Characterization and Reactivity Studies of a Terminal Copper–Nitrene Species

High‐valent terminal copper–nitrene species have been postulated as key intermediates in copper‐catalyzed aziridination and amination reactions. The high reactivity of these intermediates has prevented their characterization for decades, thereby making the mechanisms ambiguous. Very recently, the Lewis acid adduct of a copper–nitrene intermediate was trapped at ?90 °C and shown to be active in various oxidation reactions. Herein, we describe for the first time the synthesis and spectroscopic characterization of a terminal copper(II)–nitrene radical species that is stable at room temperature in the absence of any Lewis acid. The azide derivative of a triazamacrocyclic ligand that had previously been utilized in the stabilization of aryl–Cu^III intermediates was employed as an ancillary ligand in the study. The terminal copper(II)–nitrene radical species is able to transfer a nitrene moiety to phosphines and abstract a hydrogen atom from weak C?H bonds, leading to the formation of oxidized products in modest yields.     

Visible‐Light‐Promoted Polycyclizations of Dienynes

A variety of linear dienynes can deliver complex tetracyclic frameworks in the presence of an Ir^III complex and visible light. Product formation involves the generation of four new C?C bonds and six contiguous stereocenters, which decorate two [3.1.0] bicyclic units tethered through their bridging quaternary carbon atoms. The internal alkyne acts as a formal dicarbenoid for the generation of two cyclopropanes in these radical cation cascades. This behavior has not been previously observed for organic reactive intermediates and can be extended to intermolecular reactions and diendiynes.     

Mechanism and Kinetics of the Reaction of Nitrosamines with OH Radical: A Theoretical Study

The reaction of nitrosodimethylamine, nitrosoazetidine, nitrosopyrrolidine, and nitrosopiperidine with the hydroxyl radical has been studied using electronic structure calculations in gas and aqueous phases. The rate constant was calculated using variational transition state theory. The reactions are initiated by H‐atom abstraction from the αC─H group of nitrosamines and leads to the formation of alkyl radical intermediate. In the subsequent reactions, the initially formed alkyl radical intermediate reacts with O₂ forming a peroxy radical. The reaction of peroxy radical with other atmospheric oxidants, such as HO₂ and NO radicals, is studied. The structures of the reactive species were optimized by using the density functional theory methods, such as M06‐2X, MPW1K, and BHandHLYP, and hybrid methods G3B3. The single‐point energy calculations were also performed at CCSD(T)/6‐311+G(d,p)// M062X/6‐311+G(d,p) level. The calculated thermodynamical parameters show that the reactions corresponding to the formation of intermediates and products are highly exothermic. We have calculated the rate constant for the initial H‐atom abstraction and subsequent favorable secondary reactions using canonical variational transition state theory over the temperature range of 150–400 K. The calculated rate constant for initial H‐atom abstraction reaction is ∼3 × 10⁻¹² cm³ molecule⁻¹ s⁻¹ and is in agreement with the previous experimental results. The calculated thermochemical data and rate constants show that the reaction profile and kinetics of the reactions are less dependent on the number of methyl groups present in the nitrosoamines. Furthermore, it has been found that the atmospheric lifetime of nitrosamines is around 5 days in the normal atmospheric OH concentration.     

REACTION MECHANISM OF BENZOPHENONE-PHOTOINITIATED CROSSLINKING OF POLYETHYLENE

The radical intermediates, the crosslink microstructures, and the reaction mechanism of benzophenone (BP)-photoinitiated crosslinking of low-density polyethylene (LDPE) and model compounds (MD) have been reviewed in detail.The spin-trapping electron spin resonance (ESR) spectra obtained from the LDPE/BP systems with spin-trap agents showthat two kinds of polymer radical intermediates are mainly formed: tertiary carbon and secondary carbon radicals. The spin-trapping ESR studies of MD/BP systems give further evidence that photocrosslinking reactions of PE predominantly takeplace at sites of tertiary carbon, secondary carbon, and especially allylic carbon when available. The high resolution ~(13)C-NMR spectra obtained from LDPE and MD systems show that the crosslink microstructures have H- and Y-type links andthat their concentrations are of the same order. The fluorescence, ESR ~(13)C and ~1H-NMR spectra from the PE and MDsystems demonstrate that the main photoreduction product of BP(PPB) is benzpinacol formed by the recombination of twodiphenylhydroxymethyl (K·) radical intermediates. Two new PPB products: an isomer of benzpinacol with quinoid structure,1-phenylhydroxymethylene-4-diphenylhydroxymethyl-2, 5-cyclohexadiene and three kinds of α-alkyl-benzhydrols have beendetected and identified. These results provide new experimental evidence for elucidating the reaction mechanism in the BP-photoinitiated crosslinking of polyethylene.     

Visible‐Light‐Promoted Iminyl‐Radical Formation from Acyl Oximes: A Unified Approach to Pyridines,Quinolines, and Phenanthridines

A unified strategy involving visible‐light‐induced iminyl‐radical formation has been established for the construction of pyridines, quinolines, and phenanthridines from acyl oximes. With fac‐[Ir(ppy)₃] as a photoredox catalyst, the acyl oximes were converted by 1 e^? reduction into iminyl radical intermediates, which then underwent intramolecular homolytic aromatic substitution (HAS) to give the N‐containing arenes. These reactions proceeded with a broad range of substrates at room temperature in high yield. This strategy of visible‐light‐induced iminyl‐radical formation was successfully applied to a five‐step concise synthesis of benzo[c]phenanthridine alkaloids.     

Visible‐Light‐Promoted Iminyl‐Radical Formation from Acyl Oximes: A Unified Approach to Pyridines,Quinolines, and Phenanthridines

A unified strategy involving visible‐light‐induced iminyl‐radical formation has been established for the construction of pyridines, quinolines, and phenanthridines from acyl oximes. With fac‐[Ir(ppy)₃] as a photoredox catalyst, the acyl oximes were converted by 1 e⁻ reduction into iminyl radical intermediates, which then underwent intramolecular homolytic aromatic substitution (HAS) to give the N‐containing arenes. These reactions proceeded with a broad range of substrates at room temperature in high yield. This strategy of visible‐light‐induced iminyl‐radical formation was successfully applied to a five‐step concise synthesis of benzo[c]phenanthridine alkaloids.     

Calculated and measured transient product yields in pulse radiolysis of aqueous solutions: Concentration dependence

Pulse radiolysis of aqueous solutions was modeled by using 54 equations for the reaction of water radiolysis intermediates with carefully selected rate coefficients. Yields of products formed in the hydrated electron+solute and hydroxyl radical+solute reactions were calculated and compared with the measured yields in wide concentration range. These reactions are in competition with the reactions of the water radiolysis intermediates with each other and with H₂O, H⁺ and H₂O₂. An empirical equation was developed for the calculation of scavenged product yields that can be used in cases when due to low rate coefficient, low solubility or very high absorbance, low solute concentrations are applied and a considerable fraction of the water radiolysis intermediates does not react with the solute.     

Radiation induced degradation of ketoprofen in dilute aqueous solution

The intermediates and final products of ketoprofen degradation were investigated in 0.4 mmol dm^?3 solution by pulse radiolysis and gamma radiolysis. For observation of final products UV?vis spectrophotometry and HPLC separation with diode array detection were used, and for identification MS was used. The reactions of ^?OH lead to hydroxycyclohexadienyl type radical intermediates, in their further reactions hydroxylated derivatives of ketoprofen form as final products. The hydrated electron is scavenged by the carbonyl oxygen and the electron adduct protonates to ketyl radical ^?OH is more effective in decomposing ketoprofen than hydrated electron. Chemical oxygen demand and total organic carbon content measurements on irradiated aerated solutions showed that using irradiation technology ketoprofen can be mineralised. The initial toxicity of the solution monitored by the Daphnia magna test steadily decreases with irradiation. Using 5 kGy dose no toxicity of the solution was detected with this test.     

The photoyellowing of stilbene-derived fluorescent whitening agents--mass spectrometric characterization of yellow photoproducts.

The application of fluorescent whitening agents (FWAs) significantly accelerates the photoyellowing of wool and silk under exposure to the ultraviolet and visible components of sunlight <500 nm. The photochemistry involved in this process is poorly understood, particularly the role of photoproducts derived directly from the FWA itself. Hydroxylation was identified as the key initial mechanism of photodegradation leading to coloration of the solution in the irradiation of the stilbene-derived FWA 4,4'-bis(2-sulfostyryl)biphenyl (DSBP) in the presence of hydrogen peroxide (H2O2). Polyhydroxylated DSBP derivatives were implicated as critical intermediates in the formation of yellow photoproducts under these conditions. The formation of trace quantities of DSBP quinone derivatives subsequent to hydroxylation was identified as the key cause of DSBP photoyellowing. These results are the first successful characterization of yellow photoproducts resulting directly from irradiation of a stilbene-based FWA. Formation of these yellow stilbene-based FWA-derived photoproducts may occur on the surface of FWA-treated wool exposed to simulated sunlight, as previous work has shown that H2O2 is photogenerated when wet FWA-treated wool is exposed to light. These results therefore suggest that yellow FWA-derived photoproducts contribute to the accelerated photoyellowing of FWA-treated wool.     

The photostability of wool doped with photocatalytic titanium dioxide nanoparticles

Photoyellowing of wool is a serious problem for the wool industry. This study assessed the role of photocatalytic nanocrystalline titanium dioxide (P-25) as a potential antagonist or catalyst in the photoyellowing of wool. Untreated, bleached and bleached and fluorescent-whitened wool slivers were processed into fine wool powders for the purpose of even and intimate mixing with the TiO₂ nanoparticles in the solid state. Pure wool and wool/TiO₂ mixtures were then compressed into solid discs for a photoyellowing study under simulated sunlight and under UVB and UVC radiations. Yellowness and photo-induced chemiluminescence (PICL) measurements showed that nanocrystalline TiO₂ could effectively reduce the rate of photoyellowing by inhibiting free radical generation in doped wool, and that a higher concentration of TiO₂ contributed to a lower rate of photooxidation and reduced photoyellowing. Hence nanocrystalline TiO₂ acts primarily as a UV absorber on wool in dry conditions and not as a photocatalyst.     

Use of Lewis Acids in Free Radical Reactions

The neglect of free radicals over the past few years has been overcome, and they are no longer only considered as interesting reactive intermediates with limited synthetic potential. New opportunities are opened up by performing radical reactions in the presence of Lewis acids. Rate enhancement of radical addition to olefins as well as stereochemical control of such reactions can be achieved in a unique manner. Recent examples of enantioselective radical reactions and perspectives for applications in catalysis are discussed.     

Photo- and radiation chemical induced degradation of lignin model compounds

The basic mechanistic aspects of the photo- and radiation chemistry of lignin model compounds (LMCs) are discussed with respect to important processes related to lignin degradation. Several reactions occur after direct irradiation, photosensitized or radiation chemically induced oxidation of LMCs. Direct irradiation studies on LMCs have provided supportive evidence for the involvement of hydrogen abstraction reactions from phenols, beta-cleavage of substituted alpha-aryloxyacetophenones and cleavage of ketyl radicals (formed by photoreduction of aromatic ketones or hydrogen abstraction from arylglycerol beta-aryl ethers) in the photoyellowing of lignin rich pulps. Photosensitized and radiation chemically induced generation of reactive oxygen species and their reaction with LMCs are reviewed. The side-chain reactivity of LMC radical cations, generated by radiation chemical means, is also discussed in relation with the enzymatic degradation of lignin.     

Philicity of Acetonyl and Benzoyl Radicals: A Comparative Experimental and Computational Study

In this work, the reactivities of acetonyl and benzoyl radicals in aromatic substitution and addition reactions have been compared in an experimental and computational study. The results show that acetonyl is more electrophilic than benzoyl, which is rather nucleophilic. A Hammett plot analysis of the addition reactions of the two radicals to substituted styrenes clearly support the nucleophilicity of benzoyl, but in the case of acetonyl, no satisfactory linear correlation with a single substituent-related parameter was found. Computational calculations helped to rationalize this effect, and a good linear correlation was found with a combination of polar parameters (σ⁺) and the radical stabilization energies of the formed intermediates. Based on the calculated philicity indices for benzoyl and acetonyl, a quantitative comparison of these two radicals with many other reported radicals is possible, which may help to predict the reactivities of other aromatic radical substitution reactions.     

Radical cyclisation reactions with indoles

Several radical cyclisation reactions involving indoles are described. Most notably, we have shown that radical additions to C3 of an indole are frequently facile. A dichotomy in the course of radical cyclisation reactions to C2 of the indole has also been exposed wherein 6-endo-trig cyclisations are propagated by the loss of a hydrogen atom from C2 while 5-exo-trig cyclisations are propagated by hydrogen atom abstraction at C3 from tributyltin hydride. Cyclisations involving the addition of indolyl radical intermediates to arenes have also been demonstrated.     

Free-radical polymerization of methyl methacrylate in the presence of dithiobenzoates as reversible addition-fragmentation chain transfer agents

The pseudoliving radical polymerization of methyl methacrylate in bulk mediated by dithiobenzoates with various leaving groups as reversible addition-fragmentation chain-transfer agents has been studied. It has been shown that polymerization proceeds under conditions of the low steady-state concentration of radical intermediates; as a result, the steady-state of the process is rapidly achieved even at low conversions. Retardation of polymerization observed at high concentrations of reversible addition-fragmentation chain-transfer agents is apparently associated with the occurrence of chain termination reactions involving intermediates, as evidenced by the model reaction. The autoacceleration of polymerization is suppressed with an increase in the concentration of reversible addition-fragmentation chain-transfer agents. An efficient approach to the synthesis of a narrow-dispersed PMMA with the controlled molecular mass has been suggested.     

A theoretical characterization of reactions of HOOO radical with guanine: formation of 8-oxoguanine

Hydrogen trioxide (HOOO) radical and other polyoxides of general formula, RO_nR (where R stands for hydrogen, other atoms or groups and n?≥?3), are believed to be key intermediates in atmospheric chemistry and biological oxidation reactions. In this contribution, DFT calculations using M06-2X density functional and the 6-31G(d,p) and 6-311+G(d,p) basis sets have been carried out to study different reactions of HOOO radical with guanine such as addition of HOOO radical at the C2, C4, C5, and C8 sites of guanine, abstraction of hydrogen atoms (H1, H2a, and H8) of guanine, and the mechanisms of oxidation of guanine with HOOO radical yielding 8-oxoguanine(a highly mutagenic derivative of guanine) and its radical in gas phase and aqueous media. The polarizable continuum model (PCM) has been used for solvation calculations in aqueous media. Our calculations reveal that the C8 site of guanine is the most reactive site for addition of HOOO radical, and adduct formed at this site would be appreciably stable. The rate constant (\( =\frac{K_bT}{h}{e}^{-\frac{\Delta {E}^b}{RT}} \)) at the C8 site is found to be 6.07?×?10⁷ (2.89?×?10⁷) s^?1 at the M06-2X/6-311+G(d,p) level of theory in gas phase (aqueous media). The calculated barrier energy and heat of formation of hydrogen abstraction reactions show that HOOO radical would not abstract hydrogen atoms of guanine. Oxidation of guanine with HOOO radical can occur following two schemes (Scheme 1 and Scheme 2). It is found that formation of 8-oxoguanine radical via Scheme 1 would predominate over formation of 8-oxoguanine via Scheme 2, in a reaction of HOOO radical and guanine. Thus, HOOO radical can be treated as a member of reactive oxygen species (ROS) which play key roles in biological oxidation reactions, in agreement with previous literature reports.     

Irradiation treatment of azo dye containing wastewater: An overview

The radiation-induced decolouration and degradation of aqueous solutions of azo dyes and their model compounds (anilines, phenols, triazines) are reviewed together with practical applications and the experimental methods (pulse radiolysis, steady-state gamma radiolysis, as well as end-product analysis) used for studying the reactions. The proposed mechanisms and the rate coefficients for the reactions of OH, e_aq⁻ and H water radiolysis intermediates with the dye molecules and with model compounds are summarized.     

Solvent‐Enabled Radical Selectivities: Controlled Syntheses of Sulfoxides and Sulfides

Controlling selectivity is of central importance to radical chemistry. However, the highly reactive and unstable radical intermediates make this task especially challenging. Herein, a strategy for taming radical redox reactions has been developed, in which solvent‐bonding can alter the reactivity of the generated radical intermediates and thereby drastically alter the reaction selectivity at room temperature. Various β‐oxy sulfoxides and β‐hydroxy sulfides can be facilely obtained, some of which are difficult to synthesize by existing methods. Notably, neither a metal catalyst nor any further additives are necessary in these processes.     

Electron spin resonance studies of electron addition to 2-chloro- and 2-bromo-2-nitropropane

The products of electron addition to 2-chloro- and 2-bromo-2-nitropropane, which include their radical anions, have been detected by electron spin resonance spectroscopy.Radical anions of various aliphatic α-substituted nitro compounds have been proposed as reactive intermediates in radical-anion chain substitution reactions (S_RN1) (Scheme 1) and other related reactions.