Free-radical Substitution Research Articles

P-NP mineralization, total nitrogen removal and toxicity reduction using an alternating-electroreduction-electrooxidation system

Electrochemical degradation has emerged as a sustainable technology for treating p-Nitrophenol (p-NP). However, most studies have focused only on electrochemical oxidation or electrochemical reduction, and limited research on alternating-electroreduction-electrooxidation system (AEES). The process of p-NP degradation in AEES system has not been understood. In this work, the effect of AEES on p-NP treatment was investigated by regulating the reduction and oxidation time ratios as well as the alternating cycles. The results indicated that the different modes had a significant effect on p-NP degradation rate and TN removal, while the effect on mineralization was limited. Obviously, the sequential reduction–oxidation system performed better than the sequential oxidation–reduction system, with p-NP degradation rates increasing by 34.2 % and 147.9 % over the oxidation and reduction modes alone, respectively. Moreover, an increase in the number of alternating cycles (R60O120 × 1 to R10O20 × 6) results in the improved mineralization from 84.4 % ± 2.1 % to 88.7 % ± 1.7 % and TN removal from 35.1 % ± 1.9 % to 46.6 % ± 2.1 %. The increased mineralization, decreased nitrogen compounds, and reduced complex by-products also contributed to 10.0 % ± 1.4 % reduction of acute toxicity. On the basis of intermediates identification, a degradation pathway for AEES including free radical substitution and ring opening process were proposed.

Interplay of diruthenium catalyst in controlling enantioselective propargylic substitution reactions with visible light-generated alkyl radicals

Transition metal-catalyzed enantioselective free radical substitution reactions have recently attracted attention as convenient and important building tools in synthetic chemistry, although construction of stereogenic carbon centers at the propargylic position of propargylic alcohols by reactions with free radicals remains unchallenged. Here we present a strategy to control enantioselective propargylic substitution reactions with alkyl radicals under photoredox conditions by applying dual photoredox and diruthenium catalytic system, where the photoredox catalyst generates alkyl radicals from 4-alkyl-1,4-dihydropyridines, and the diruthenium core with a chiral ligand traps propargylic alcohols and alkyl radicals to guide enantioselective alkylation at the propargylic position, leading to high yields of propargylic alkylated products containing a quaternary stereogenic carbon center at the propargylic position with a high enantioselectivity. The result described in this paper provides the successful example of transition metal-catalyzed enantioselective propargylic substitution reactions with free alkyl radicals.

Homogeneous functionalization of light alkanes

Light alkanes are the most abundantly available carbon-based feedstocks on earth and as major constituents of natural gas and oil-associated gas. Compared with oil, natural gas is considered a greener fuel and promising alternative for energy and chemical production. However, the grand challenge in the chemistry of light alkanes utilization is the development of an efficient method for the selective activation C–H bond. The transportation issue comes from the low boiling point. On the other hand, these unactivated molecules are hard to be activated under mild conditions because of their high bond dissociation energy and low acidity. Moreover, the functionalized products, such as ethylene or methanol, have higher reactivity than light alkanes. Due to these challenges, it is vital that develop efficient processes to achieve value-added convert under economic and green reaction conditions. In this review, we summarized the recent progress in the field of homogeneous functionalizations of light alkanes and was subtitled to four different aspects according to the mechanism, including electrophilic substitution, oxidative addition, carbene insertion, and free radical substitution. However, developing a new, lower temperature, economic, and green direct light alkanes conversion process is still needed to address.

Quantum chemical calculation of free radical substitution reaction mechanism of camptothecin

Free radical substitution reaction, which has low energy barrier and takes place in mild reaction conditions, is an important method for camptothecin's modification. The experimental data show that the free radical substitution reaction of camptothecin has high site selectivity, and prefers to take place at site 7. Up to now, few researches focus on the mechanism of it. In this study, the differences of the reaction rate constant (k) for the reactions at different sites, such as site of 7, 9, 10, 11, 12, were investigated with B3LYP of density functional theory at the 6-31+G (d, p) base set level and CPCM aqueous solvent model. It was found that the substitution reaction can be carried out in two steps in acidic condition. First, the methyl radical attacks the corresponding site to form an intermediate having methyl radical combined with the camptothecin skeleton, and then a hydrogen atom was abstracted by the singlet oxygen to form methyl camptothecin, wherein the first step was the rate control step of the reaction. The results show that site 7 has the higherreaction rate constant (k) than other examined sites, indicating that the reaction tends to take place on site 7 position, which is in agreement with the experimental results.

Structure of a product of free radical substitution of a chlorine atom for 1,3,5-trioxane in an iron(II) chlathrochelate

A tert-BuOOH/Fe(II) system is used for the free radical substitution of the chlorine atom at the rib of a boron capped iron(II) tris-dioximate chlathrochelate for 1,3,5-trioxane. The structure of the compound is determined by single crystal XRD and solution NMR.

Строение продукта свободно-радикального замещения атома хлора в клатрохелате железа(II) на фрагмент 1,3,5-триоксана

Строение продукта свободно-радикального замещения атома хлора в клатрохелате железа(II) на фрагмент 1,3,5-триоксана

Reactions of 2-Propanol Radical with Halogenated Organics in Aqueous Solution: Theoretical Evidence for Proton-Coupled Electron Transfer and Competing Mechanisms.

The reactions of α-hydroxyalkyl radicals in aqueous medium are of interest because they exhibit a rich variety of fundamentally important competing mechanisms, such as proton-coupled electron transfer (PCET), hydrogen atom transfer, free radical substitutions, abstractions and additions, etc. We present a theoretical study of the mechanism and kinetics of the aqueous reactions of α-hydroxyisopropyl (2-propanol) radical with four halogenated organic substrates: iodoacetate (IAc), iodoacetamide (IAm), 5-bromouracil (5-BrU), and carbon tetrachloride (CCl4). The reactions are studied using density functional theory (DFT) (M06-2X), and the solvent is modeled as a polarizable continuum, either without the explicit solvent molecules or with one added water molecule. For an additional refinement, the double hybrid DFT B2PLYP energies were calculated at the M06-2X stationary points. Within this framework, for each substrate, we determine the most favorable radical-induced decomposition pathway among the several found and compare the thermochemical predictions against the experimental kinetics. The following dominant decomposition mechanisms are inferred: PCET for IAc, PCET-H2O and the I-atom abstraction for IAm, the ortho-addition to the double bond for 5-BrU, and the Cl-atom abstraction for CCl4. These pathways are invariably characterized by the negative apparent activation energies. Whereas for 5-BrU and CCl4 the transition state theory rate constants are in good agreement with the experiment, the rate constants for IAc and IAm-the two substrates reacting preferably via the PCET-are difficult to predict correctly. Consequently, the corresponding reaction barriers necessitate lowering by 1-3 kcal mol-1 to bring them in accord with experiment. The B2PLYP method provides a worthwhile improvement over the M06-2X energetics although the largest errors remain for the two PCET processes.

Improved Halogenation of Methyl Aromatics and Methyl Heteroaromatics: Unexpected Reactivity of Tetrahalogeno‐diphenylglycolurils

ABSTRACT1,3,4,6‐Tetrachloro (TCDGU) and 1,3,4,6‐tetrabromo‐3α,6α‐diphenylglycolurils smooth halogen oxidizers have been exploited in a new direction as reagents for free radical substitution toward some N‐halosuccinimide nonreactive bis‐heterocycles. An unexpected selectivity and reactivity were observed with methyl benzenes, methyl heterocycles, and methyl‐bis‐heterocycles of interest. A chemometric study has been performed to optimize five independent factors of the chlorination reaction with TCDGU. The predictive model was established either for the halogenation conversion and the ratio of monochlorination.

Synthesis and performance comparison of perfluorobenzoic acid and perfluorobenzenesulfonic acid membranes

Perfluorobenzoic acid (PFBA) and perfluorobenzenesulfonic acid (PFBSA) membranes were synthesized from perfluorosulfonyl iodide via novel free-radical substitution reactions. Despite its lower ion-exchange capacity, the PFBA film has proton conductivities typically more than two times higher than PFBSA under various temperatures. For example, at 80°C, PFBA has a proton conductivity of 109.6 mS/cm while the PFBSA membrane is only 44.1mS/cm in water. The PFBA membrane also shows better performance than PFBSA in a 5cm2 PEMFC setting. The formation of unique dimerized/trimerized nano-clusters of –CO2H facilitates the hopping of protons across the membrane—leading to the higher ion-conductivities of PFBA. In addition, free-radical substitution reactions were found to be a general method for grafting aromatics or heteroaromatics to the terminals of fluoropolymer side chains via stable C–C bonds that can tolerate hydrolytic conditions. Such perfluoro-aromatic and perfluoro-heteroaromatic membranes may find applications in numerous fields like chemical, chlor-alkali, electric/electronic, optical, and construction industries.

Mechanisms and Competition of Free Radical Addition and Substitution Reactions of Propylene with HBr and Br·

Mechanisms and Competition of Free Radical Addition and Substitution Reactions of Propylene with HBr and Br·

Structure of the product of free-radical substitution of chlorine atoms in iron(II) clathrochelate with two ethanol fragments

The homolytic substitution of chlorine atoms afforded iron(II) clathrochelate bearing two ethanol fragments. The structure of the compound was determined by XRD in the crystalline state and by NMR in solution.

Convenient peripheral aroyloxylation reactions of porphyrins and chlorophyll-a-based chlorins with benzoyl peroxide

A practical and efficient methodology for the formation of C–O bonds on the porphyrin/chlorin periphery was developed. The aroyloxy-substituted porphyrins and chlorins related to chlorophyll-a at the β- and meso-positions, respectively, were conveniently synthesized by the free radical substitution reaction with benzoyl peroxide and its homologs.

New fluorinated poly(ether sulfone imide)s with high thermal stability and low dielectric constant

A new aromatic diamine monomer with four pendant trifluoromethyl groups, 2,2′-bis{3-[3,5-di(trifluoromethyl)phenyl]-4-[4-amino-phenoxy]phenyl}sulfone (3), was successfully synthesized through free-radical substitution, Suzuki coupling and nucleophilic substitution reactions using bis(4-fluorophenyl)sulfone and N-bromosuccinimide as starting materials. Then it was employed to prepare a series of fluorinated poly(ether sulfone imide)s (PESIs 5a–c) with various commercial aromatic dianhydrides via a one-step high-temperature polycondensation. These polymers could be easily dissolved in some strong polar organic solvents, such as N-methyl-2-pyrrolidone, N,N-dimethylacetamide, and N,N-dimethylformamide, chloroform, dichloromethane and tetrahydrofuran at room temperature. Flexible and transparent films can be obtained easily by solution casting. They had high thermal stability and didn't show significant weight loss up to temperature of approximately 530 °C in nitrogen and air atmospheres. They also revealed low dielectric constants with the values in the range of 2.74–2.90 at 1 MHz measured for their capacitance.

ChemInform Abstract: Covalent Chemistry on Graphene

AbstractReview: 45 refs.

Stereoselective C-alkylation of an iron(II) dichloroclathrochelate via free-radical reactions with alcohols

Abstract Free-radical substitution of one of the two chlorine atoms of the tris-dioximate iron(II) dichloroclathrochelate with ethanol and iso-propanol radicals proceeding with high regioselectivity at their α-carbon atoms afforded the monofunctionalized macrobicyclic products, which were characterized both in solution and in solid state by multinuclear one- and two-dimensional NMR spectroscopy and X-ray crystallography, respectively.

An efficient buffer-mediated control between free radical substitution and proton-coupled electron transfer: dehalogenation of iodoethane by the α-hydroxyethyl radical in aqueous solution

A remarkable buffer-mediated control between free-radical substitution (FRS) and proton-coupled electron transfer (PCET) is demonstrated for the reaction between iodoethane and the α-hydroxyethyl radical in neutral aqueous solution in the presence of bicarbonate or phosphate buffer. The reaction is initiated by the γ-radiolysis of the water solvent, and the products, either the iodine atom (FRS) or anion (PCET), are analysed using ion chromatographic and spectrophotometric techniques. A detailed insight into the mechanism is gained by employing density functional theory (M06-2X), Møller-Plesset perturbation treatment to the second order (MP2), and multireference methods (CASSCF/CASPT2). Addition of a basic buffer anion is indispensable for the reaction to occur and the competition between the two channels depends subtly on its proton accepting affinity, with FRS being the dominant channel in the phosphate and PCET in the bicarbonate containing solutions. Unlike the former, the latter channel sustains a chain-like process which significantly enhances the dehalogenation. The present systems furnish an example of the novel PCET/FRS dichotomy, as well as insights into possibilities of its efficient control.

Covalent chemistry on graphene

The recent growth in graphene has witnessed the utilization of graphene as promising materials in a plethora of applications. The potential of graphene could be improved exponentially provided that the processability and band gap could be finely controlled. To this aim, chemical functionalization of graphene remains an important and fundamental approach. This tutorial review aims to provide a broad-based coverage on the recent solution-based functionalization methods of graphene in a concise and mechanistic manner. We focus on the reactions of the graphene sp(2) backbone, such as nucleophilic addition, cycloaddition, free radical additions, substitutions and rearrangements.

Advanced ultrafiltration membranes based on functionalized poly(arylene ether sulfone) block copolymers

In this study, poly(arylene ether sulfone) (PAES) block copolymer was synthesized from fluoride-terminated oligomer (A16) and hydroxyl terminated oligomer (B12) by aromatic nucleophilic substitution (SNAr) polycondensation reaction. Then allylic free radical substitution reaction at methyl side group using N-bromosuccinimide and benzoyl peroxide was performed to synthesize PAES–CH2Br block copolymer. The chemical structure of PAES and PAES–CH2Br block copolymers was confirmed by 1H-NMR spectroscopy. Gel permeation chromatography was applied to determine the molecular weight and the degree of polymerization of A16 and B12 oligomers, PAES and PAES–CH2Br block copolymers. The synthesized block copolymer had high molecular weight and multiblock structure. Non-solvent induced phase separation of blends of PAES–CH2Br and PAES block copolymers followed by heterogeneous quaternization using trimethyl amine led to hydrophilic positively charged ultrafiltration membranes. Fourier transform infrared spectroscopy in the attenuated total reflection mode, scanning electron microscopy, contact angle, and outer surface zeta potential studies were performed to characterize the resulting membranes. The results confirmed that the prepared membranes were porous, hydrophilic and positively charged. Membranes were also characterized with respect to permeability and then used in ultrafiltration of bovine serum albumin and lysozyme model solutions at varied pH values. Membrane performance depended on the positive charge density which could be adjusted by varying the fraction of PAES–CH2Br in the membrane. Due to the combined effects of size exclusion and charge repulsion, permeability, antifouling properties and protein separation selectivity of membranes could be increased simultaneously with increasing charge density.

A simple, general synthesis of carbonimidic dichlorides

The reaction of aliphatic and aromatic isonitriles with sulfuryl chloride provides an efficient, general route to the corresponding dichlorides without byproducts of free-radical substitution.

Direct trifluoro-methoxylation of aromatics with perfluoro-methyl-hypofluorite

The reactivity of CF3OF (FTM) has been widely studied especially in halogenated olefinic systems and its use in pharmaceutical synthesis as a mild radical and electrophilic fluorinating agent is well documented. On the other hand, the chemical behavior of the perfluoro-methyl-hypofluorite with aromatic substrates is much less studied. Up to now few and scattered data regarding its use as electrophilic fluorinating agent of variously substituted aromatic compounds are found in the literature. In this work the reactivity of CF3OF with simple electron rich and electron poor aromatics (α,α,α-trifluoro-toluene, toluene, benzene, chloro-benzene, methoxybenzene) has been investigated. The possibility of selectively bind the trifluoro-methoxy group (via radical mechanism) or the fluorine atom (via electrophilic addition) by varying the reaction conditions has been explored. In particular we have observed that the trifluoro-methoxy free radical substitution can be the main synthetic pathway if the reaction is promoted by an independent and steady source of CF3O radical.