Catalytic Radical Research Articles

2,2,6,6-Tetramethyl-1-piperidinyl-oxyl/[bis(acetoxy)-iodo]benzene-mediated oxidation: A versatile and convenient route to poly(ethylene glycol) aldehyde or carboxylic acid derivatives

A new and very convenient route to oxidized poly(ethylene glycol), with a catalytic nitroxyl radical oxidant (2,2,6,6-tetramethyl-1-piperidinyl-oxyl) regenerated in situ by stoichiometric amounts of [bis(acetoxy)-iodo]benzene, is described. Under these conditions, the reaction is quantitative and leads to the pure product by a simple process such as precipitation and washing with diethyl ether. © 2001 John Wiley & Sons, Inc. J Polym Sci Part A: Polym Chem 39: 4022–4024, 2001

Mechanisms for midlatitude ozone loss: Heterogeneous chemistry in the lowermost stratosphere?

The question of midlatitude ozone erosion by chlorine free radical catalysis is examined. We present and analyze simultaneous, high‐resolution observations of ClO, H2O, tropopause height, particle reactive surface area, and ice saturation occurrence frequency obtained from the NASA ER‐2 aircraft. The objective is to test the hypothesis that the existence of cirrus clouds or cold aerosols in the first few kilometers above the tropopause at midlatitudes is responsible for increasing the ratio of chlorine free radicals to total inorganic chlorine, thus amplifying the rate of catalytic ozone destruction. The observations reveal a sharp decrease in ice saturation frequency at the tropopause, a marked degree of under‐saturation just above the tropopause, a corresponding sharp gradient in the product of cold aerosol reactive surface area and reaction probability, γSa, and, finally, the consistent absence of enhanced concentrations of ClO immediately above the tropopause. These results suggest that midlatitude ozone loss is not controlled in situ by the mechanism of cirrus cloud and/or cold aerosol enhancement of chlorine radicals in the vicinity of the tropopause.

Evolution of Polycopperorganosiloxanes in the Course of a Catalytic Reaction of CCl4Addition to a Multiple Bond

The catalytic activity of linear and network oligomer copperorganosiloxanes immobilized on the surface of Silochrome in a reaction of CCl4addition to olefins was studied. It is likely that the reduction of Cu(II) ions to Cu(I) is a step in the catalytic radical reaction studied. This process also occurs in the absence of reagents at relatively low temperatures. The reaction occurs more readily in linear polycopperorganosiloxanes than in the network oligomers, which is consistent with the catalytic activity. The studied catalytic reaction is characterized by an unusual dependence of the activity of polycopperorganosiloxanes on metal concentration.

The kinetic nature of sulfur’s chemistry in flames

Radi et al. recently presented the first quantitative multiplexed measurements of S 2 and OH concentrations in flames using degenerate four-wave mixing (DFWM). Although the absorption-calibrated OH measurements were in agreement with expectations, very severe discrepancies were reported for the indirectly calibrated S 2 densities. Magnitudes ranged up to 236-fold larger than predicted. The researchers have questioned the adequacy of previous flame work and the current kinetic model of the mechanisms of sulfur’s flame chemistry. The implications of their suggestion are extensive and severe, and their result unexpected. They have suggested that the kinetic modeling is either incomplete, or that an additional species such as NS may be responsible. As a consequence, a reexamination of their work and all previous studies in H 2, C 3H 8, and CH 3OH flames has been made. The resulting conclusions are that it will be very difficult to modify the kinetic model of sulfur in flames to encompass such results. Suggestions that other species such as NS may be playing a significant role are shown to have little merit. In addition, an analysis of potential roles for OCS, CS, CS 2, and one recently suggested for HCS in fossil-fueled flames also indicates these to be very minor. A closer examination of the recent DFWM measurements implies various other disquieting aspects. One is that the reported S 2 densities are essentially close to or above expected flame equilibrium values. Numerous independent measurements all agree that S 2 concentrations are depressed by flame nonequilibrium, and increase with downstream time as flame radical concentrations relax towards their equilibrium values. In the present case, however, measured OH concentrations still are in the range of 42- to 10-fold above their equilibrium values. A second aspect of concern is that the measurements imply an insensitivity to S 2, recording levels at several hundreds of ppm with some difficulty. Other DFWM measurements, for example, with CH, C 2, CN, NO, and OH, all report sensitivities down to a few ppm [3–8]. Even CH 3 can be measured in flames at levels of 65–70 ppm [9, 10]. The fact that under saturation conditions, DFWM intensities fall off as the square of the concentration indicates the severity of these differences. On the other hand, the degree of theoretical understanding now is quite sound for DFWM, and the levels of approximation involved would not appear to conceivably introduce the magnitude of change that is required to bring this body of data together. The DFWM spectroscopy involved, per se, appears to be reasonable and valid. An indirect calibration method, however, is used to scale the S 2 intensities, and has never been validated. One plausible explanation that is proposed herein relates to the very efficient collision free coupling that occurs between the S 2( B 3Σ u −) and S 2( B″ 3Π u ) states. There is evidence in the literature that the latter long-lived state can act as a pseudometastable state. Instantaneous depletion of the pumped S 2( X 3Σ g −, v = 2) into this state would modify the data by lowering the resultant values. If correct, this appears to be the first such reported interference with DFWM monitoring. A review of the current status of our understanding of the behavior of the major sulfur species in flames indicates that any possible need for modification should be only minor. Major remaining uncertainties, which cannot noticeably perturb the sulfur chemistry itself, center on the exact nature of the mechanisms by which sulfur modifies NO x formation and, to a lesser extent, to reexamine the validity of the exact mechanisms involved in the catalytic flame radical recombination cycles. Definitive studies of these have yet to be done.

Catalytic radical polymerization of vinyl monomers by cobalt porphyrin complex and metamorphosis of block-into-block copolymer

This is the first report on the catalytic room temperature free-radical polymerization of vinyl monomers, namely styrene (STY) and methyl methacrylate (MMA) using Co(II)TPP(Py) and aerial oxygen as a catalyst and a co-catalyst, respectively. The mechanism of polymerization has been investigated using 1H NMR, differential scanning calorimeter (DSC), thermogravimetric (TGA) and gel permeation chromatography (GPC) analysis and a radical pathway for the polymer formation is proposed. The rate of polymerization for the synthesis of active polymers of STY is higher than that of MMA, which has also been supported by computational studies. The active polymer, polystyrene (PS)–poly(styrene peroxide) (PSP) was also used for the synthesis of another block copolymer, PS-b-PMMA, by reacting PS–PSP with MMA. The mechanism of block copolymerization is discussed.

Effect of Sodium Hydroxide Addition on Polycyclic Aromatic Hydrocarbon Emissions of Toluene Flames

AbstractAddition of sodium hydroxide solution into organic flames has been extensively investigated to reduce soot formation; nevertheless, data on the effect of metal salt solution on the formation of hazardous air pollutants of polycyclic aromatic hydrocarbons (PAHs) are rather scarce. A laboratory‐scale liquid‐injection incinerator was used to incinerate toluene, which resembled a waste or used solvent. Solid‐phase and gas‐phase PAHs in the flue gases were sampled, treated, and analyzed using a high performance liquid chromatograph (HPLC). The experimental parameters included equivalence ratio and concentration of added sodium hydroxide solution. The incineration temperature was 750°C and the nominal gas residence time was approximately 2.5 s. Results indicated that particles might play a very active role in the formation of PAHs under all experimental conditions. Further, under oxygen‐rich incineration, addition of sodium hydroxide solution caused a considerable increase of the sixteen solid‐phase priority pollutant PAHs (pp‐PAHs); while for oxygen‐lean toluene flame the effect of addition of NaOH solution on solid‐phase PAH was only slight. This greater solid‐phase PAH content on soot of the oxygen‐rich NaOH‐seeded toluene flames suggested that the radical catalysis cycle of NaOH to NaO2 to Na to NaO, then back to NaOH (Zamansky et al., 1999) might have affected the solid‐phase PAH measurements. We speculate that hydrogen atoms were abstracted from hydrocarbons to form H2O and 16 pp‐PAHs by OH radicals that were produced via the sodium catalysis cycle.

ChemInform Abstract: Catalytic Tin Radical Mediated Tricyclizations. Part 2.

AbstractChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 100 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select a “Full Text” option. The original article is trackable via the “References” option.

ChemInform Abstract: Catalytic Tin Radical Mediated Tricyclizations. Part 1. Monocyclization Studies.

ChemInform Abstract: Catalytic Tin Radical Mediated Tricyclizations. Part 1. Monocyclization Studies.

Formation of cyclic fatty acids during the frying process

Cyclic fatty acids formed during frying were at one time thought to be highly toxic. Fears have receded somewhat, but this stimulated considerable research. In our work, the structures of all the cyclic monomers formed from oleic, linoleic, and linolenic acids in heated frying oils were determined. The compounds were first isolated by urea fractionation and reversed-phase high-performance liquid chromatography (HPLC). The mixtures were simplified by silver ion HPLC, and then examined by gas chromatography-mass spectrometry in the form of the picolinyl esters and 4,4-dimethyloxazoline derivatives. In addition, hydrogenation and deuteration aided the characterization. Assignment of double bond configurations was accomplished by gas chromatography linked to Fourier-transform infrared (IR) spectroscopy. Surprisingly, a simpler range of compounds was formed from the trienoic acid than from the diene. Linolenic acid gave four basic cyclic diene structures (two with cyclopentene and two with cyclohexene rings), each of which existed as isomers with different double bond and ring conformations. Some bicyclic fatty acids were formed from linoleate. The mechanism of the reaction is believed to involve intramolecular rearrangements via free radical catalysis with traces of hydroperoxides as the initiators.

How a protein generates a catalytic radical from coenzyme B 12: X-ray structure of a diol-dehydratase–adeninylpentylcobalamin complex

Background: Adenosylcobalamin (coenzyme B 12) serves as a cofactor for enzymatic radical reactions. The adenosyl radical, a catalytic radical in these reactions, is formed by homolysis of the cobalt–carbon bond of the coenzyme, although the mechanism of cleavage of its organometallic bond remains unsolved. Results: We determined the three-dimensional structures of diol dehydratase complexed with adeninylpentylcobalamin and with cyanocobalamin at 1.7 Å and 1.9 Å resolution, respectively, at cryogenic temperatures. In the adeninylpentylcobalamin complex, the adenine ring is bound parallel to the corrin ring as in the free form and methylmalonyl-CoA-mutase-bound coenzyme, but with the other side facing pyrrole ring C. All of its nitrogen atoms except for N(9) are hydrogen-bonded to mainchain amide oxygen and amide nitrogen atoms, a sidechain hydroxyl group, and a water molecule. As compared with the cyanocobalamin complex, the sidechain of Serα224 rotates by 120° to hydrogen bond with N(3) of the adenine ring. Conclusions: The structure of the adenine-ring-binding site provides a molecular basis for the strict specificity of diol dehydratase for the coenzyme adenosyl group. The superimposition of the structure of the free coenzyme on that of enzyme-bound adeninylpentylcobalamin demonstrated that the tight enzyme–coenzyme interactions at both the cobalamin moiety and adenine ring of the adenosyl group would inevitably lead to cleavage of the cobalt–carbon bond. Rotation of the ribose moiety around the glycosidic linkage makes the 5′-carbon radical accessible to the hydrogen atom of the substrate to be abstracted.

ChemInform Abstract: Convenient Catalytic Free Radical Reductions of Alkyl Halides Using an Organotin Reagent on Non‐Cross‐Linked Polystyrene Support.

AbstractChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 100 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select a “Full Text” option. The original article is trackable via the “References” option.

Radical catalysis of B12 enzymes: structure, mechanism, inactivation, and reactivation of diol and glycerol dehydratases.

Enzymatic radical catalysis is defined as a mechanism of catalysis by which enzymes catalyze chemically difficult reactions by utilizing the high reactivity of free radicals. Adenosylcobalamin (coenzyme B12) serves as a cofactor for enzymatic radical reactions. The recent structural analysis of adenosylcobalamin-dependent diol dehydratase revealed that the substrate 1,2-propanediol and an essential potassium ion are located inside a (beta/alpha)8 barrel. Two hydroxyl groups of the substrate coordinate directly to the potassium ion which binds to the negatively charged inner part of the cavity. Cobalamin bound in the base-on mode covers the cavity to isolate the active site from solvent. Based on the three-dimensional structure and theoretical calculations, a new mechanism for diol dehydratase is proposed in which the potassium ion plays a direct role in the catalysis. The mechanisms for generation of a catalytic radical by homolysis of the coenzyme Co-C bond and for protection of radical intermediates from undesired side reactions during catalysis are discussed based on the structure. The reactivating factors for diol and glycerol dehydratases have been identified. These factors are a new type of molecular chaperone which participate in reactivation of the inactivated holoenzymes by mediating ATP-dependent exchange of the modified coenzyme for free intact coenzyme.

Catalytic tin radical mediated tricyclisations. Part 1. Monocyclisation studies †

A general strategy for catalytic tin radical mediated, radical cascade reactions is proposed in which three rings are constructed in a single step. The initial step in the tricyclisation process has been examined using 2,3-dideoxy-α-D-erythro-hex-2-enopyranosides bearing unsaturated substituents at the 1-O and/or 4-O-positions. Substrates for cyclisation of substituents at the 1-O-position were prepared by a novel zinc chloride catalysed Ferrier rearrangement of tri-O-acetyl-D-glucal with unsaturated alcohols, whereas substrates for cyclisation of substituents at the 4-O-position were prepared by alkylation or acylation of ethyl 6-O-protected 2,3-dideoxy-α-D-erythro-hex-2-enopyranosides. Propargyl substituents cyclise efficiently, but propenyl substituents less so. Propioloyl substituents undergo hydrostannylation without cyclisation.

Catalytic tin radical mediated tricyclisations. Part 2 †‡

Propenyl 4-O-propargyl-, propargyl 4-O-propenyl-, and propargyl 4-O-propargyl-2,3-dideoxy-α-D-erythro-hex-2-enopyranosides undergo catalytic, tin radical initiated, cascade reactions, in which three rings are constructed in a single reaction. In each case, a lack of stereoselectivity in the second cyclisation results in an additional product which is produced non-catalytically. The dienes which result from catalytic cyclisation of propargyl 4-O-propargyl-2,3-dideoxy-α-D-erythro-hex-2-enopyranosides, undergo in situ hydrostannylation to give unusual allylstannanes.

Catalytic α-hydroxy carbon radical generation and addition. Synthesis of α-hydroxy-γ-lactones from alcohols, α,β-unsaturated esters and dioxygen

A catalytic method for α-hydroxy carbon radical generation from alcohols has been developed and a convenient and synthetically useful approach to α-hydroxy-γ-lactones constructed.

Catalytic radical addition of ketones to alkenes by a metal–dioxygen redox system

Radical addition of ketones to alkenes catalyzed by Mn(OAc)2 combined with Co(OAc)2 using dioxygen as oxidant was developed; for instance, the reaction of cyclohexanone with oct-1-ene in the presence of very small amounts of Mn(OAc)2 and Co(OAc)2 under air (1 atm) gave 2-octylcyclohexanone in good selectivity; from styrene, a six-membered cyclic peroxide was isolated in good yield.

Copper(I)-Catalyzed Intramolecular Addition of N-Chloroamines to Double Bonds under Aprotic Conditions. Towards a Stereoselective Catalytic Radical Reaction

Copper(I)-Catalyzed Intramolecular Addition of N-Chloroamines to Double Bonds under Aprotic Conditions. Towards a Stereoselective Catalytic Radical Reaction

Flame inhibition by ferrocene and blends of inert and catalytic agents

The production of the fire suppressant CF3Br has been banned, and finding a replacement with all of its desirable properties is proving difficult. Iron pentacarbonyl has been found to be up to several orders of magnitude more effective than CF3Br, but it is flammable and highly toxic. Ferrocene [Fe(C5H5)2], which is much less toxic and flammable than Fe(CO)5, can also be used to introduce iron into a flame. We present the first experimental data and numerical modeling for flame inhibition by ferrocene and find it to behave similarly to Fe(CO)5. A ferrocene mole fraction of 200 ppm reduced the burning velocity of slightly preheated premixed methane/air flames by a factor of two, and the effectiveness dropped off sharply at higher mole fractions. For air with a higher oxygen mole fraction, the burning velocity reduction was less. We also present experimental data and modeling for flames with ferrocene blended with CO2 or CF3H. The combination of the thermally acting agent CO2 with ferrocene mitigated the loss of effectiveness experienced by ferrocene alone at higher mole fractions. An agent consisting of 1.5% ferrocene in 98.5% CO2 performed as effectively as CF3Br in achieving a 50% reduction in burning velocity. Likewise, four times less CO2 was required to achieve the 50% reduction if 0.35% ferrocene was added to the CO2. In contrast, addition of 0.35% ferrocene to the hydrofluorocarbon CF3H reduced the CF3H required to achieve the 50% reduction in burning velocity by only about 25%. Thermodynamic equilibrium calculations predict that the formation of iron/fluoride compounds can reduce the concentrations of the iron-species oxide and hydroxide intermediates which are believed to be responsible for the catalytic radical recombination cycles.

Extinction conditions of non-premixed flames with fine droplets of water and water/NaOH solutions

Interactions of fine droplets of water and water/NaOH solutions with a steady, laminar counterflow methane/air nonpremixed flame are investigated experimentally and numerically. A water atomizer generating a polydisperse distribution of droplet sizes with a median diameter of 20 lm is used in experiments with steady feed rate. Comparisons of the measured flame extinction condition as a function of droplet mass fraction in the air stream indicate a trend similar to that predicted previously using 20 lm monodisperse water droplets. The hybrid Eulerian-Lagrangian numerical model previously developed is generalized to include polydisperse distribution of drop sizes; however, the differences seen between experiments and the numerical predictions at high water mass fractions could not be attributed to variation in size distribution alone. Present experiments support the conclusions of an earlier modeling work that on a mass basis, fine water mist can be as effective as the now-banned gaseous fire suppressant halon 1301. Inclusion of NaOH in water (up to 17.5% by mass) is shown to significantly enhance the fire suppression ability of water by complementing its thermal effects with chemical catalytic radical recombination effects of NaOH.

ChemInform Abstract: Catalytic Radical Acetylation of Adamantanes with Biacetyl by a Cobalt Salt under Atmospheric Dioxygen.

AbstractChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 100 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select a “Full Text” option. The original article is trackable via the “References” option.