Radicals 2a Research Articles

Die Oxidation zweier neuer 1,3-Dihydroxyimidazolidine – Darstellung und Struktur von Nitronylnitroxid-Radikalen

The Oxidation of Two New 1,3-Dihydroxyimidazolidines – Synthesis and Structure of Nitronyl Nitroxide Radicals The two new 1,3-dihydroxyimidazolidines 3a and 3b were prepared by condensation of 4-formylbenzoic acid (2a) and sodium glyoxylate (2b) with 2,3-dimethyl-2,3-butanediylbis-(hydroxylamine) sulfate (1). The 1,3-dihydroxyimidazolidines were oxidized with sodium periodate in water to form nitronyl nitroxide radicals. Oxidation of the 1,3-dihydroxyimidazolidine 3a yielded the new radical 4a, whereas the product obtained from the 1,3-dihydroxyimidazolidine 3b was, due to decarboxylation, the unsubstituted radical 4c. The crystal structure of 4a was solved by single-crystal X-ray diffraction, showing the molecules to be arranged in zigzag chains as a consequence of intermolecular hydrogen bonds.

Syntheses and ESR spectra of radicals produced in the thermal decomposition of tetraarylsuccinonitriles

AbstractTetraarylsuccinonitriles 2a–e were synthesized via oxidative dimerization of diarylacetonitriles 1a–e in basic media. The thermal decomposition of 2 results in two identical cyanodiarylmethyl radicals 3. Both 1H and 13C NMR analyses of 1a–e and 2a–e and ESR spectra of 3a–e are presented and discussed. The results of ESR measurements provide a plausible explanation for the different reactivity of cyanodiarylmethyl radicals 3a–e in free radical polymerization.

Free radical induced oxidation of phloroglucinol. A pulse radiolysis and EPR study

Phloroglucinol-derived radicals have been studied using pulse radiolysis and EPR spectroscopy. Phloroglucinol 1a(pKa= 8.0) and its anion 1b(pKa= 9.2) have phenolic structures while the 3,5-dihydroxycyclohexa-2,5-dienone structure predominates in the dianion 2c. The neutral OH-adduct radicals (λmax= 345 nm) rapidly eliminate water (k= 2 × 105 s–1) yielding the 3,5-dihydroxyphenoxyl radical 4a(λmax= 495 nm, pKa= 6.5). This radical as well as its monoanion 4b(λmax= 550 nm, pKa= 8.6), its isomer 5b, derived from 2c(λmax > 800 nm), and the dianion 4c(λmax= 640 nm) can be generated directly with the N3 radical (k= 1.4 × 109 dm3 mol–1 s–1 at pH 6). All four radicals have been characterized by EPR spectroscopy. Radical 5b reacts with the phloroglucinol monoanion 1b with a rate constant of 2 × 107 dm3 mol–1 s–1. Formation of an adduct is excluded by EPR. Therefore, electron transfer from the phloroglucinol monoanion 1b to radical 5b is favoured as an explanation for this reaction.While 4a does not react with O2(k < 4 × 105 dm3 mol–1 s–1), the anions 4b and 5b do so quite rapidly (k= 2.1 × 108 and 1.9 × 108 dm3 mol–1 s–1, respectively) and at pH 7, O2 is consumed with G= 15 × 10–7 mol J–1.Although Br2˙– mainly produces radicals 4a and b, bromination occurs with an efficiency of at least 10%.

Preparation and ESR‐Spectroscopical Investigation of Remarkably Persistent Oxoisobenzofuranyl Radicals

AbstractThe synthesis of the diastereoisomeric 1,1′‐diaryl‐1,1′‐bi(isobenzofuran)‐3,3′(1H,1′H)‐diones 3a–d starting from the readily available 2‐aroylbenzoic acids 1a–d is described (Scheme 1). Of the colourless dimers 3a–d, only the sterically congested 3a and 3b dissociate at ambient temperature in solution to the deep red free 3‐oxoisobenzofuran‐1‐yl radicals 4a and 4b, respectively. The radicals 4a, b are extremely persistent in the absence of O2. The structures of these radicals are confirmed and the coupling constants assigned by ESR and ENDOR spectroscopy and computer simulation of their ESR spectra. The dissociation equilibrium constant at 20° in toluene for 3a is determined to be 1.18 · 10−5 M. By studying the steady‐state radical concentration as a function of temperature, the enthalpy and entropy changes for the homolytic dissociation of 3a are determined.

Sterically Hindered Free Radicals, 21. 1,2‐ and 1,4‐Additions of Diphenylmethyl Radicals to Substituted Acrylonitriles

AbstractAdditions of the substituted diphenylmethyl radicals Ar11‐Ar2CR 2 (RCMe3, SiMe3, GeMe3, SnMe3, OSiMe3, CF3, CO2Me, CN) to various acrylonitriles CH2C(X)CN 3 (XSMe, SiPr, StBu, OAc, OSiMe3, OSiEt3, OMe, OEt) lead to 1,2‐5 or 1,4‐adducts 6 (ketenimines), depending mainly on the steric hindrance by the substituents R and X. Bulky substituents like tBu in 2 and tBuS in the acrylonitrile favour the formation of the extended and nearly strainless ketenimine system 6 (1,4‐adduct); smaller substituents like OSiMe3 (radical 2) and SMe, OAc, OSiMe3, OSiEt3, OMe, OEt (acrylonitrile) allow isolation of the sterically crowded 1,2‐adducts. Substituents of intermediate bulkiness like CF3 (radical 2) and SiPr (acrylonitrile) give a mixture of 1,2‐adducts (6cb, hb) and dimers 7 of the adduct radicals 4 (7cb, hb). The voluminous tBu group directly bound to the olefin (3j,k) prevents addition. The latter is generally reversible, and the various adducts 5, 6, or 7 dissociate to the adduct radicals 4 and/or fragment to the initial radicals 2 at temperatures which reflect the steric strains of the corresponding substituents R, X. The complete inertness, even toward the electron‐rich olefin 3i, of the electrophilic α1‐carbonyl‐substituted radicals 2q – s (RCHO, COMe, COPh) in the above additions is discussed. Additions of the radicals 2a – c, f, i, n, o to the conjugated olefin 3n are described and are in accordance with the conclusion that steric effects predominate in adduct formation, whereas electronic effects are of distinct but minor importance.

Radikalreaktionen an N-Heterocyclen. XI. ESR-spektroskopischer Nachweis von Radikalen der 3-Methyl-pyrazolin-5-one

Free Radical Reactions of N-Heterocyclic Compounds. XI. ESR-Study of Radicals of 3-Methyl-pyrazolin-5-ones The oxidation of 1,4-substituted 3-methylpyrazolin-5-ones (1a – p) led to pyrazolinonyl radicals (2a – p), which were detected directly or after trapping with nitrosobenzene as phenyl nitroxides. In a few cases, the products of oxidation of pyrazolin-5-ones (1) with electron-withdrawing groups in 4-position by peroxy radicals were endo cyclic nitroxides (8), which were also detected by ESR spectroscopy.

Radikalreaktionen an N-Heterocyclen. XI [1] Reaktionen von 3-Methyl-pyrazolin-5-onen mit Phenoxyl-Radikalen

Free Radical Reactions of N-Heterocyclic Compounds. XI. Reaction of 3-Methyl-pyrazolin-5-ones with Phenoxy Radicals Pyrazolin-5-ones (3a–i) were oxidized with 2,4,6-trisubstituted phenoxy radicals (2a–d) to the corresponding radicals (4a–i), which dimerised or combined with phenoxy radical (2a) depending on the R1- and R4-substituents in (3). In the case of 3-methyl-1-phenyl-pyrazolin-5-one (3f) the primary radical combination products were not found, but the corresponding quinone methide (17) and the o-phenol derivative (18) were isolated. Products and yields have been investigated as a function of mol ratio substrate: oxidant and solvent. The radical combination products (7–10) could de-tertbutylated in the presence of aluminium chloride or in the presence of trifluoroarcetic acid, forming heterocyclic substituted phenols (21) and (22).

Picosecond radical kinetics. Ring openings of phenyl-substituted cyclopropylcarbinyl radicals

Rate constants for ring openings of the trans-(2-phenylcyclopropyl)carbinyl radical (1a), the cis-(2-phenylcyclopropyl)carbinyl radical (1b), and the (2,2-diphenylcyclopropyl)carbinyl radical (1c) were studied by competition kinetics using PTOC esters as radical precursors and hydrogen atom transfer trapping from benzeneselenol as the basis reaction. Radical 1a was studied in two solvents, toluene and THF; the experimental Arrhenius function for ring opening of 1a was log (κ r .s)=13.9-3.3/2.3RT (R in kcal/mol)

Polar Effects in Halogen Abstraction Reactions of Alkyl Radicals

AbstractIn a series of structurally similar alkyl radicals 1a–c the tertiary 1,1‐dimethyl‐5‐hexenyl radical 1c reacts 30 times faster with carbon tetrachloride than the primary 5‐hexenyl radical 1a. The reactivity of the secondary 1‐methyl‐5‐hexenyl radical 1b aligns itself in between the primary and the tertiary radical 1a and 1c. The results indicate that the increasing nucleophilicity of the alkyl radicals is the major factor contributing to the reactivity.

Dehydrodimers of arylaldoxime—‐a convenient source of iminoxy radicals

AbstractArylaldoxime dehydrodimers 3a—1 have been prepared from the corresponding oximes 1a—1 by using (HCrO4)2(Py)4Co as oxidant. Their structures were assigned to be bisnitrone by XPS studies. ESR studies of 3a—1 in hot chloroform show that iminoxy radicals 2a—1 are formed. Therefore the dehydrodimers can provide a convenient source of these radicals.

1,2-Stereoinduction in acyclic radicals: allylic strain effects

ESR Experiments and AM1 calculations of ester substituted radicals show that acyclic radicals 3a,b exist in preferred conformations I and II. The stereoselectivity of the hydrogen abstraction can be explained by the attack anti to the Bu t group of conformers I and II, respectively.

Formation of sulfur-centered cation radicals by photofragmentation

The selected dialkyl dithioethers 1,5-dithiocane, 1,4-dithiepane, 1,4-dithiane, and 2,6-dithiaheptane are readily monoalkylated in nitromethane by tert-butyl O-trifluoromethanesulfonyl-3-hydroxyperoxypropanoate, 6, to give the corresponding sulfonium salt peresters 2a-c, and 3 in good yield. Laser flash photolysis of these compounds affords the known two-sulfur, three-electron stabilized cation radicals 2a, b, and 5 which were characterized by optical absorption spectroscopy.

Manganese(III)-mediated tandem oxidative cyclizations of .beta.-diesters. Influence of copper(II) upon the chemoselectivity of the reaction

Mn(III)-mediated oxidative cyclization of esters derived from allylmalonic and benzylmalonic acids afforded through two consecutive cyclizations bislactones (17,5) and bicyclic lactones (19,21) or tricyclic lactones (7,9). The ratio of these products is controlled by the stereoselectivity of the first step involving the intramolecular addition of a carbon-centered radical to a double bond. Oxidations conducted either in the presence of stoichiometric cupric ion or without added Cu(II) provide a comprehensive view of the respective influences of Mn(III) and Cu(II) upon the chemoselectivity of the reaction, which are related to their ability to oxidize radicals of different classes. For example, the primary and secondary radicals 3a and 3b, which are not efficiently oxidized by Mn(III), undergo cyclization leading to the trans fused tricyclic γ-lactones 9a and 9b exclusively in the absence of cupric ion

Kinetics and mechanism of some vinyl radical cyclisations

The vinyl radicals 2a, 2b, and 11 each undergo fast exo ring closure to give 5a, 5b, and 12, the first two of which readily rearrange to ring-expanded radicals.

Elimination of Ammonium Ion from the a-Hydroxyalkyl Radicals of Serine and Threonine in Aqueous Solution and the Difference in the Reaction Mechanism

Abstract The zwitterionic radicals HO-ĊH-CH(COO-)NH3 + (4a) and HO-Ċ(CH3)-CH(COO-)NH3 + (4b) are the main species produced upon OH· radical attack in aqueous solutions at pH 3-7 at the amino acids serine, HO-CH2-CH(COO-)NH3 +, or threonine, HO-CH(CH3)-CH(COO-)NH3 +, respectively. Both radicals undergo elimination of NH4 + ion to form the radicals O=CH-ĊH-COO- (7) or CH3-CO-ĊH-COO- (9) respectively. The pKa of the serine-derived cationic radical HO-ĊH-CH(COOH)NH3 + (3a) (3a ⇄ 4a + H+), was determined by ESR spectroscopy to 2.2 ± 0.1 at 276 K. From kinetic data the pKa(OH) of radical 4a (4a ⇄ O-ĊH-CH(COO-)NH3 + (5a) + H+) was calculated to 7.0. The elimination of NH3 takes place from the ketyl radical 5a (type-B mechanism), the rate constant was calculated from kinetic data to 2.4 × 106 s-1 at 290 K. The half-lives of radicals 4a and 4b were measured by time-resolved conductivity changes upon pulse radiolysis, 170 ± 10 μs for 4a and 26 ± 2 μs for 4b, at 290 K and pH 5.8 . With the threonine derived radicals elimination of NH3 takes place at the stage of the α-hydroxyalkyl radical 4b (type-A mechanism). In this series the pKa of the product radical CH3-CO-ĊH-COOH (8) (8 ⇄ 9 + H+), was determined by ERS spectroscopy to 2.7 ± 0.1. The reasons for the observed mechanistic differences (type-A versus type-B decay) are discussed. As further examples for a type-B decay some preliminary data on the elimination of HF from the radicals CF3-Ċ(OH)-CF3 and CF3-ĊH-OH have been added.

Benzenesulphenanilidyl radicals. Part 3. Reactions of 4′-substituted benzenesulphenanilides with t-butoxyl radicals

Decomposition products of the 4′-substituted benzenesulphenanilidyl radicals (2a, b, d, and e), generated from the corresponding benzenesulphenanilides (1) with t-butoxyl radicals, have been investigated in benzene, acetonitrile, and acetone. Results indicate that the decomposition modes exhibited by these radicals are not essentially influenced by variation of the solvent polarity; comparison of the findings with present and previous results from the oxidation of (1) with lead dioxide show that the chemical reactivity trend displaced by the thioaminyls (2) can be greatly influenced both by the reaction medium and the 4′-substituent. Evidence is presented that the benzenesulphenanilides (1b) and (1e) react in acetone in the presence of di-t-butyl hyponitrite to give products ascribable to homolytic substitution at the sulphenanilide sulphur by acetonyl radicals.

Benzenesulphenanilidyl radicals. Reactivity of 4′-methoxy- and 4′-methoxy-2-nitro-benzenesulphenanilidyl radicals

Oxidation of 4′-methoxybenzenesulphenanilide (1a) with lead dioxide at 30 °C afforded the NN-bis(phenylthio)p-anisidine (7a) and 2,7-dimethoxyphenazine (3a) together with minor amounts of diphenyl disulphide (4a), whereas at 10 °C almost exclusive formation of compound (7a) and the o-benzoquinone di-imine (8a) was observed; compound (8a) was found to be thermally unstable and to give, quantitatively, compounds (4a) and (3a). The reaction products are rationalized by assuming that 4′-methoxybenzenesulphenanilidyl radicals (2a), produced by oxidation of compound (1a), undergo Cortho–N coupling to give the dimer (9a), which is responsible for the reaction products observed. A comparable trend was observed by producing the radical (2a) from reaction of compound (1a) with t-butoxy-radicals and by thermal decomposition of compound (7a). These findings rule out the nitrene mechanism previously suggested for the formation of compound (3) from oxidation of the reagent (1a). Evidence has been found that Cortho–N coupling is the main reaction path, exhibited also by 4′-methoxy-2-nitrobenzenesulphenanilidyl radicals (2c) generated from the corresponding anilide (1c) by reaction with t-butoxy-radicals or oxidation with lead dioxide, which thus rejects a previous claim that the radicals (2)(Ar = Ph and Ar = C6H4NO2-o) exhibit different chemical behaviour.

Structural information from liquid- and solid-phase ESR: The example of triphenyl-substituted radicals (C6H5)3A of Group IVB Elements

The objective of this article is to show in terms of a specific radical system how ESR data can be used to obtain structural information.

Formation of N‐Hydroxy‐amines of Spin Labeled Nucleosides for 1H‐NMR. Analysis

AbstractAmine‐oxide radical 1a was efficiently converted to the corresponding N‐hydroxy‐amine 2a with sodium dithionite in acetone/water. This reaction was used to develop a procedure for monitoring the NMR. spectra of sodium dithionite reduced amine‐oxide radicals and of novel reduced amine‐oxide‐labeled nucleosides.

Intramolecular addition of aryl radicals to the azo-group. Part 2. Production of some N-(carbazol-9-yl)arylaminyl radicals and their reactivity

The N-(carbazol-9-yl)arylaminyl radicals (2a–d) have been produced by intramolecular addition of the 2′-arylazobiphenyl-2-yl radicals (1a–d) generated by reduction of the corresponding arenediazonium tetrafluoroborates (3a–d) and their reactivity has been investigated. N-(Carbazol-9-yl)-4-tolylaminyl (2a) and N-(carbazol-9-yl)4-chiorophenylaminyl (2b) exhibit N,N-coupling at –20° C, whereas hydrogen abstraction and Cortho,N-coupling appear to be the main reaction paths at room temperature. A comparable trend has been observed with 1-(4-chlorophenyl)-2,2-diphenylhydrazyl radicals (14) generated by oxidation of the corresponding hydrazine (13) with lead dioxide. From the N-(carbazol-9-yl)biphenyl-2-ylaminyl radicals (2c) and (2d) some evidence has been obtained that they are capable of undergoing intramolecular addition to aromatic rings.