Subtle ligand effects in oxidative photocatalysis with iridium complexes: application to photopolymerization

Iridium(III) complexes were designed and evaluated as efficient photoinitiators of polymerization reactions in combination with iodonium salts and silanes. Mechanistically, these reactions were shown to proceed through oxidative photoredox catalysis, generating aryl and silyl radicals under very soft irradiation conditions (blue LED, xenon lamp, and even sunlight). These radicals can initiate the free radical polymerization of acrylates or can be oxidized during the catalytic cycle to promote the ring-opening polymerization of epoxy monomers. Remarkably, both the (photo)chemical reactivity and the practical efficiency are dramatically affected by the ligands. In addition, the central role played by the oxidation ability of the excited state of the photocatalyst is discussed.     

Mobile protons versus mobile radicals: Gas-phase unimolecular chemistry of radical cations of cysteine-containing peptides

A combination of electrospray ionization (ESI), multistage, and high-resolution mass spectrometry experiments are used to examine the gas-phase fragmentation reactions of radical cations of cysteine containing di- and tripeptides. Two different chemical methods were used to form initial populations of radical cations in which the radical sites were located at different positions: (1) sulfur-centered cysteinyl radicals via bond homolysis of protonated S-nitrosocysteine containing peptides; and (2) α-carbon backbone-centered radicals via Siu’s sequence of reactions (J. Am. Chem. Soc. 2008, 130, 7862). Comparison of the fragmentation reactions of these regiospecifically generated radicals suggests that hydrogen atom transfer (HAT) between the α C-H of adjacent residues and the cysteinyl radical can occur. In addition, using accurate mass measurements, deuterium labeling, and comparison with an authentic sample, a novel loss of part of the N-terminal cysteine residue was shown to give rise to the protonated, truncated N-formyl peptide (an even-electron x_n ion). DFT calculations were performed on the radical cation [GCG]^.+ to examine: the relative stabilities of isomers with different radical and protonation sites; the barriers associated with radical migration between four possible radical sites, [G.CG]⁺, [GC.G]⁺, [GCG.]⁺, and [GC(S.)G]⁺; and for dissociation from these sites to yield b₂-type ions.     

Acid-, base-, and lewis-acid-catalyzed heterolysis of methoxide from an alpha-hydroxy-beta-methoxy radical: models for reactions catalyzed by coenzyme B12-dependent diol dehydratase

[Reaction: see text].A model for glycol radicals was employed in laser flash photolysis kinetic studies of catalysis of the fragmentation of a methoxy group adjacent to an alpha-hydroxy radical center. Photolysis of a phenylselenylmethylcyclopropane precursor gave a cyclopropylcarbinyl radical that rapidly ring opened to the target alpha-hydroxy-beta-methoxy radical (3). Heterolysis of the methoxy group in 3 gave an enolyl radical (4a) or an enol ether radical cation (4b), depending upon pH. Radicals 4 contain a 2,2-diphenylcyclopropane reporter group, and they rapidly opened to give UV-observable diphenylalkyl radicals as the final products. No heterolysis was observed for radical 3 under neutral conditions. In basic aqueous acetonitrile solutions, specific base catalysis of the heterolysis was observed; the pK(a) of radical 3 was determined to be 12.5 from kinetic titration plots, and the ketyl radical formed by deprotonation of 3 eliminated methoxide with a rate constant of 5 x 10(7) s(-1). In the presence of carboxylic acids in acetonitrile solutions, radical 3 eliminated methanol in a general acid-catalyzed reaction, and rate constants for protonation of the methoxy group in 3 by several acids were measured. Radical 3 also reacted by fragmentation of methoxide in Lewis-acid-catalyzed heterolysis reactions; ZnBr2, Sc(OTf)3, and BF3 were found to be efficient catalysts. Catalytic rate constants for the heterolysis reactions were in the range of 3 x 10(4) to 2 x 10(6) s(-1). The Lewis-acid-catalyzed heterolysis reactions are fast enough for kinetic competence in coenzyme B12 dependent enzyme-catalyzed reactions of glycols, and Lewis-acid-catalyzed cleavages of beta-ethers in radicals might be applied in synthetic reactions.     

Transient free radicals in viscous solvents

The majority of free radicals are highly reactive species which participate in bimolecular reactions with each other. Validation of the theory of molecular diffusion and reactivity in the liquid state requires knowledge of rate constants of radical–radical reactions (recombination, disproportionation) and their viscosity dependencies. An accurate comparison of theory and experiment has become available due to experimentally measured diffusion coefficients of reactive radicals by transient grating technique. Initial distribution of radicals in solution can be not random but pair-wise as in photo- or thermoinitiation of free radical polymerization reactions. Probability of a radical escape of a partner (cage escape) characterizes the initiator efficiency. Despite decades of measurement of cage effect values, cage effect dynamics with free radicals have only been investigated quite recently. The present tutorial review considers the effect of viscosity of Newtonian liquid on two types of recombination—in the solvent bulk and in a cage. Further, since radicals are paramagnetic species, external magnetic field affects probability of their reactions in pairs. These effects are also observed in viscous liquids, and reasons for such observations are explained. The recently discovered low magnetic field effect is also observed on radical pairs in viscous liquids.     

Products resulting by free radical attack on aqueous genistein

Genistein (GEN) is a phytoestrogen of the isoflavonoid group, which, in addition to its antitumor properties of breast cancer, also shows various distinguished biological effects. Products resulting from GEN by free radicals reactions in aqueous media were analyzed. Their yields are studied as a function of radical concentration (absorbed radiation dose) in neutral solution. The knowledge of GEN-degradation products is of special interest, since they can initiate biological side effects.     

ESR study of the mechanism of radical formation during liquid-and solid-phase radiolyses of ethylamines

The formation of radicals during the liquid-phase radiolysis of ethylamine, diethylamine, and triethylamine was studied by means of the spin trapping technique. The radicals produced in ion-molecule reactions and in the rearrangement and fragmentation reactions of the primary radical cations of the amines were identified. The structure and reactions of the primary radical cations were studied in a low-temperature CFCl₃ freonic matrix in which amine radical cations were generated via charge transfer from matrix radical cations to amines during freon irradiation. The results of experiments in the liquid and solid phases are consistent with one another. The structure of neutral radicals and radical cations of the ethylamines was corroborated by quantum-chemical calculations.     

The Role of Sulphate and Phosphate Ions in the Recovery of Benzoic Acid Self-Enhanced Ozonation in Water Containing Bromides

The ozonation of aromatic compounds in low-pH water is ineffective. In an acidic environment, the decomposition of ozone into hydroxyl radicals is limited and insufficient for the degradation of organic pollutants. Radical processes are also strongly inhibited by halogen ions present in the reaction medium, especially at low pH. It was shown that even under such unfavorable conditions, some compounds can initiate radical chain reactions leading to the formation of hydroxyl radicals, thus accelerating the ozonation process, which is referred to as so-called “self-enhanced ozonation”. This paper presents the effect of bromides on “self-enhanced ozonation” of benzoic acid (BA) at pH 2.5. It is the first report to fully and quantitatively describe this process. The presence of only 15 µM bromides in water inhibits ozone decomposition and completely blocks BA degradation. However, the effectiveness of this process can be regained by ozonation in the presence of phosphates or sulphate. The addition of these inorganic salts to the bromide-containing solution helps to recover ozone decomposition and BA degradation efficiency. As part of this research, the fractions of hydroxyl, sulphate and phosphate radicals reacting with benzoic acid and bromides were calculated.     

Mechanism and selectivity of 2,3-dimethyl-2,3-diphenylbutane mediated addition of vinyltriethoxysilane to polyethylene

The thermolysis of 2,3-dimethyl-2,3-diphenylbutane (bicumene) at temperatures ranging from 220 to 310 °C is used to initiate the radical-mediated graft addition of vinyltriethoxysilane (VTEOS) to polyethylene. Model hydrocarbon studies indicate that the cumyl radicals generated by the slow decomposition of bicumene are capable of direct hydrogen atom abstraction at levels that are sufficient to sustain a graft propagation sequence of high kinetic chain length. The interaction of O₂ with cumyl radicals can lead to oxidation of the initiator and the hydrocarbon substrate, thereby enhancing the macroradical population and improving grafting rates and yields. In addition to providing remarkable kinetic chain lengths for VTEOS additions, high-temperature bicumene-based processes can induce HDPE and LDPE fragmentation such that the effects of radical combination on melt viscosity are counteracted. As a result, alkoxysilane-modified polymers that moisture-cure efficiently can be produced without incurring the undesirable increases in molecular weight that accompany conventional grafting processes.     

Computational study of the chemistry of 3-phenylpropyl radicals

Density functional theory (DFT) and G3-type (G3(MP2)-RAD) composite calculations were performed on a series of substituted 3-phenylpropyl radicals, to determine the relative importance of fragmentation and cyclization reactions in the chemistry of such species. Our studies indicate that cyclization is generally the more important of these reactions, with exceptions where fragmentation yields highly stabilized benzylic species. The energetic barriers for the cyclization reactions (enthalpies of activation) were found to be determined largely by the stability of the reactant radical and to a lesser but significant extent, by steric factors. Polarity effects in the transition state (modeled by SOMO-LUMO gaps of the products) appear to be less important. The data obtained indicated that the addition of benzyl radical to alkenes may be considered to be irreversible, but calculations for α-substituted styrenic systems indicate that reversibility of addition may become a factor in dilute polymerizing solutions for select systems.     

Chemical reactions in clouds

Summary In contrast to a fairly extensive knowledge of gas phase chemical reactions in the atmosphere, our current understanding of the chemistry in the aqueous phase of clouds is still inadequate. Particularly for continental clouds the difficulties arise primarily from uncertainties in reaction mechanisms for the oxidation of various sulfur(IV) species originating from the dissolution of sulfur dioxide in cloud water and the role of the ions of transition metals in this oxidation process. The importance of OH and other radicals in gas-phase reactions has led to models, in which radical reactions are held responsible for much of the chemical change also in the liquid phase. This viewpoint has gained support specifically from the identification of iron(III)-hydroxo complexes as a photochemical source of OH radicals in continental clouds. Their reactions with sulfur(IV) compounds in the aqueous phase initiate chain processes, which are currently being examined by laboratory studies.     

Sulfonyl Nitrene and Amidyl Radical: Structure and Reactivity

Photocatalytic generation of nitrenes and radicals can be used to tune or even control their reactivity. Photocatalytic activation of sulfonyl azides leads to the elimination of N₂ and the resulting reactive species initiate C−H activations and amide formation reactions. Here, we present reactive radicals that are generated from sulfonyl azides: sulfonyl nitrene radical anion, sulfonyl nitrene and sulfonyl amidyl radical, and test their gas phase reactivity in C−H activation reactions. The sulfonyl nitrene radical anion is the least reactive and its reactivity is governed by the proton coupled electron transfer mechanism. In contrast, sulfonyl nitrene and sulfonyl amidyl radicals react via hydrogen atom transfer pathways. These reactivities and detailed characterization of the radicals with vibrational spectroscopy and with DFT calculations provide information necessary for taking control over the reactivity of these intermediates.     

p-Nitrobenzenesulfenate esters as precursors for laser flash photolysis studies of alkyl radicals

A series of p-nitrobenzenesulfenate esters was used in laser flash photolysis (LFP) studies to generate alkoxyl radicals that fragmented to give the (2,2-diphenylcyclopropyl)methyl radical. Rate constants for the beta-scission reactions increased as a function of the carbonyl compound produced in the fragmentation reaction in the order CH2O < MeCHO < Me2CO < PhCHO < Ph2CO and increased with increasing solvent polarity. For alkoxyl radicals that fragment to produce benzaldehyde and benzophenone, the beta-scission reactions are faster than 1,5-hydrogen atom abstractions when the incipient carbon radical is as stable as a secondary alkyl radical, and this entry to carbon radicals can be used in LFP kinetic studies.     

Inorganic radicals in organic synthesis

Inorganic radicals have so far led a shadowy existence in synthetic organic radical chemistry. This article briefly reviews the synthetic applications of the most important inorganic radicals. In addition, a new synthetic concept is presented, which should demonstrate that with inorganic, oxygen-centered radicals of the type X-O*, in which X is NO2, SO3-, and H, respectively, novel oxidative radical reactions could be performed, which in turn are difficult or impossible with their organic counterparts, the alkoxyl radicals R-O*.     

The role of hydroperoxides in photo-oxidative degradation of cis-1,4-polybutadiene

Hydroperoxides undergo various types of homolytic reactions on exposure to u.v. radiation. Free radicals formed from the photodecomposition of the hydroperoxide group (OOH) are oxy (HO.) and peroxy (HOO.) radicals which participate in further reactions. In cis-1,4-polybutadiene, they may initiate free radical oxidations. Cleavage of alkoxy (RO.) radicals and crosslinking of polymer radicals through polymer peroxides in the presence of air in solid film nearly balance. Most polymer radicals produced in the absence of oxygen undergo cross-linking but form peroxy radicals (POO.) in its presence. This paper presents results on the photodecomposition of tert-butyl hydroperoxide, cumyl hydroperoxide and 2,5-dimethyl-2,5-dihydroperoxyhexane in cis-1,4-polybutadiene in film and in solution.     

Fragmentation reactions of optically active trisubstituted cyclopropylcarbinyl radicals.

Fragmentation reactions of optically active trisubstituted cyclopropylcarbinyl radicals and their application to the synthesis of natural products are described. Preparation of the optically pure substrates for radical fragmentation reactions was efficiently accomplished by lipase-mediated desymmetrization of sigma-symmetrical 3-substituted-1,2-cyclopropanedimethanols. In the presence of a radical stabilizing group, e.g., aryl, ester, or alpha,beta-unsaturated ester, the fragmentation occurs selectively to generate the radical on the alpha-carbon of the group and provide the optically pure alkene derivatives. These derivatives possess three chemically distinct functionalities, making them excellent chiral building blocks for the construction of biologically active molecules. The synthetic usefulness of the procedure developed here has been demonstrated by an application to the enantioselective synthesis of both enantiomers of the key intermediate, 4-(3,4-methylenedioxybenzyl)dihydrofuran-2(3H)-one (54), for the total synthesis of biologically active lignans.     

Computational studies on the reactions of 3‐butenyl and 3‐butenylperoxy radicals

The reactions of 3‐butenyl (^?CH₂CH₂CH?CH₂) radicals—unimolecular decomposition, isomerization, as well as reaction with O₂—and the subsequent unimolecular rearrangement reactions of the 3‐butenylperoxy radicals have been investigated and are compared to the analogous reactions of butyl (^?CH₂CH₂CH₂CH₃) and butylperoxy radicals using transition‐state theory based on the quantum chemical calculations at the CBS‐QB3 level. For alkyl‐analogue processes, the reactions of 3‐butenyl and 3‐butenylperoxy radicals can be well characterized by the decreased and increased bond dissociation energies at the allylic and vinylic sites, respectively. The intramolecular addition reactions of the radical center atoms to the double bonds were found to be important non‐alkyl‐analogue reactions of 3‐butenyl and 3‐butenylperoxy radicals. As a consequence, the thermal decomposition of 3‐butenyl radicals was found to be slower than that of butyl radicals by one order of magnitude at temperature near 1000 K. Intramolecular addition reactions are suggested to be the predominant unimolecular rearrangement processes of 3‐butenylperoxy radicals over the entire temperature range investigated (500–1200 K). The intramolecular addition reactions of the alkenyl peroxy radicals, which have not been included in combustion kinetic models, and their implications for the autoignition of alkenes are discussed. © 2010 Wiley Periodicals, Inc. Int J Chem Kinet 42: 273–288, 2010     

Regioselectivity of enzymatic and photochemical single electron transfer promoted carbon-carbon bond fragmentation reactions of tetrameric lignin model compounds

New types of tetrameric lignin model compounds, which contain the common β-O-4 and β-1 structural subunits found in natural lignins, have been prepared and carbon-carbon bond fragmentation reactions of their cation radicals, formed by photochemical (9,10-dicyanoanthracene) and enzymatic (lignin peroxidase) SET-promoted methods, have been explored. The results show that cation radical intermediates generated from the tetrameric model compounds undergo highly regioselective C-C bond cleavage in their β-1 subunits. The outcomes of these processes suggest that, independent of positive charge and odd-electron distributions, cation radicals of lignins formed by SET to excited states of sensitizers or heme-iron centers in enzymes degrade selectively through bond cleavage reactions in β-1 vs β-O-4 moieties. In addition, the findings made in the enzymatic studies demonstrate that the sterically large tetrameric lignin model compounds undergo lignin peroxidase-catalyzed cleavage via a mechanism involving preliminary formation of an enzyme-substrate complex.     

Allyl sulfonium salt as a novel initiator for active cationic polymerization of epoxide by shooting with radicals species

A rapid cationic polymerization of cyclohexene oxide that completed within a few minutes was achieved by a new initiation system that involves (1) a copper‐catalyzed reduction of benzoyl peroxide by an ascorbic acid derivative that generates free radicals and (2) capture of the radicals by allyl sulfonium salt having hexafluoroantimonate (SbF) as a counter anion, followed by fragmentation of sulfonium radical cation, from which a super acid HSbF₆ was produced to initiate the rapid polymerization. The key factor in designing an efficient allyl sulfonium salt was attachment of an electron withdrawing ester group at the allyl group, of which ability to stabilize the formed radical can enhance the efficiency in trapping radicals by the allylic salt. © 2010 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 48: 4178–4183, 2010     

Hypochlorite-mediated fragmentation of hyaluronan, chondroitin sulfates, and related N-acetyl glycosamines: evidence for chloramide intermediates, free radical transfer reactions, and site-specific fragmentation

Myeloperoxidase released from activated phagocytes reacts with H(2)O(2) in the presence of chloride ions to give hypochlorous acid. This oxidant has been implicated in the fragmentation of glycosaminoglycans, such as hyaluronan and chondroitin sulfates. In this study it is shown that reaction of HOCl with glycosaminoglycans and model compounds yields chloramides derived from the N-acetyl function of the glycosamine rings. The results of EPR spin trapping and product studies are consistent with the formation of amidyl radicals from these chloramides via both metal ion-dependent and -independent processes. In the case of glycosaminoglycan-derived amidyl radicals, evidence has been obtained in studies with model glycosides that these radicals undergo rapid intramolecular abstraction reactions to give carbon-centered radicals at C-2 on the N-acetyl glycosamine rings (via a 1,2-hydrogen atom shift) and at C-4 on the neighboring uronic acid residues (via 1,5-hydrogen atom shifts). The C-4 carbon-centered radicals, and analogous species derived from model glycosides, undergo pH-independent beta-scission reactions that result in glycosidic bond cleavage. With N-acetyl glucosamine C-1 alkyl glycosides, product formation via this mechanism is near quantitative with respect to chloramide loss. Analogous reactions with the glycosaminoglycans result in selective fragmentation at disaccharide intervals, as evidenced by the formation of "ladders" on gels; this selectivity is less marked under atmospheric oxygen concentrations than under anoxic conditions, due to competing peroxyl radical reactions. As the extracellular matrix plays a key role in mediating cell adhesion, growth, activation, and signaling, such HOCl-mediated glycosaminoglycan fragmentation may play a key role in disease progression and resolution, with the resulting fragments modulating the magnitude and quality of the immune response in inflammatory conditions.     

Computational study on the 1,2-rearrangement in beta-(nitroxy)vinyl and beta-(acetoxy)vinyl radicals

The 1,2-nitroxyl and 1,2-acetoxyl rearrangement in beta-(nitroxy)vinyl and beta-(acetoxy)vinyl radicals 13a and 13b, respectively, has been studied for the gas phase with various ab initio and density functional methods. The energetically most favorable pathway for 13a is calculated to proceed via reversible fragmentation/radical addition through transition state I-19a. In the case of 13b, rearrangement through a five-membered ring transition state III-16b and the fragmentation/radical addition pathway via transition state I-19b are competing processes. Mulliken and natural population analysis reveal a certain degree of charge separation in III-16a/b that may indicate a potential solvent effect on the rearrangement rate. A stepwise group migration through a cyclic radical intermediate V-18a/b or rearrangement through a three-membered ring transition state II-15a/b can be ruled out for both vinyl radicals. A comparison of the results of the calculations with experimental findings provides important insights into the kinetics of "self-terminating radical oxygenations". A significant method dependence on the outcome of the calculations was observed, which revealed the unsuitability of the UHF, MP2, B3LYP, and mPW1PW91 methods for computing these radical rearrangement processes. The results from BHandHLYP/cc-pVDZ calculations showed the best agreement with single-point energy calculations performed at the QCISD and CCSD(T) levels of theory.