Radical Coupling Process Research Articles

Catalytic enantioselective radical coupling of activated ketones with N-aryl glycines.

Asymmetric H-bonding catalysis as a viable strategy for enantioselective radical coupling of ketones is demonstrated. With a visible-light-mediated dual catalytic system involving a dicyanopyrazine-derived chromophore (DPZ) photosensitizer and a chiral phosphoric acid (CPA), N-aryl glycines with a variety of 1,2-diketones and isatins underwent a redox-neutral radical coupling process and furnished two series of valuable chiral 1,2-amino tertiary alcohols in high yields with good to excellent enantioselectivities (up to 97% ee). In this catalysis platform, the formation of neutral radical intermediates between ketyl and H-bonding catalyst CPA is responsible for presenting stereocontrolling factors. Its success in this work should provide inspiration for expansion to other readily accessible ketones to react with various radical species, thus leading to a productive approach to access chiral tertiary alcohol derivatives.

An Efficient and Modular Route to C3*-TunePhos-Type Ligands

An efficient and modular synthetic route to the bidentate C3*-TunePhos was developed, which allowed tunable steric and electronic effects of the ligands. This novel chemical technology highlights a versatile C3*-dibromodiphenyl intermediate that was accomplished by in situ Grignard exchange and subsequent Cu(II)-mediated intra­molecular oxidative radical coupling process. It is worth noting that this advanced intermediate not only could be easily prepared by a diverse array of C3*-TunePhos-type ligands, but also could be used to facile synthesis of other novel type of N,S-centered bidentate ligands.

ChemInform Abstract: Metal‐Free Carbonyl C(sp2)—H Oxidative Alkynylation of Aldehydes Using Hypervalent Iodine Reagents Leading to Ynones.

AbstractAccording to mechanistic studies, the reaction is based on a carbonyl C(sp2)—H oxidative radical coupling process.

Engineering Plant Biomass Lignin Content and Composition for Biofuels and Bioproducts

Lignin is an aromatic biopolymer involved in providing structural support to plant cell walls. Compared to the other cell wall polymers, i.e., cellulose and hemicelluloses, lignin has been considered a hindrance in cellulosic bioethanol production due to the complexity involved in its separation from other polymers of various biomass feedstocks. Nevertheless, lignin is a potential source of valuable aromatic chemical compounds and upgradable building blocks. Though the biosynthetic pathway of lignin has been elucidated in great detail, the random nature of the polymerization (free radical coupling) process poses challenges for its depolymerization into valuable bioproducts. The absence of specific methodologies for lignin degradation represents an important opportunity for research and development. This review highlights research development in lignin biosynthesis, lignin genetic engineering and different biological and chemical means of depolymerization used to convert lignin into biofuels and bioproducts.

Laccase-Catalyzed Degradation of Perfluorooctanoic Acid

This study examined the decomposition of perfluorooctanoic acid (PFOA) in enzyme-catalyzed oxidative humification reactions (ECOHRs). ECOHRs make up a class of reactions that are ubiquitous in the environment. Approximately 50% of PFOA in a mineral buffer solution decomposed upon addition of laccase and 1-hydroxybenzotriazole after 157 days with a pseudo-first-order rate constant of 0.0044 day–1 (r2 = 0.89). No shorter carbon-chain perfluorocarboxylic acids were detected as degradation products during the experiment. However, partially fluorinated shorter-chain alcohols and aldehydes were identified by high-resolution mass spectrometry. These partially fluorinated compounds were likely products resulting from PFOA degradation via a combination of free radical decarboxylation, rearrangement, and coupling processes. Fluoride was detected in the reaction solution, and the concentration indicated a 28.2% defluorination ratio during the treatment. This finding suggests that PFOA may be transformed during humif...

Metal-free Mizoroki-Heck type reaction: a radical oxidative coupling reaction of 2-chloro-dithiane with substituted olefins.

An efficient metal-free Mizoroki-Heck type reaction of di- and tri-substituted alkenes with 2-chloro-dithiane has been developed under ambient pressure of air or using a relatively low loading of BF3·Et2O. This study represents a new environmentally friendly method for the syntheses of dithianyl-substituted alkene derivatives via a radical oxidative coupling process.

ChemInform Abstract: Iron‐Catalyzed Alkenylation of Cyclic Ethers via Decarboxylative sp3(C)—sp2(C) Coupling.

Abstract An efficient Fe(acac)3-catalyzed decarboxylative C(sp2)–C(sp3) coupling reaction via oxidation of C–H bond adjacent to an oxygen atom has been developed successfully, in which cyclic ethers are selectively transformed into the corresponding alkenylation products with good chemical yields and excellent stereoselectivities. The mechanism was studied and the reaction was supposed to proceed through a radical oxidative coupling process.

Iron-catalyzed alkenylation of cyclic ethers via decarboxylative sp3(C)–sp2(C) coupling

An efficient Fe(acac)3-catalyzed decarboxylative C(sp2)–C(sp3) coupling reaction via oxidation of C–H bond adjacent to an oxygen atom has been developed successfully, in which cyclic ethers are selectively transformed into the corresponding alkenylation products with good chemical yields and excellent stereoselectivities. The mechanism was studied and the reaction was supposed to proceed through a radical oxidative coupling process.

Synthesis of H-shaped complex macromolecular structures by combination of atom transfer radical polymerization, photoinduced radical coupling, ring-opening polymerization, and iniferter processes

Three controlled/living polymerization processes, namely atom transfer radical polymerization (ATRP), ring opening polymerization (ROP) and iniferter polymerization, and photoinduced radical coupling reaction were combined for the preparation of ABCBD-type H-shaped complex copolymer. First, a-benzophenone functional polystyrene (BP-PS) and poly( methyl methacrylate) (BP-PMMA) were prepared independently by ATRP. The resulting polymers were irradiated to form ketyl radicals by hydrogen abstraction of the excited benzophenone moieties present at each chain end. Coupling of these radicals resulted in the formation of polystyrene-b-poly(methyl methacrylate) (PS-b-PMMA) with benzpinacole structure at the junction point possessing both hydroxyl and iniferter functionalities. ROP of e-caprolactone (CL) by using PS-b-PMMA as bifunctional initiator, in the presence of stannous octoate yielded the corresponding tetrablock copolymer, PCL-PSPMMA- PCL. Finally, the polymerization of tert-butyl acrylate (tBA) via iniferter process gave the targeted H-shaped block copolymer

Cobalt porphyrins as homogeneous catalysts for water oxidation

Three water-soluble cobalt porphyrins have been investigated as water oxidation catalysts via photo-initiation using Ru(II)(bpy)3(2+)/Na2S2O8. The pH dependence of the turnover frequency revealed maximum activity at pH 11. Based on the second order dependence on catalyst concentration for the rate of water oxidation, we suggest a bimolecular radical coupling process as the rate determining step.

Synthesis of Macrocyclic Polymers Formed via Intramolecular Radical Trap-Assisted Atom Transfer Radical Coupling.

The synthesis of cyclic polystyrene (PSt) with an alkoxyamine functionality has been accomplished by intramolecular radical coupling in the presence of a nitroso radical trap. Linear α,ω-dibrominated polystyrene, produced by the atom transfer radical polymerization (ATRP) of styrene using a dibrominated initiator, was subjected to chain-end activation via the atom transfer radical coupling (ATRC) process under pseudodilute conditions in the presence of 2-methyl-2-nitrosopropane (MNP). This radical trap-assisted, intramolecular ATRC (RTA-ATRC) produced cyclic polymers in greater than 90% yields, possessing ⟨G⟩ values in the 0.8-0.9 range as determined by gel permeation chromatography (GPC). Thermal-induced opening of the cycles, made possible by the incorporated alkoxyamine, resulted in a return to the original apparent molecular weight, further supporting the formation of cyclic polymers in the RTA-ATRC reaction. Liquid chromatography-mass spectrometry (LC-MS) provided direct confirmation of the cyclic architecture and the incorporation of the nitroso group into the macrocycle. RTA-ATRC cyclizations carried out with faster rates of polymer addition into the redox active solution and/or in the presence of a much larger excess of MNP (up to a 250:1 ratio of MNP:C-Br chain end) still yielded cyclic polymers that contained alkoxyamine functionality.

CCoAOMT suppression modifies lignin composition in Pinus radiata

A cDNA clone encoding the lignin-related enzyme caffeoyl CoA 3-O-methyltransferase (CCoAOMT) was isolated from a Pinus radiata cDNA library derived from differentiating xylem. Suppression of PrCCoAOMT expression in P.radiata tracheary element cultures affected lignin content and composition, resulting in a lignin polymer containing p-hydroxyphenyl (H), catechyl (C) and guaiacyl (G) units. Acetyl bromide-soluble lignin assays revealed reductions in lignin content of up to 20% in PrCCoAOMT-deficient transgenic lines. Pyrolysis-GC/MS and 2D-NMR studies demonstrated that these reductions were due to depletion of G-type lignin. Correspondingly, the proportion of H-type lignin in PrCCoAOMT-deficient transgenic lines increased, resulting in up to a 10-fold increase in the H/G ratio relative to untransformed controls. 2D-NMR spectra revealed that PrCCoAOMT suppression resulted in formation of benzodioxanes in the lignin polymer. This suggested that phenylpropanoids with an ortho-diphenyl structure such as caffeyl alcohol are involved in lignin polymerization. To test this hypothesis, synthetic lignins containing methyl caffeate or caffeyl alcohol were generated and analyzed by 2D-NMR. Comparison of the 2D-NMR spectra from PrCCoAOMT-RNAi lines and synthetic lignins identified caffeyl alcohol as the new lignin constituent in PrCCoAOMT-deficient lines. The incorporation of caffeyl alcohol into lignin created a polymer containing catechyl units, a lignin type that has not been previously identified in recombinant lignin studies. This finding is consistent with the theory that lignin polymerization is based on a radical coupling process that is determined solely by chemical processes.

Synthesis of block and star copolymers by photoinduced radical coupling process

AbstractThe general design for the synthesis of AB diblock, and A2B and AB2 star copolymers based on the statistical coupling of poly(styrene) (PSt) and poly (methyl methacrylate) (PMMA) macromolecules containing photoreactive benzophenone is presented. For this purpose, mono‐ and bifunctional initiators for Atom Transfer Radical Polymerization (ATRP) bearing benzophenone group were synthesized and characterized. End‐ and mid‐chain benzophenone functional PSt and PMMA with low molecular weights were obtained by ATRP using these initiators in the presence of CuBr/N,N,N′,N″,N″‐pentamethyldiethylenetriamine (PMDETA) catalytic complex. Poly(styrene‐block‐methyl methacrylate) (PSt‐b‐PMMA) copolymers were prepared by photolysis of the solutions containing end functional PSt and PMMA in THF at λ = 350 nm for 60 min in the presence of a hydrogen donor such as N‐methyldiethanolamine (NMDEA). The proposed mechanism assumes hydrogen abstraction of photoexcited benzophenone moiety by NMDEA. Ketyl radicals resulting from abstraction reaction undergo radical‐radical coupling to form benzpinacol structure at the core. Formation of A2B and AB2 type star copolymers upon irradiation of solutions containing appropriate combinations of end‐ and mid‐chain functional polymers was also demonstrated. © 2009 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 47: 2938–2947, 2009

Ti‐Catalyzed Barbier‐Type Allylations and Related Reactions

Titanocene(III) complexes, easily generated in situ from commercial Ti(IV) precursors, catalyze Barbier-type allylations, intramolecular crotylations (cyclizations), and prenylations of a wide range of aldehydes and ketones. The reaction displays surprising and unprecedented mechanistic subtleties. In cyclizations a fast and irreversible addition of an allyl radical to a Ti(III)-coordinated carbonyl group seems to occur. Intermolecular additions to conjugated aldehydes proceed through a coupling of a Ti(IV)-bound ketyl radical with an allyl radical. Reactions of ketones with allylic halides take place by the classical addition of an allylic organometallic reagent. The radical coupling processes enable transformations such as the highly regioselective alpha-prenylation that are otherwise difficult to achieve. The mild reaction conditions and the possibility to employ titanocene complexes in only catalytic quantities are highly attractive features of our protocol. These unusual properties have been taken advantage of for the straightforward synthesis of the natural products rosiridol, shikalkin, and 12-hydroxysqualene.

Solvent‐Mediated Generation of Cobalt‐Cluster‐Stabilised Propargyl Cations and Radicals: Allyl Migration versus Peroxide Formation

The protonation of [Co(2)(CO)(6){9-[(allyldimethylsilyl)ethynyl]-9H-fluoren-9-ol}] (4), with HBF(4) in CH(2)Cl(2) led to migration of the allyl group from silicon to the cobalt-stabilised cationic site to furnish [Co(2)(CO)(6){9-allyl-9-[(dimethylfluorosilyl)ethynyl]-9H-fluorene}] (17). However, under the same conditions, [Co(2)(CO)(6){9-[(benzyldimethylsilyl)ethynyl]-9H-fluoren-9-ol}] (5) underwent desilylation and rearrangement of the resulting terminal alkyne-dicobalt complex to give [Co(3)(CO)(9)(9H-fluorenylmethylcarbynyl)] (24); moreover, dimerisation of the (benzyldimethylsilyl)ethynyl-9H-fluorenyl moiety led to the propargyl-allene 26. In contrast, protonation of 5 in THF yielded [{Co(2)(CO)(6){[(benzyldimethylsilyl)ethynyl-9H-fluorenyl])}(2)peroxide] (27) through a radical coupling process. Analogously, protonation of [Co(2)(CO)(6){9-[(vinyldimethylsilyl)ethynyl]-9H-fluoren-9-ol}] (6) yields the corresponding peroxide 28. X-ray crystallographic data are reported for, among others, complexes 17, 24, 26, 27 and 28.

Electrocarboxylation and related radical coupling processes of aryl and benzyl halides in microemulsion

Electrocarboxylation of aryl halides and benzyl halides in cetyltrimethylammonium bromide (CTAB) based bicontinuous microemulsion is reported. Benzyl halides exhibited higher conversion of coupling products between the benzyl radical and the phenyl acetate radical. Benzylphenylacetate is found to be the predominant product in galvanostatic electrolysis experiments. Electrocarboxylation of aryl halides is conventionally carried out in dimethylformamide (DMF) using nickel cathode and magnesium or aluminium anodes [1, 2]. Further improvements in the electrocarboxylation processes employing redox mediators [3–5] and silver cathodes [6, 7] have been reported. Poly halogenated aromatic compounds [8], iodo aromatic compounds [9] and aliphatic halides [10, 11] also undergo facile electrocarboxylation. Al ion generated at the anode appears to have a catalytic influence on the electrocarboxylation reactions [12]. Highly pure and moisture free DMF still remains the medium of choice for electrocarboxylation reactions [13]. Quite recently both cyclo addition of carbon dioxide to epoxides [14] and electrocarboxylation of cinnamate ester [15] were achieved in moisture free ionic liquid. In contrast to dimethylformamide and ionic liquids, bicontinuous microemulsions contain significant amount of water and are indeed much greener media for chemical and electrochemical processes [16]. Rusling et al. have reported free radical generation and coupling processes in bicontinuous microemulsion [17–19]. Similar electro generated free radical reactions have also been reported [20, 21]. Electrochemical coupling of benzyl free radicals generated from benzyl bromide with active methylene compounds under galvanostatic experimental conditions in cationic microemulsions was reported from this laboratory [21]. The objective of this communication is to explore the possibility of another important coupling reaction, namely electrocarboxylation, involving the carbon dioxide and the electro generated aryl and benzyl free radicals in a bicontinuous microemulsion.

Synthesis via ATRP and Anchoring Properties of Ammonium‐Terminated Monofunctional or Telechelic Polystyrenes

AbstractMono‐ and di‐(trimethylammonium)‐terminated polystyrenes with similar molar masses were synthesized. Bromopolystyrenes were first prepared by atom transfer radical polymerization (ATRP) using (N‐Boc‐propylamino)‐2‐bromoisobutyrate as initiator and Cu(I)Br/2,2′‐bipyridine as catalytic system in bulk at 110 °C. α,ω‐Telechelic polymer with double molecular weight with respect to the starting polystyrene was then obtained under atom transfer radical coupling process using 2,2′‐bipyridine and copper(0) mediated reductive conditions in anisole. Final (trimethylammonium)‐terminated polystyrenes were eventually obtained after removal of the tert‐butyloxycarbonyl group and permethylation reactions. Intermediate and final polymers were characterized by 1H NMR. The aligning properties of the obtained polymers for liquid crystals have been investigated. Pretilt and zenithal anchorings on ITO/glass substrates were measured. Finally, the possibility for the telechelic polystyrene to form a network in an inverted microemulsion was also considered.magnified image

Characterization of carbofuran photodegradation by‐products by liquid chromatography/hybrid quadrupole time‐of‐flight mass spectrometry

The structural elucidation of by-products arising from carbofuran photodegradation using a high-pressure UV lamp has been investigated by liquid chromatography/electrospray ionization tandem mass spectrometry (LC/ESI-MS/MS) employing a quadrupole time-of-flight mass spectrometer. Exact mass measurements of the [M + H]+ ions of the by-products and of product ions allowed the elemental formulae and related structures of seven photodegradation by-products (resulting, respectively, from photo-Fries rearrangement, hydroxylation of the benzene ring, oxidation of the 2,3-dihydrobenzofuran ring, cleavage of the carbamate group, hydrolysis of the ether group and the newly observed radical coupling and decarboxylation processes) to be determined confidently. Accurate mass measurements of product ions allowed ambiguities to be removed concerning neutral losses having the same nominal mass, namely CO and C2H4, allowing the fragmentation patterns to be rationalized.

UV degradation of carbofuran insecticide in aqueous solution: identification and toxicity evolution of by-products

The paper reports the results of an investigation about the UV degradation of carbofuran, a widely used insecticide in Europe. Specific objectives were the identification of the by-products formed and the evaluation of the toxicity of the irradiated solution compared to that of carbofuran. The experimental results, obtained treating an aqueous carbofuran solution (50 mg/L) by high pressure UV lamp (125 W), show that the insecticide is completely removed within 120 min. Several intermediate by-products have been identified by liquid chromatography-mass spectrometry (LC-MS) as a result of hydroxylation process of the 2,3-dihydro benzofurane ring and other reactions such as the cleavage of the carbamate group, the hydrolysis of ethereal moiety, radical coupling and decarboxylation processes. After 270 min of reaction the identified by-products were completely degraded and COD and TOC removals of 35 and 20% were measured, respectively. Toxicological analyses performed using the comparison procedure of the MicrotoxTM assay provided interesting clues concerning toxic effects of the photodegradation by-products. The results revealed a substantial increase of the toxicity during the first 15 min proving that photodegradation of organic contaminant could even lead to an increase of the toxicity of treated solution.

Photolysis of the ion pair triphenylsulfonium thiophenolate induced by outersphere charge transfer excitation

Abstract The ion pair Ph3S+PhS− can be viewed as a sulfur-based mixed-valence system which is characterized by a PhS− to Ph3S+ outersphere charge transfer (OSCT) absorption at λmax=390 nm (in CHCl3). OSCT excitation leads to the radical pair Ph3S/PhS which undergoes subsequent elimination and radical coupling processes with the final formation of Ph2S. The photolysis proceeds with a quantum yield of φ=0.23 at λirr=436 nm.