, were trapped by the in situ formed CH₃NO. O‐centered radicals could not add to the freshly formed C‐nitroso compounds. Possible mechanisms are suggested. Copyright © 2002 John Wiley & Sons, Ltd.     

Radical‐Based C−C Bond‐Forming Processes Enabled by the Photoexcitation of 4‐Alkyl‐1,4‐dihydropyridines

We report herein that 4‐alkyl‐1,4‐dihydropyridines (alkyl‐DHPs) can directly reach an electronically excited state upon light absorption and trigger the generation of C(sp³)‐centered radicals without the need for an external photocatalyst. Selective excitation with a violet‐light‐emitting diode turns alkyl‐DHPs into strong reducing agents that can activate reagents through single‐electron transfer manifolds while undergoing homolytic cleavage to generate radicals. We used this photochemical dual‐reactivity profile to trigger radical‐based carbon–carbon bond‐forming processes, including nickel‐catalyzed cross‐coupling reactions.