New Radical Borylation Pathways for Organoboron Synthesis Enabled by Photoredox Catalysis

Radical borylation using N‐heterocyclic carbene (NHC)‐BH₃ complexes as boryl radical precursors has emerged as an important synthetic tool for organoboron assembly. However, the majority of reported methods are limited to reaction modes involving carbo‐ and/or hydroboration of specific alkenes and alkynes. Moreover, the generation of NHC‐boryl radicals relies principally on hydrogen atom abstraction with the aid of radical initiators. A distinct radical generation method is reported, as well as the reaction pathways of NHC‐boryl radicals enabled by photoredox catalysis. NHC‐boryl radicals are generated via a single‐electron oxidation and subsequently undergo cross‐coupling with the in‐situ‐generated radical anions to yield gem‐difluoroallylboronates. A photoredox‐catalyzed radical arylboration reaction of alkenes was achieved using cyanoarenes as arylating components from which elaborated organoborons were accessed. Mechanistic studies verified the oxidative formation of NHC‐boryl radicals through a single‐electron‐transfer pathway.     

Research Progress on Triarylmethyl Radical-Based High-Efficiency OLED

Perchlorotrityl radical (PTM), tris (2,4,6-trichlorophenyl) methyl radical (TTM), (3,5-dichloro-4-pyridyl) bis (2,4,6 trichlorophenyl) methyl radical (PyBTM), (N-carbazolyl) bis (2,4,6-trichlorophenyl) methyl radical (CzBTM), and their derivatives are stable organic radicals that exhibit light emissions at room temperature. Since these triarylmethyl radicals have an unpaired electron, their electron spins at the lowest excited state and ground state are both doublets, and the transition from the lowest excited state to the ground state does not pose the problem of a spin-forbidden reaction. When used as OLED layers, these triarylmethyl radicals exhibit unique light-emitting properties, which can increase the theoretical upper limit of the OLED’s internal quantum efficiency (IQE) to 100%. In recent years, research on the luminescent properties of triarylmethyl radicals has attracted increasing attention. In this review, recent developments in these triarylmethyl radicals and their derivatives in OLED devices are introduced.     

Reductive tert-Butylation of Anils by tert-Butylmercury Halides(1)

tert-Butyl radicals add to the carbon atom of benzylideneanilines to form anilino radicals, which are protonated in the presence of PTSA or NH(4)(+) in Me(2)SO. Reduction of the resulting aniline radical cations occurs readily by the ate complex, t-BuHgI(2)(-). In the absence of a proton donor, t-BuHgI will also transfer a hydrogen atom to the anilino radical to give the reductive alkylation product. Protonation can promote a free radical chain process involving electron transfer by substrate activation and/or by increasing the electron affinity of the intermediate radicals. Since the adduct radicals formed from benzylideneanilines are more easily protonated than the parent Schiff bases, PTSA but not NH(4)(+) demonstrates substrate activation, although both proton donors promote the free radical reaction.     

Electron transfer between a tyrosyl radical and a cysteine residue in hemoproteins: spin trapping analysis

We investigated electron transfer between a tyrosyl radical and cysteine residue in two systems, oxyhemoglobin (oxyHb)/peroxynitrite/5,5-dimethyl-1-pyrroline N-oxide (DMPO) and myoglobin (Mb)/hydrogen peroxide/DMPO, using a combination of techniques including ESR, immuno-spin trapping (IST), and ESI/MS. These techniques show that the nitrone spin trap DMPO covalently binds to one or more amino acid radicals in the protein. Treating oxyHb with peroxynitrite and Mb with H2O2 in the presence of a low DMPO concentration yielded secondary Cys-DMPO radical adduct exclusively, whereas in the presence of high DMPO, more of the primary Tyr-DMPO radical adduct was detected. In both systems studied, we found that, at high DMPO concentrations, mainly tyrosyl radicals (Hb-Tyr42/Tyr24 and Mb-Tyr103) are trapped and the secondary electron-transfer reaction does not compete, whereas in the presence of low concentrations of DMPO, the secondary reaction predominates over tyrosyl trapping, and a thiyl radical is formed and then trapped (Hb-Cys93 or Mb-Cys110). With increasing concentrations of DMPO in the reaction medium, primary radicals have an increasing probability of being trapped. MS/MS was used to identify the specific Tyr and Cys residues forming radicals in the myoglobin system. All data obtained from this combination of approaches support the conclusion that the initial site of radical formation is a Tyr, which then abstracts an electron from a cysteine residue to produce a cysteinyl radical. This complex phenomenon of electron transfer from one radical to another has been investigated in proteins by IST, ESR, and MS.     

Thermal-induced conversion of maleic and fumaric acid anion radicals in poly(methyl methacrylate)

Thermal-induced conversion of maleic and fumaric acid anion radicals produced by γ irradiation at 77 K in poly(methyl methacrylate) (PMMA) was studied by electron spin resonance (ESR) and optical absorption spectroscopic measurements. The ESR spectra of these acid anion radicals change into two-line spectra with a line separation of ca. 10 G by thermal annealing. This spectrum is assigned to a protonated radical of each acid anion radical. Anion radicals of the solutes are relatively stable below the γ transition point of PMMA and the conversion reaction takes place near this point. This means that the molecular motion of matrix molecule affects the radical conversion reaction.     

Interaction of polypyromellitimide with nitrogen dioxide

The mechanism of interaction of nitrogen dioxide with aromatic polyimides is considered by the example of polypyromellitimide. The formation of stable radicals of acylarylaminoxyl, iminoxyl and phenoxyl types has been detected by electron paramagnetic resonance spectroscopy. Acylarylaminoxyl radicals were detected in polypyromellitimide after its exposure to nitrogen dioxide at room temperature followed by pumping nitrogen dioxide from the samples. Iminoxyl and phenoxyl radicals were formed during thermolysis of the nitration products of the polymer at 373 K. The proposed mechanism is based on the reaction of dimers of nitrogen dioxide in the form of nitrosyl nitrate. It was observed that intermediate radical cations and nitric oxide were formed in the primary reaction of electron transfer from the polyimide to nitrosyl nitrate. The subsequent cage reactions with participation of radical cations and nitric oxide give nitroso compounds and nitrates which are precursors of stable nitrogen-containing and phenoxyl radicals.     

Hydrogen Transfer in SAM‐Mediated Enzymatic Radical Reactions

S‐Adenosylmethionine (SAM) plays an essential role in a variety of enzyme‐mediated radical reactions. One‐electron reduction of SAM is currently believed to generate the C5′‐desoxyadenosyl radical, which subsequently abstracts a hydrogen atom from the actual substrate in a catalytic or a non‐catalytic fashion. Using a combination of theoretical and experimental bond dissociation energy (BDE) data, the energetics of these radical processes have now been quantified. SAM‐derived radicals are found to react with their respective substrates in an exothermic fashion in enzymes using SAM in a stoichiometric (non‐catalytic) way. In contrast, the catalytic use of SAM appears to be linked to a sequence of moderately endothermic and exothermic reaction steps. The use of SAM in spore photoproduct lyase (SPL) appears to fit neither of these general categories and appears to constitute the first example of a SAM‐initiated radical reaction propagated independently of the cofactor.     

On the direct scavenging activity of melatonin towards hydroxyl and a series of peroxyl radicals

The reactions of melatonin (MLT) with hydroxyl and several peroxyl radicals have been studied using the Density Functional Theory, specifically the M05-2X functional. Five mechanisms of reaction have been considered: radical adduct formation (RAF), Hydrogen atom transfer (HAT), single electron transfer (SET), sequential electron proton transfer (SEPT) and proton coupled electron transfer (PCET). It has been found that MLT reacts with OH radicals in a diffusion-limited way, regardless of the polarity of the environment, which indicates that MLT is an excellent OH radical scavenger. The calculated values of the overall rate coefficient of MLT + ˙OH reaction in benzene and water solutions are 2.23 × 10(10) and 1.85 × 10(10) M(-1) s(-1), respectively. MLT is also predicted to be a very good ˙OOCCl(3) scavenger but rather ineffective for scavenging less reactive peroxyl radicals, such as alkenyl peroxyl radicals and ˙OOH. Therefore it is concluded that the protective effect of MLT against lipid peroxidation does not take place by directly trapping peroxyl radicals, but rather by scavenging more reactive species, such as ˙OH, which can initiate the degradation process. Branching ratios for the different channels of reaction are reported for the first time. In aqueous solutions SEPT was found to be the main mechanism for the MLT + ˙OH reaction, accounting for about 44.1% of the overall reactivity of MLT towards this radical. The good agreement between the calculated and the available experimental data, on the studied processes, supports the reliability of the results presented in this work.     

Radical identification by liquid chromatography/thermospray mass spectrometry

Radical adducts of 5,5-dimethyl-1-pyrroline N-oxide (DMPO) with hydroxyl, methanol-derived, and ethanol-derived radicals were detected by a combination of liquid chromatography with either electron paramagnetic resonance or thermospray mass spectrometry (LC/EPR or LC/TSP-MS) in the Fenton system (with methanol or ethanol). One radical adduct was observed in the reaction of DMPO with the hydroxyl radical or the methanol-derived radical, while two adducts were detected in the reaction of DMPO with ethanol-derived radicals. The LC/TSP-MS spectra showed quasi-molecular ions [M + H]+ at m/z 146 and m/z 160 for the methanol-derived and ethanol-derived radical adducts, respectively, and an apparent molecular ion M+ at m/z 130 for the hydroxyl radical adduct. Use of methyl-D3 alcohol (CD3OH) and ethyl-D5 alcohol (CD3CD2OH) indicated that carbon-centered radicals are formed. Experiments with partially deuterated ethanol (CD3CH2OH and CH3CD2OH) indicated that the two adducts observed in the reaction of DMPO with ethanol-derived radicals correspond to the two diastereomeric adducts of DMPO with the alpha-hydroxyethyl free radical.     

Compatible injection and detection systems for studying the kinetics of excess electron transfer

[reaction: see text] A design for fast kinetic studies of electron transfer in radical anions is reported. alpha-Hydroxy radicals formed by 355 nm laser flash photolysis of alpha-phenacyl alcohols are deprotonated under basic conditions to give ketyl radical anions that serve as electron injectors in inter- and intramolecular electron-transfer reactions. The 2,2-diphenylcyclopropyl group serves as a reporter. When an electron is injected and transferred such that spin character is adjacent to the reporter, cyclopropyl ring opening gives a readily detected diphenylalkyl radical.     

High performance liquid chromatography/electron spin resonance/mass spectrometry analyses of radicals formed in an anaerobic reaction of 9- (or 13-) hydroperoxide octadecadienoic acids with ferrous ions

The oxidation of linoleic acid yields isomeric acyl hydroperoxides. In order to clarify the relation between the lipid peroxide-derived radicals and the toxicity of the lipid peroxide, identification of the lipid-derived radicals is essential. In this paper, high performance liquid chromatography/electron spin resonance/mass spectrometry (HPLC/EPR/MS) analysis of the radicals was performed for the reaction mixture containing 9-hydroperoxy-(10E,12E)-octadeca-10,12-dienoic acid (9EE-OOH) [or 13-hydroperoxy-(9Z,11E)-octadeca-9,11-dienoic acid (13ZE-OOH)] under an aerobic condition or an anaerobic condition. Following radicals were identified from 9EE-OOH (or 13EZ-OOH) by using high performance liquid chromatography/electron spin resonance spectrometry (HPLC/EPR) and HPLC/EPR/MS: pentyl radical and isomers of epoxylinoleic acid radicals from 13EZ-OOH under an anaerobic condition; 7-carboxyheptyl radical and pentyl radical from 13EZ-OOH under an aerobic condition; 7-carboxyheptyl radical and pentyl radical from 9EE-OOH under an aerobic condition; 7-carboxyheptyl radical from 9EE-OOH under an anaerobic condition. These results showed that the formation of the respective radical species depends on oxygen concentration in the reaction mixtures to a great extent.     

Antioxidant activity of trans-resveratrol toward hydroxyl and hydroperoxyl radicals: a quantum chemical and computational kinetics study

In this work, we have carried out a systematic study of the antioxidant activity of trans-resveratrol toward hydroxyl ((?)OH) and hydroperoxyl ((?)OOH) radicals in aqueous simulated media using density functional quantum chemistry and computational kinetics methods. All possible mechanisms have been considered: hydrogen atom transfer (HAT), proton-coupled electron transfer (PCET), sequential electron proton transfer (SEPT), and radical adduct formation (RAF). Rate constants have been calculated using conventional transition state theory in conjunction with the Collins-Kimball theory. Branching ratios for the different paths contributing to the overall reaction, at 298 K, are reported. For the global reactivity of trans-resveratrol toward (?)OH radicals, in water at physiological pH, the main mechanism of reaction is proposed to be the sequential electron proton transfer (SEPT). However, we show that trans-resveratrol always reacts with (?)OH radicals at a rate that is diffusion-controlled, independent of the reaction pathway. This explains why trans-resveratrol is an excellent but very unselective (?)OH radical scavenger that provides antioxidant protection to the cell. Reaction between trans-resveratrol and the hydroperoxyl radical occurs only by phenolic hydrogen abstraction. The total rate coefficient is predicted to be 1.42 × 10(5) M(-1) s(-1), which is much smaller than the ones for reactions of trans-resveratrol with (?)OH radicals, but still important. Since the (?)OOH half-life time is several orders larger than the one of the (?)OH radical, it should contribute significantly to trans-resveratrol oxidation in aqueous biological media. Thus, trans-resveratrol may act as an efficient (?)OOH, and also presumably (?)OOR, radical scavenger.     

Photoinduced Five-Component Radical Relay Aminocarbonylation of Alkenes

Radical single carbonylation reactions with CO constitute a direct and robust strategy toward various carbonyl compounds from readily available chemicals, and have been extensively studied over the past decades. However, realizing highly selective catalytic systems for controlled radical double carbonylation reactions has remained a substantial challenge, particularly for the more advanced multicomponent variants, despite their great potential value. Herein, we report a visible-light-driven radical relay five-component radical double aminocarbonylation reaction of unactivated alkenes using CO under metal-free conditions. This protocol provides direct access to valuable γ-trifluoromethyl α-ketoamides with good yields and high chemoselectivity. Crucial was the identification of distinct dual roles of amine coupling partners, sequentially acting as electron donors for the formation of photoactive electron donor-acceptor (EDA) complexes with radical precursors and then as a CO acceptor via nitrogen radical cations to form carbamoyl radicals. Cross-coupling of carbamoyl radicals with the acyl radicals that are formed in an alkene-based relay process affords double aminocarbonylation products.     

Reactions of charged phenyl radicals with aliphatic amino acids in the gas phase

Gas-phase reactivity of five differently substituted positively charged phenyl radicals was examined toward six amino acids by using Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR). The reactivity of the radicals studied was determined by the electrophilicity of the radical, which can be characterized by the radical's electron affinity (EA). The larger the electron affinity of the radical, the higher the overall reaction rate. In addition to the expected H-atom abstraction, several unprecedented reaction pathways were observed, including NH2 abstraction, SH abstraction, and SCH3 abstraction. These reaction pathways dominate for the most electrophilic radicals, and they may not follow radical but rather nucleophilic addition-elimination mechanisms. Hydrogen abstraction from glycine was also investigated theoretically. The results indicate that hydrogen abstraction from alphaC of glycine is both kinetically and thermodynamically favored over the NH2 group. The ordering of transition state energies for hydrogen abstraction from the alphaC and NH2 groups was found to reflect the radicals' EA ordering.     

Photosensitive decomposition of organic solvents at 77 degrees K by an aromatic amine, diphenylparaphenylenediamine

Abstract— –By e.s.r. we have studied the photoexcitation of an aromatic amine to its triplet state at 77°K, its photoionization to a radical cation and the simultaneous formation of solvent radicals proceeding from the photosensitization of the organic glassy matrix. In the case of methanol and ethanol matrix we observe approximately one solvent radical per solute radical cation. In the case of isopropanol and methyltetrahydrofuran we find respectively three and two solvent radicals per solute radical cation. The results suggest two possible processes of photosensitization. By successive absorption of two photons, the amine reaches an excited triplet state which is able either to dissociate giving one electron and one cation radical or to transfer its energy to the solvent, this last being decomposed. It is assumed that in the case of methanol and ethanol, the radicals from the solvent are only formed by reaction on the matrix by the released electron, whereas in the case of isopropanol and methyltetrahydrofuran, the second process is prevalent or exclusive.     

Quenching of nucleotide-derived radicals by bis benzimidazole derivative Hoechst-33258 in aqueous solution

The pulse radiolysis technique has been employed to investigate the reaction of DNA-minor-groove ligand bisbenzimidazole Hoechst 33258 with pyrimidine and purine nucleotide-derived radicals. Formation of an N-centred Hoechst-33258 radical is observed. Bimolecular rate constants and the yields of Hoechst-33258 radical have been evaluated. While the rate constant for the reaction of pyrimidine-derived radicals with Hoechst-33258 remained the same (1–2) × 10⁹ dm³ mol⁻¹ s⁻¹, the yields of the Hoechst-33258 radical varied from 25% (5′-cytidine monophosphate) to 75% (5′-guanosine monophosphate) under anoxic conditions. The rate constant values for the reaction of purine-derived radicals with Hoechst-33258, under oxic and anoxic conditions, remained the same whereas with pyrimidine-derived radicals, the rate constant value under oxic conditions was about two orders of magnitude lower than under anoxic conditions. The difference in the yields of Hoechst-33258 radical with various nucleotide-derived radicals suggest the formation of different types of radicals and that the reaction mainly occurs by electron transfer from Hoechst-33258 to the nucleotide radicals.     

Proton transfer reactions of photogenerated cyanoaromatic-methylaromatic radical ion pairs2

The photochemical reactions of a number of cyanoaromatic (acceptor) and methylaromatic (donor) molecules have been investigated. These reactions can result in the formation of photosubstitution products or benzyl radical coupling products. A survey of our results and previously published data indicates that exergonic photostimulated electron transfer is a necessary but not sufficient condition for the observation of reaction products. The efficiency of proton transfer from the donor cation radical to the acceptor anion radical is determined by the kinetic acidity and basicity of the radical ion pair. Mechanistic evidence is presented which indicates that proton transfer requires diffusion apart and reencounter of the initially formed radical ion pair. Predominant radical pair combination is observed for anion radicals which yield electron-deficient free radicals upon protonation, whereas predominant cage escape and benzyl radical coupling is observed for anion radicals which yield electron-rich free radicals upon protonation.     

Reduction of Transient Histidine Radicals by Tyrosine: Influence of the Protonation State of Reactants

The role of tyrosine radicals as mediators of electron transfer reactions in enzymes is well established, as is the involvement of histidine as a binding partner. But how environmental factors affect these reactions remains poorly explored. In the study presented here, kinetic data on the influence of the protonation state of the reactants on the reduction of transient histidine radicals by tyrosine were obtained in neutral and basic aqueous solution (pH 6–12) using time-resolved chemically induced dynamic nuclear polarization (CIDNP). The histidine radicals were generated in the photo-induced reaction with the photosensitizer 3,3′,4,4′-tetracarboxy benzophenone. From model simulations of the detected CIDNP kinetics, pH dependent second-order rate constants of the reduction of histidine radicals were obtained for four possible combinations of the amino acids and their N-acetyl derivatives, and also for the systems histidine-phenylalanine dipeptide/N-acetyl tyrosine, and N-acetyl histidine/tyrosine-glutamine dipeptide. The pH dependences of the rate constant of the reduction reaction are explained accounting for the protonation states of reactants, and also protonation state of the equilibrium form of the product - reduced form of histidine radical, which is histidine with neutral or a positively charged imidazole.     

Electron spin polarization in a three-electron spin system. An application to bacterial photosynthesis

Calculations on a system consisting of three electron spins and one nuclear spin are presented and their implications for bacterial photosynthesis discussed. Comparison with experimental measurements of electron spin polarization in pre-reduced photosynthetic reaction centres leads to conclusion that the exchange interaction within the primary radical pair is positive and less than 0.8 mT when the g values of the photoinduced radicals are taken to be those measured for the isolated radical species.     

Purine N-oxides—LII: ESR studies on photochemically induced radicals from N-hydroxyxanthines

UV, gamma-, or X-irradiation of several N-hydroxyxanthines as powdered solids produces radicals that are indefinitely stable in the solid state at room temperature, but are highly unstable in protic solvents. The ESR spectra are not sufficiently resolved to be definitive but are compatible with an amidogen radical, the unpaired electron of which is partially delocalized through the aromatic π system. Structural characterization was obtained by comparing the UV induced radicals from 3-hydroxyxanthine and 3-hydroxy-8-methylxanthine with chemically generated nitroxyl radicals from the same compounds. These two radical species show differences in their ESR spectra, in the extent of interaction of the unpaired electron with the methyl group at position 8, and in the products resulting upon reaction in water.The amidogen radical reacts instantaneously with water to yield the parent xanthines. Parallels are drawn between this reduction of the amidogen radical, the photoreduction of 3-hydroxyxanthine when solutions of it are irradiated with UV light, and the reduction of 3-acetoxyxanthine in aqueous solution in the absence of light.The synthesis of a requisite derivative, 3-hydroxy-7,8-dimethylxanthine, is reported.