Reaction Of Aryl Radicals Research Articles

A Hierarchy of Ligands Controls Formation and Reaction of Aryl Radicals in Pd-Catalyzed Ground-State Base-Promoted Coupling Reactions.

Palladium salts and complexes were tested separately and in the presence of added ligands as potential sources of aryl radicals in ground-state coupling reactions of aryl halide with arenes under basic conditions (KOtBu). Our recently developed assay for aryl radicals was employed to test for aryl radicals. In this assay, aryl radicals derived from the test substrate, 1-iodo-2,6-dimethylbenzene 7, undergo base-promoted homolytic aromatic substitution (BHAS) with benzene to produce 2,6-dimethylbiphenyl 8 and biphenyl 9 in an approximately 1:4 ratio as well as m-xylene 10. The biphenyl arises from a diagnostic radical transfer reaction with the solvent benzene. Using substrate 7 with a range of Pd sources as potential initiators led to formation of 8, 9, and 10 in varying amounts. However, when any one of a range of diphosphinoferrocenes (e.g., dppf or dippf) or BINAP or the monophosphine, diphenylphosphinoferrocene, was added as a ligand to Pd(OAc)2, the ratio of [2,6-dimethylbiphenyl 8: biphenyl 9] moved decisively to that expected from the BHAS (radical) pathway. Further studies were conducted with dppf. When dppf was added to each of the other Pd sources, the ratio of coupled products was also diverted to that expected for radical BHAS chemistry. Deuterium isotope studies and radical trap experiments provide strong additional support for the involvement of aryl radicals. Accordingly, under these ground-state conditions, palladium sources, in the presence of defined ligands, convert aryl iodides to aryl radicals. A rationale is proposed for these observations.

Elaborating E/Z‐Geometry of Alkenes via Cage‐Confined Arylation Catalysis of Terminal Olefins

AbstractA visible light photosensitizing metal‐organic cage is applied as an artificial supramolecular reactor to control the reaction of aryl radicals with terminal olefins under green light/solvent conditions, which facilitates selective transformation in the confined enzyme‐mimicking environment to give a series of geometrically defined E/Z‐alkenes. The hydrophobic cage displays good host–guest inclusion with aromatic substrates, promoting Meerwein arylation and protecting E‐isomeric products during reaction; while a small amount of benzonitrile can turn on efficient E→Z isomerization. Besides π–π stacking, the hydrogen bonding and halogen bonding interactions also act as control forces for the arylation of aliphatic terminal olefins known as poor acceptors in classic Meerwein arylation. The application of this switchable cage‐confined arylation catalysis has been demonstrated by the syntheses of Tapinarof and a marine natural product from the same substrate via controllable E/Z selectivity.

Elaborating E/Z-Geometry of Alkenes via Cage-Confined Arylation Catalysis of Terminal Olefins.

A visible light photosensitizing metal-organic cage is applied as an artificial supramolecular reactor to control the reaction of aryl radicals with terminal olefins under green light/solvent conditions, which facilitates selective transformation in the confined enzyme-mimicking environment to give a series of geometrically defined E/Z-alkenes. The hydrophobic cage displays good host-guest inclusion with aromatic substrates, promoting Meerwein arylation and protecting E-isomeric products during reaction; while a small amount of benzonitrile can turn on efficient E→Z isomerization. Besides π-π stacking, the hydrogen bonding and halogen bonding interactions also act as control forces for the arylation of aliphatic terminal olefins known as poor acceptors in classic Meerwein arylation. The application of this switchable cage-confined arylation catalysis has been demonstrated by the syntheses of Tapinarof and a marine natural product from the same substrate via controllable E/Z selectivity.

Reaction kinetics of phenyl + phenylacetylene at combustion-relevant intermediate temperatures

The reaction between phenyl radical and phenylacetylene is a prototype for the reactions of aryl radicals and alkynyl peri‑condensed aromatic hydrocarbons (PCAHs), which may help explain the dimerization of PCAHs by covalent bond with acetylene assistance. In this work, we have experimentally and theoretically investigated reaction kinetics of phenyl radical and phenylacetylene. Gas chromatography-mass spectrometry (GC–MS) is applied to separate and identify molecular structures of reaction products in a jet-stirred reactor (JSR). Diphenylacetylene is observed as the major product of this reaction, which shows dominant concentration near 1000 K. Formation of phenanthrene increases with temperature, while 9-methylene-fluorene is a minor product, and 2-ethynyl-biphenyl is negligible. The potential energy surface and rate coefficients of phenyl and phenylacetylene reaction are calculated by quantum chemistry and transition state theory. 1,2-Diphenylvinyl is the only resonance-stabilized radical (RSR) among the adducts formed via all six possible addition reaction channels. Its formation reaction channel has a much lower energy barrier and deeper potential well and, therefore, it wins the competition in the reaction of phenyl and phenylacetylene at combustion-relevant intermediate temperatures.

Study on soot nucleation and growth from PAHs and some reactive species at flame temperatures by ReaxFF molecular dynamics

The transformations from polycyclic aromatic hydrocarbons (PAHs) to polymers and finally to young soot were simulated under different initial conditions at flame temperatures using Reax-FF MD. At low temperatures, PAH heterogeneous nucleation is mainly through physical interactions, but these clusters are not stable at higher temperatures. The molecular dynamics simulation results at 1450 K suggested that soot growth at intermediate temperature and low radical concentration conditions mainly controlled by chemical reactions involving carbon addition directly, without hydrogen abstraction. Some branched chains can be added to the PAH edges and small PAHs grow into dimers, trimers, or larger molecules through combination with unsaturated aliphatic hydrocarbons. At higher temperatures, radical-molecule reactions, such as via the HACA mechanism, reactions of aryl radicals and aryl-acetylene, and radical-radical reactions involving the combination of two aryl radicals, are dominant in soot formation because of high radical concentrations.

ChemInform Abstract: An Expedient Route to Heterocycles Through α‐Arylation of Ketones and Arylamides by Microwave Induced Thermal SRN1 Reactions.

Abstract2‐Aryl‐1‐phenylethanones are prepared by the reaction of aryl radicals that are generated from aryl iodides and ketone enolate anions which are generated by deprotonation of ketones with tBuOK in DMSO.

Gas-phase reactions of aryl radicals with 2-butyne: experimental and theoretical investigation employing the N-methyl-pyridinium-4-yl radical cation

Aromatic radicals form in a variety of reacting gas-phase systems, where their molecular weight growth reactions with unsaturated hydrocarbons are of considerable importance. We have investigated the ion-molecule reaction of the aromatic distonic N-methyl-pyridinium-4-yl (NMP) radical cation with 2-butyne (CH(3)C≡CCH(3)) using ion trap mass spectrometry. Comparison is made to high-level ab initio energy surfaces for the reaction of NMP and for the neutral phenyl radical system. The NMP radical cation reacts rapidly with 2-butyne at ambient temperature, due to the apparent absence of any barrier. The activated vinyl radical adduct predominantly dissociates via loss of a H atom, with lesser amounts of CH(3) loss. High-resolution Fourier transform ion cyclotron resonance (FT-ICR) mass spectrometry allows us to identify small quantities of the collisionally deactivated reaction adduct. Statistical reaction rate theory calculations (master equation/RRKM theory) on the NMP+2-butyne system support our experimental findings, and indicate a mechanism that predominantly involves an allylic resonance-stabilized radical formed via H atom shuttling between the aromatic ring and the C(4) side-chain, followed by cyclization and/or low-energy H atom β-scission reactions. A similar mechanism is demonstrated for the neutral phenyl radical (Ph˙)+2-butyne reaction, forming products that include 3-methylindene. The collisionally deactivated reaction adduct is predicted to be quenched in the form of a resonance-stabilized methylphenylallyl radical. Experiments using a 2,5-dichloro substituted methyl-pyridiniumyl radical cation revealed that in this case CH(3) loss from the 2-butyne adduct is favoured over H atom loss, verifying the key role of ortho H atoms, and the shuttling mechanism, in the reactions of aromatic radicals with alkynes. As well as being useful phenyl radical analogues, pyridiniumyl radical cations may form in the ionosphere of Titan, where they could undergo rapid molecular weight growth reactions to yield polycyclic aromatic nitrogen hydrocarbons (PANHs).

ChemInform Abstract: Radical Addition of Arylboronic Acids to Various Olefins under Oxidative Conditions.

AbstractThe reaction of aryl radicals, generated from the boronic acids, with olefins proceeds using Mn(OAc)3 via double olefin addition and aromatic substitution to provides tetrahydronaphthalene derivatives with moderate yields and diastereoselectivities under the inert atmosphere [cf.

ChemInform Abstract: Replacing Tin in Radical Chemistry: N-Ethylpiperidine Hypophosphite in Cyclization Reactions of Aryl Radicals.

ChemInform Abstract: Replacing Tin in Radical Chemistry: N-Ethylpiperidine Hypophosphite in Cyclization Reactions of Aryl Radicals.

Steric effects of nucleophile-radical coupling reaction. Determination of rate constants for the reaction of aryl radicals with 2-naphthoxide anion

The absolute rate constants for the reaction of different aryl radicals with 2-naphthoxide anion were determined using an indirect method, a competition of the coupling reaction with the H-atom abstraction. We here show that the radical-ambident nucleophile reactions are sensitive to steric hindrance. A lower reactivity is found for 2-anisyl radical with respect to 4-anisyl and for 2-methoxy-1-naphthyl radical with respect to 1-naphthyl (k2b < k2a and k2d < k2c). The ortho substitution to the radical centre decreases the rate constant. The reactivity is in agreement with energy barriers determined by theoretical calculations.

Steric Effects in the Reaction of Aryl Radicals on Surfaces

Steric effects are investigated in the reaction of aryl radicals with surfaces. The electrochemical reduction of 2-, 3-, 4-methyl, 2-methoxy, 2-ethyl, 2,6-, 2,4-, and 3,5-dimethyl, 4-tert-butyl, 3,5-bis-tert-butyl benzenediazonium, 3,5-bis(trifluoromethyl), and pentafluoro benzenediazonium tetrafluoroborates is examined in acetonitrile solutions. It leads to the formation of grafted layers only if the steric hindrance at the 2- or 2,6-position(s) is small. When the 3,5-positions are crowded with tert-butyl groups, the growth of the organic layer is limited by steric effects and a monolayer is formed. The efficiency of the grafting process is assessed by cyclic voltammetry, X-ray photoelectron spectroscopy, infrared, and ellipsometry. These experiments, together with density functional computations of bonding energies of substituted phenyl groups on a copper surface, are discussed in terms of the reactivity of aryl radicals in the electrografting reaction and in the growth of the polyaryl layer.

Iminyl Radicals from α-Azido o-Iodoanilides via 1,5-H Transfer Reactions of Aryl Radicals: New Transformation of α-Azido Acids to Decarboxylated Nitriles

The radical reaction of tributyltin hydride with o-iodo- N-methylanilides derived from alpha-azido acids provides an excellent access to alpha-(aminocarbonyl)iminyl radicals through 1,5-hydrogen transfer reaction of initially formed aryl radicals followed by beta-elimination of dinitrogen from ensuing alpha-azido-alpha-(aminocarbonyl)alkyl radicals. The outcoming iminyls display a peculiar tendency to form corresponding nitriles by beta-elimination of aminocarbonyl radicals.

Quantitating the effect of an ortho substituent on cyclization and intramolecular hydrogen-transfer reactions of aryl radicals.

Reduction of allyl 2-iodobenzyl malonates with triphenyltin hydrides generates aryl radicals that partition between 6-exo cyclization, 7-endo cyclization, and 1,5-hydrogen atom transfer. Rate constants for all of these processes are high (>10(8) M(-)(1) s(-)(1)), and the rates are only marginally reduced (<33%) by the introduction of methyl group ortho to the reacting radical.

N‐Azinylpyridinium N‐Aminides: An Approach to Pyrazolopyridines via an Intramolecular Radical Pathway.

AbstractFor Abstract see ChemInform Abstract in Full Text.

N-Azinylpyridinium N-Aminides: An Approach to Pyrazolopyridines via an Intramolecular Radical Pathway

Intramolecular radical arylation, under thermal condi- tions, to a -deficient pyridinium, linked to a -excessive 2-azi- nyliminopyridine moiety is described. The method allows a new entry to pyrazolo(1,5-a)pyridine nucleus. The generation and subsequent reactions of aryl radicals, derived from aryl halides using tri-n-butyltin hydride (Bu3SnH) and azobisisobutyronitrile (AIBN) is now well documented, and several syntheses, based on aryl radical cyclisations have been reported. 1 Few examples of het- eroaryl radicals are known, and presumably they would behave similarly to aryl radicals, since the lone electron would be in an orbital orthogonal to the aromatic -system and hence, its nature ( -excessive or -deficient) should have little or no effect on the reactivity of such radicals. 2 Particular attention has been devoted to pyridyl radicals; both Snieckus 3 and Nadin 4 have reported pyridyl radical cyclisations. Harrowven 5 has published some papers, which include pyridyl radical cyclisations and aryl radical cyclisations onto pyridines. Jones 6 has disclosed the use of radicals derived from 3-bromopyridine and the exten- sion of this chemistry to pyridinium radicals. 7 However, to the best of our knowledge, pyrazinyl radical cyclisa- tions have not been reported before.

Determination of Rate Constants for the Reaction of Aryl Radicals with Enolate Ions

The rate of ring closure (kC) of o-(ω-alkenyl)aryl radicals by means of an intramolecular attack of the intermediate phenyl-type radical at the tethered double bond was found to be 4.2 × 108 s−1 (for 1•) and 7.6×107 s−1 (for 8•), both being 6-exo-trig processes, and 9.6 × 109 s−1 for the 5-exo-trig process of 3•. The kC rate constant of these radicals was calibrated with respect to a known rate of H-atom abstraction (kH). The photostimulated SRN1 reactions of radical clock precursor 1 with anions PhS− and (EtO)2PO− in Me2SO at 25 °C provided the rates of addition of these nucleophiles (kY) to intermediate 1• (3.2 × 108 and 2.5 × 109 M−1 s−1, respectively). In contrast, the analogous reaction of a ketone enolate ion with precursor 1 did not take the expected SRN1 course; instead an elimination reaction was favoured. Similarly, the reactions of radical clock precursors 3 or 8 with the enolate ion failed. However, investigation of the distribution of 9-anthracenyl (11•) or 1-naphthyl (12•) radicals between two competing reactions, namely combination with a nucleophile and H abstraction from the solvent (Me2SO), was successful and eventually enabled us to find the kY values for the addition of the enolate ion to these two aryl radical intermediates (4.4 × 108 and 2.9 × 109 M−1 s−1, respectively).

Replacing tin in radical chemistry: N-ethylpiperidine hypophosphite in cyclisation reactions of aryl radicals

A detailed study of aryl radical cyclisations using N-ethylpiperidine hypophosphite shows that the reagent has advantages over tributyltin hydride in radical generation and reaction.

Modification of the Reductive Pathway in Gamma-Irradiated DNA by Electron Scavengers: Targeting the Sugar-Phosphate Backbone

Several electron scavengers that irreversibly form potential hydrogen-abstracting species upon one-electron reduction have been tested as agents for conversion of reductive damage to DNA bases into damage to the sugar-phosphate backbone. Electron spin resonance spectroscopy is employed to follow the production of radicals and transformations after irradiation. The scavengers tested included neutral (acrylamide, iodoacetamide) and cationic [triphenylsulfonium (Ph3S+), o,o'-diphenylenebromonium (DPB) and o,o'-diphenyleneiodonium (DPI)] compounds. Modification of reductive radiation damage in DNA is found to occur by scavenging of initial mobile electrons at low temperatures as well as thermally activated electron transfer from DNA electron-gain centers upon annealing. Electron transfer from the bases to hydrogen-bonded acrylamide has the smallest activation energy among other scavengers but produces a secondary alkyl radical incapable of abstracting hydrogen from the sugar-phosphate backbone. A primary alkyl radical generated from iodoacetamide has been shown to abstract preferentially from the thymine methyl group but not from deoxyribose moieties. Aryl radicals generated from aromatic onium salts such as Ph3S+, and especially DPI and DPB, are found to be the agents which best abstract hydrogen atoms from the deoxyribose portion of DNA. The use of DPB and DPI as radiation modifiers allows the elimination of undesirable side reactions of aryl radicals and through hydrogen abstraction results in high yields of a species identified as the DNA C1'. sugar radical. The second reaction pathway found for DPI and DPB in DNA is addition of an aryl radical to the thymine 5,6 double bond. Cysteamine is shown to preferentially eliminate sugar radicals upon annealing and to have little impact on the thermal stability of the thymine adduct radical.

Oxidation of Phenylhydrazones with Benzeneseleninic Anhydride: A New Mechanistically Interesting Observation

The effects by benzeneseleninic anhydride were studied. An unexpected by-product, 1,4-bis(phenylselanyl)benzene, was isolated from the oxidation reactions. Its formation was suppressed when pyridine was used as the co-solvent. Aryl phenyl selenides were synthesized by reaction of aryl radicals, generated from reduction of aryl diazonium salts, with diphenyl diselenide.

The Scope and Limitations of Intramolecular Radical Cyclizations of Acylsilanes with Alkyl, Aryl, and Vinyl Radicals

5-Exo cyclizations of primary and secondary radicals with acylsilanes successfully give cyclopentyl silyl ethers. The corresponding 6-exo cyclizations are sensitive to changes of the size of silyl groups. Secondary radicals undergo 6-exo cyclizations with acylsilanes more slowly. Reaction of aryl radical with acylsilane proceeds well for 5-exo cyclization but not for 6-exo cyclization. Vinyl radicals give best results in 5-exo cyclizations with acylsilanes but give low yields (∼30%) in 6-exo cyclizations. Intramolecular cyclizations of vinyl radicals with acylsilanes give enol silyl ethers regiospecifically.