Radical Addition Reactions Research Articles

Difunctionalization of Dienes, Enynes and Related Compounds via Sequential Radical Addition and Cyclization Reactions.

Radical reactions are powerful in creating carbon-carbon and carbon-heteroatom bonds. Designing one-pot radical reactions with cascade transformations to assemble the cyclic skeletons with two new functional groups is both synthetically and operationally efficient. Summarized in this paper is the recent development of reactions involving radical addition and cyclization of dienes, diynes, enynes, as well as arene-bridged and arene-terminated compounds for the preparation of difunctionalization cyclic compounds. Reactions carried out with radical initiators, transition metal-catalysis, photoredox, and electrochemical conditions are included.

Degradation of Bisphonel AF (BPAF) by zero-valent iron activated persulfate: Kinetics, mechanisms, theoretical calculations, and effect of co-existing chloride

The removal of Bisphonel AF (BPAF) by zero-valent iron activated persulfate (Fe0/PS) system was systematically evaluated in this work. 30.0 μM BPAF was removed by 94.4% in 60 min of treatment under optimal conditions of pH = 3.0 and [PS] = [Fe0] = 3.0 mM. Cl− significantly accelerated the removal of BPAF, resulting from accelerated Fe2+ release and reactive chlorine species (RCS) formation. Liquid chromatography-time-of-flight-mass spectrometry identified thirteen degradation products, and bond breaking, coupling reactions, hydroxylation and sulfate addition were considered as the major transformation pathways. When Cl− was present, six new chlorinated byproducts were also generated. Based on density functional theory (DFT) calculations, the occurrence of radical addition reactions was verified and the preferential reaction channels were determined. Significantly BPAF degradation products were less toxic, according to toxicity assessment by the ECOSAR program. Moreover, a high removal efficiency of BPAF (>90%) was also obtained in the three actual water matrixes. The present work demonstrates the feasibility of Fe0/PS system for treating BPAF, which could also provide new insights into the influence of coexisting Cl− on the environmental fate of organic pollutants in sulfate radicals based advanced oxidation processes.

Exceptional Light Sensitivity by Thiol–Ene Click Lithography

Lithographic patterning, which utilizes the solubility switch of photoresists to convert optical signals into nanostructures on the substrate, is the primary top-down approach for nanoscale fabrication. However, the low light/electron-energy conversion efficiency severely limits the throughput of lithography. Thiol-ene reaction, as a photoinitiated radical addition reaction, is widely known as click reaction in the field of chemistry due to its extremely high efficiency. Here, we introduce a click lithography strategy utilizing the rapid thiol-ene click reaction to realize ultraefficient nanofabrication. This novel approach facilitated by the implementation of ultrahigh-functionality material designs enables high-contrast patterning of metal-containing nanoclusters under an extremely low deep-ultraviolet exposure dose, e.g., 7.5 mJ cm-2, which is 10-20 times lower than the dose used in the photoacid generator-based photoresist system. Meanwhile, 45 nm dense patterns were also achieved at a low dose using electron beam lithography, revealing the great potential of this approach in high-resolution patterning. Our results demonstrated the high-sensitivity and high-resolution features of click lithography, providing inspiration for future lithography design.

Design and characterization of molecular, crystal and interfacial structures of PVDF-based dielectric nanocomposites for electric energy storage.

PVDF-based polymers with polar covalent bonds are next-generation dielectric materials for electric energy storage applications. Several types of PVDF-based polymers, such as homopolymers, copolymers, terpolymers and tetrapolymers, were synthesized by radical addition reactions, controlled radical polymerizations, chemical modifications or reduction with the monomers of vinylidene fluoride (VDF), tetrafluoroethylene (TFE), trifluoroethylene (TrFE), hexafluoropropylene (HFP) and chlorotrifluoroethylene (CTFE). Owing to rich molecular structures and complicated crystal structures, PVDF-based dielectric polymers can show versatile dielectric polarization properties, including normal ferroelectrics, relaxor ferroelectrics, anti-ferroelectrics and linear dielectrics, which are beneficial for designing polymer films with high capacity and high charge-discharge efficiency for capacitor applications. Furthermore, to satisfy the requirements of practical high-capacity capacitors, the polymer nanocomposite method is another promising strategy to achieve high-capacitance dielectric materials by the addition of high-dielectric ceramic nanoparticles, moderate-dielectric nanoparticles (MgO, and Al2O3), high-insulation nanosheets (BN), etc. It is concluded with the current problems and future perspectives of interfacial engineering, such as core-shell strategies and hierarchical interfaces in polymer-based composite dielectrics for high-energy-density capacitor applications. In addition, an in-depth understanding of the roles of interfaces on the dielectric properties of nanocomposites can be achieved by indirect analysis techniques (theoretical simulation) and direct analysis techniques (scanning probe microscopy). Our systematic discussions on molecular, crystal and interfacial structures provide guidance for designing fluoropolymer-based nanocomposites for high-performance capacitor applications.

Strategic design of gold nanocatalysts for effective photocatalytic organic transformation.

We demonstrate the design strategy of free-standing Au nanocatalysts by correlating their physicochemical characteristics with photocatalytic performance. By tailoring the particle size and surface characteristics, we found that small Au nanocatalysts called Au nanoclusters with discrete energy levels are more effective than large metallic Au nanoparticles, while the microenvironments (e.g., charge status and hydrophilicity/hydrophobicity) around the surface of Au-nanoclusters are crucial in determining the performance. With the optimized Au nanocatalyst, under visible light, decarboxylative radical addition reactions for C-C bond formation (i.e., Giese reaction) were first achieved with high yields and further utilized for the preparation of one of the bioactive γ-aminobutyric acid derivatives, pregabalin (Lyrica®), demonstrating its potential in pharmaceutical applications.

Research Progress in Radical Addition Reaction of Alkenes Involving Chloroform

Research Progress in Radical Addition Reaction of Alkenes Involving Chloroform

Heteroleptic copper(I) complexes [Cu(dmp)(N^P)]BF4 for photoinduced atom-transfer radical addition reactions.

Earth-abundant copper(I) coordination complexes of an imine-phosphine and a diimine have been developed as visible-light photocatalysts. Reaction of [Cu(MeCN)4]BF4 with hetero-bidentate phosphinopyrazole (phpz) ligand R1R2C3HN2PPh3 (R1 = R2 = H (1a); R1 = H, R2 = Me (1b); R1 = H, R2 = Ph (1c); R1 = R2 = Me (1d)) and 2,9-dimethyl-1,10-phenanthroline (dmp) gave four heteroleptic bis-chelate Cu(I) complexes [Cu(dmp)(R1R2C3HN2PPh3)]BF4 (R1 = R2 = H (2a); R1 = H, R2 = Me (2b); R1 = H, R2 = Ph (2c); R1 = R2 = Me (2d)) with distorted tetrahedral geometries. Complexes 2a-2d exhibited broad absorption in the visible spectrum and could facilitate photochemical intermolecular atom-transfer radical addition reactions of CBr4, or CCl3Br, CHI3 to styrenes in yields up to 91% and with a broad substrate scope. The absorption, emission, redox potential and photocatalytic activity were dependent on the substituents on the phpz ligand. Mechanistic studies supported an atom-transfer radical addition (ATRA) mechanism.

Dielectric Characteristics of Crosslinked Polyethylene Modified by Grafting Polar-Group Molecules.

Polar group-modified crosslinked polyethylene (XLPE) materials are developed with a peroxide thermochemical method of individually grafting chloroacetic acid allyl ester (CAAE) and maleic anhydride (MAH) to polyethylene molecular-chains, which are dedicated to ameliorating dielectric characteristics through charge-trapping mechanism. By free radical addition reactions, the CAAE and MAH molecules are successfully grafted to polyethylene molecular chains of XLPE in crosslinking process, as verified by infrared spectroscopy molecular characterizations. Dielectric spectra, electric conductance, and dielectric breakdown strength are tested to evaluate the improved dielectric performances. Charge trap characteristics are investigated by analyzing thermal stimulation depolarization currents in combination with first-principles electronic-structure calculations to reveal the polar-group introduced mechanisms of contributing dipole dielectric polarization, impeding electric conduction, and promoting electrical breakdown field. The grafted polar-group molecules, especially for MAH, can introduce deep-level charge traps in XLPE materials to effectively restrict charge injections and hinder charge carrier transports, which accounts for the significant improvements in electric resistance and dielectric breakdown strength.

Syn-Selective Chlorosulfonylation of Alkynes via a Copper-Powder-Initiated Atom Transfer Radical Addition Reaction and Mechanistic Studies.

Copper-powder-catalyzed syn-selective chlorosulfonylation of readily available alkynes by an atom transfer radical addition (ATAR) process has been developed, providing straightforward access to a broad range of (Z)-β-chlorovinylsulfones in good yields under mild conditions. In addition, this method is ligand-free and features excellent stereoselectivity and high atom economy. Moreover, the product was obtained without an apparent loss of yield when the reaction was performed on the gram scale at a low catalyst loading. In this reaction, the copper powder not only acts as a sulfone radical initiator but also produces the catalytically active CuCl species. Mechanistic investigations and DFT calculation studies revealed that the stereoselectivity is controlled by the thermodynamic stabilities of the in situ-generated cyclic alkenyl CuII complex intermediate, which can serve as a chlorine atom transfer agent.

Exploring low-temperature oxidation chemistry of 2- and 3-pentanone

The low-temperature oxidation chemistry of 2- and 3-pentanone was investigated over a wide range of conditions using a jet-stirred reactor (JSR) and a rapid compression machine (RCM). The JSR oxidation experiment was performed at the pressure of 93.3 kPa over a temperature range of 600-1000 K. Detailed speciation information was obtained using synchrotron vacuum ultraviolet photoionization mass spectrometry. Ignition delay times (IDTs) of 2- and 3-pentanone were measured in an RCM from 640 to 820 K at pressures of 15 and 25 bar and an equivalence ratio of 1.0. The two C5 ketones showed NTC behavior and two-stage ignition phenomena. Mole fraction time histories of intermediate species during the two-stage ignition process of both ketones were obtained using a fast-sampling system coupled with gas chromatography. There are distinct differences between 2- and 3-pentanone in species concentration profiles and IDTs. A kinetic mechanism for the low-temperature oxidation of 2- and 3-pentanone was developed, which can satisfactorily predict all available measurements. The reaction path analyses indicate that the intramolecular hydrogen migration reaction of ROO radicals tends to produce resonance-stabilized QOOH radical structures. The secondary oxygen addition reaction of resonance-stabilized QOOH radicals thus is the most important source of OH radicals in the low-temperature oxidation of ketone fuels. The intramolecular hydrogen migration reactions are slowed down in the presence of the carbonyl functional group, which makes the low-temperature reactivity of the two C5 ketones lower than that of n-pentane. The position of the carbonyl functional group affects the species pools during the oxidation of the two ketones to a great extent. Larger production of CH4, C3H6, CH3COCH3, and C2H5CHO were observed in 2-pentanone oxidation, while the production of CH3CHO was favored during 3-pentanone oxidation both in the JSR and RCM experiments. The different lengths of the carbon chain on both sides of the carbonyl group in 2- and 3-pentanone resulted in the difference in the species distribution.

Theoretical Study of Atom-Transfer Radical Addition Reactions between Perfluoroalkyl Iodides and Styrene Using a Copper Photoredox Catalyst.

The reaction mechanism of atom-transfer radical addition (ATRA) reactions of perfluoroalkyl iodides with styrene using a Cu(I) photoredox catalyst was analyzed using density functional theory calculations. From among four previously mentioned mechanisms, the ligand-transfer mechanism (ligand abstraction by the radical intermediate) was shown to be most plausible. It was also suggested that the ATRA product would also be reduced by the photoexcited Cu(I) complex.

Soot growth mechanism in C2H2 combustion with H2 addition: A reactive molecular dynamics study

A numerical investigation of the effect of H2 addition on soot formation was conducted in C2H2 combustion process from the molecular level. The results showed that the chemical effect of H2 (CE-H2) inhibited the soot formation of C2H2 combustion. With the gradual increase of H2 doping ratio, the soot morphology became sparser and the soot particle size decreased. The CE-H2 significantly inhibited the surface growth and coalescence of soot. Addition of H2 led to an increase in the H2 concentration in the combustion process, which inhibited the dehydrogenation reaction of the primary soot particle with C2H2, free radicals, and polycyclic aromatic hydrocarbons (PAHs). The CE-H2 increased the consumption rate of C2H2 and the formation rate of H2O, and it decreased the formation rate of CO. Moreover, the addition reaction of free radicals and PAHs played an important role in the soot growth process, which effectively promoted the soot surface growth process. And the coalescence process significantly increased the soot mass and affected the soot size and morphology although its frequency was relatively low.

Coke formation of heavy oil during thermal cracking: New insights into the effect of olefinic-bond-containing aromatics

The distributions of olefinic-bond-containing aromatic hydrocarbon (OAH) and the effect of OAH on coke formation during heavy oil thermal cracking have been rarely studied in the past. Here, the information of olefins in thermally cracked products derived from vacuum residue (VR) was investigated first, and then the effect of OAH on coke formation was elaborated by revealing the evolution characteristics of self/co-addition systems participated by representative OAH model compounds. The results found that OAHs were generated during VR thermal cracking, and they could participate in further reactions to promote coke formation. During thermal reaction, OAHs were more reactive for addition reactions than alkenes, and the reactivity of OAH increased roughly with the number of aromatic rings. Meanwhile, the higher concentration of OAH or the longer thermal reaction time or the higher thermal reaction temperature, the higher conversion of addition products, and the greater condensation degree of reaction products. OAHs participating in the larger structure molecule formation involves free radical addition reactions, which could occur between different OAHs as well as between OAHs and conventional aromatic molecules. More importantly, OAHs could form larger structure molecules by direct addition reaction of unsaturated double bond via non-free radical addition reaction, thus promoting fast coke formation.

The formation mechanism of highly oxygenated organic molecules produced by toluene in the urban atmosphere

Highly oxygenated organic molecules (HOMs) with low volatility from atmospheric toluene oxidation play an important role in new particle formation and contribute significantly to secondary organic aerosols in cities. In this work, quantum chemical calculations were performed to study the formation mechanism of HOM monomers and dimers from toluene oxidation initiated by OH radicals. Under 0.4 ppbv NO and 40 pptv HO2 radical conditions, the formed initial mono- and multi–OH–substituted organic peroxyl radicals (RO2) first undergo intramolecular H-shift and cyclization reactions to promote autoxidation, which is followed by bimolecular reactions of highly oxidized RO2 with NO and HO2 radical, resulting in the formation of HOM monomers. It implies that high NO concentration hardly inhibits the formation of RO2 with a low O:C ratio in the aforementioned processes, but mainly intercepts the formation of RO2 with a high O:C ratio. Except for the self- and cross-reaction of RO2, the addition reactions of RO2 and acyl peroxy radicals (RC(O)O2) with toluene-derived alkenes (RO2 + alkene and RC(O)O2 + alkene reactions) have sufficiently low potential barriers, which are non-ignorable pathways for the HOM dimers formation. The present work expands our knowledge of the mechanism of HOMs formation from the oxidation of atmospheric aromatic hydrocarbons.

Tagging Peptides with a Redox Responsive Fluorescent Probe Enabled by Photoredox Difunctionalization of Phenylacetylenes with Sulfinates and Disulfides.

Herein, we describe a photoredox three-component atom-transfer radical addition (ATRA) reaction of aryl alkynes directly with dialkyl disulfides and alkylsulfinates, circumventing the utilization of chemically unstable and synthetically challenging S-alkyl alkylthiosulfonates as viable addition partners. A vast array of (E)-β-alkylsulfonylvinyl alkylsulfides was prepared with great regio- and stereoselectivity. Moreover, this powerful tactic could be employed to tag cysteine residues of complex polypeptides in solution or on resin merging with solid phase peptide synthesis (SPPS) techniques. A sulfonyl-derived redox responsive fluorescent probe could be conveniently introduced on the peptide, which displays green fluorescence in cells while showing blue fluorescence in medium. The photophysical investigations reveal that the red shift of the emission fluorescence is attested to reduction of carbonyl group to the corresponding hydroxyl moiety. Interestingly, the fluorescence change of tagged peptide could be reverted in cells by treatment of H2O2, arising from the reoxidation of hydroxyl group back to ketone by the elevated level of reactive oxygen species (ROS).

Highly Selective Radical Relay 1,4-Oxyimination of Two Electronically Differentiated Olefins.

Radical addition reactions of olefins have emerged as an attractive tool for the rapid assembly of complex structures, and have plentiful applications in organic synthesis, however, such reactions are often limited to polymerization or 1,2-difunctionalization. Herein, we disclose an unprecedented radical relay 1,4-oxyimination of two electronically differentiated olefins with a class of bifunctional oxime carbonate reagents via an energy transfer strategy. The protocol is highly chemo- and regioselective, and three different chemical bonds (C-O, C-C, and C-N bonds) were formed in a single operation in an orchestrated manner. Notably, this reaction provides rapid access to a large variety of structurally diverse 1,4-oxyimination products, and the obtained products could be easily converted into valuable biologically relevant δ-hydroxyl-α-amino acids. With a combination of experimental and theoretical methods, the mechanism for this 1,4-oxyimination reaction has been investigated. Theoretical calculations reveal that a radical chain mechanism might operate in the reaction.

Reactions of difluoro-pentafluorosulfanyl-iodomethane (SF5CF2I) with electronically different types of alkenes

Difluoro-pentafluorosulfanyl-iodomethane (SF5CF2I) was used in Et3B-mediated radical addition reactions toward electronically different types of alkenes. Reactions with different enol ethers were nonselective in heptane as solvent, while butyl vinyl ether reacted selectively with SF5CF2I in dichloromethane to give the SF5CF2-substituted acetaldehyde dibutyl acetal in moderate yield. Reactions with less electron-rich alkenes such as dec‑1-ene, allyl acetate, and vinyl acetate led to the expected 1-(SF5CF2)-2-iodo products with fair yields. The strained dienes (Z,Z)-cycloocta-1,5-diene and norbornadiene did not yield 1,2-addition products but provided either mixtures of the diastereomeric 2-iodo-6-(CF2SF5)-bicyclo[3.3.0]octanes or the corresponding 3,5-disubstituted nortricyclene derivatives, respectively, by intermediate transannular participation of the second double bond.

Plasmon-Generated Solvated Electrons for Chemical Transformations.

Methods for generating solvated electrons─free electrons in solution─have focused primarily on alkali metal ionization or high-energy electrons or photons. Here we report the generation of solvated electrons by exciting the plasmon resonance of Al nanocrystals suspended in solution with visible light. Two chemical reactions were performed: a radical-addition reaction with the spin-trap 2-methyl-2-nitrosopropane, and a model cyclization reaction with the radical clock 6-bromohex-1-ene. A quantum efficiency of at least ∼1.1% for plasmon absorbed photon to solvated electron generation can be inferred from the measured radical clock reaction concentration. This study demonstrates a simple way to generate solvated electrons for driving reductive organic chemical reactions in a quantifiable and controlled manner.

Investigating the reaction mechanism of light tar for Shenfu bituminous coal pyrolysis

The generation mechanism of light tar during Shenfu bituminous coal pyrolysis was investigated in this work. TGA, Py-GC/MS, TGA-FTIR and reactive force field simulations (ReaxFF) were implemented to reveal the light tar reaction mechanism. The proposed coal structure model was C3198H3261N51O513S33, whose pyrolysis was simulated using the ReaxFF method. Simulation results showed that the pyrolysis process was divided into activation, pyrolysis, and condensation, which agreed well with TGA results. Based on evolutions of gases in the activation stage, it was discovered that coal pyrolysis was initiated by the cleavage of oxygen-containing groups. Simulated results with the coal unimolecular structure showed that the reaction mechanism of light tar was free radical addition reaction, where free radicals were mainly obtained from the cleavage of C–O chemical bond. Also, it was found that the temperature of 2400 K was the transition point, indicating the transition of dominating bond cleavage from C–O bond to C–C bond. The variation of chemical bond agreed well with TGA-FTIR and Py-GC/MS results, indicating that the simulated results are valid and convincing. Therefore, it was concluded that the free radical addition reaction that was mainly controlled by the rupture of C–O bond was the reaction mechanism of light tar.

Turning abundant waste sulfur to polymers for manufacturing: Exploiting role of organic crosslinkers and benign catalysts

Abundant source of sulfur produced as byproduct of refineries demands sustainable valorization solutions. Use of organic crosslinkers and benign catalysts to polymerize sulfur for use in manufacturing offers a sustainable value-added application. This paper introduces MgO as an effective benign catalyst and oleic acid as an organic crosslinker for inverse vulcanization of sulfur. We further compare efficacy of MgO with other metal catalysts (ZnO and CaO) using molecular modeling and laboratory experiments. ZnO is commonly used in elastomeric formulations; however, industries try hard to keep the ZnO level low because of its environmental impact, particularly on aquatic organisms, and for economic reasons. Our rheological characterization shows the inverse vulcanization is enhanced when MgO is present as evidenced by the increase of viscosity of sulfur-oleic acid blends by 94 % relative to the blend without catalyst and 39 % relative to the blend with ZnO catalyst. Based on our quantum-based molecular modeling, using density functional theory (DFT), coordination complexes of Mg oleate (Mg (RCOO)2), made from the reaction between oleic acid and MgO, are energetically better stabilized compared to Zn or Ca oleate. Stronger interactions in Mg oleate, leading to the more stabilized coordination complexes, could be a reason for higher viscosity for MgO/sulfur-oleic acid compared to ZnO/sulfur-oleic acid. In complexes of sulfur and metal oleate, regardless of the electronic nature of sulfur chains (ionic or radical form), coordination complexes containing MgO show binding energies that are higher than those of Zn complexes and notably higher than those of Ca complexes, indicating capability of Mg substitution in Zn oleate compounds as catalysts. Studying the addition reactions of sulfur radicals to oleic acid and the formation of CS bonds, using DFT modeling, shows the superiority of allylic (CHCHĊH) sites of oleic acid over olefinic (CC) sites in reactions of sulfur radicals. Compared to sulfur radicals, sulfur chains in ionic character provide higher binding energies with metal oleates because of the electrostatic interactions between the positively charged metal and negative sulfur atoms. The study outcomes support sustainability and resource conservation in chemical manufacturing and petroleum refining industry.