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Relevant academic research and scientific papers

ESR Studies of deuteriated Polycyclic Aromatic Radical Cations

Several deuteriated polycyclic aromatic radical cations were studied by ESR.Their hyperfine coupling constants are reported and mechanism for their formation are proposed.Key Words: Friedel-Crafts reaction; Polycyclic aromatic radical cations; ESR

The Preparation of some Specifically Tritium Labelled Carcinogenic Hydrocarbons and their Covalent Binding to DNA

Syntheses are described for the preparation of anthracene, benzopyrene, and 1>-7,12-dimethylbenzanthracene.The nature and extent of covalent binding of these hydrocarbons to calf thymus DNA resulting from ultraviolet or gamma irradiation, from oxidative processes involving iodine or hydrogen peroxide, or from oxygen in conjunction with NADPH and a rat liver microsomal preparation is investigated.The displacement of tritium on hydrocarbon bonding to DNA identifies position-6 in benzopyrene and position-9 in anthracene as sites for DNA binding by irradiation processes or by chemical oxidation.Metabolic binding of benzopyrene to DNA shows an altered regioselectivity while that for 7,12-dimethylbenzanthracene appears to involve the 7-methyl group.

Photochemical Transformations. 40. Syn and Anti Migration in Photo-Wagner-Meerwein Rearrangements

The photochemistry of the trans and cis isomers of 7,8-dichloro-2,3:5,6-dibenzobicycloocta-2,5-diene (1 and 3) has been explored.The singlet excited states of these compounds are photoactive.In acetonitrile, mixtures of exo- and endo-4,anti-8-dichloro-2,3:6,7-dibenzobicycloocta-2,6-diene (2-Cl), the syn-8-chloro epimers exo- and endo-4-Cl, N-(anti-8-chloro-2,3:6,7-dibenzobicycloocta-2,6-dien-exo-4-yl)acetamide (2-NHAc) and its syn-8-chloro epimer exo-4-NHAc were produced.In acetic acid, the dichloro compounds and a mixture of the anti- andsyn-8-chloro-2,3:6,7-dibenzobicycloocta-2,6-dien-exo-4-yl acetates (2-OAc and 4-OAc) were produced.In cyclohexane, irradiation of 2 gave the dichlorides epimeric at C-4 and C-8.All of the photoreactions proceeded with a preponderance of migration with retention at the migration terminus (syn migrations), in contrast to the ground-state reactions which proceed with clean inversion at the migration terminus (anti migration).Failure to see stereospecificity was shown not to be due to C-8 bridge migration.Quantum yields for the various reactions are reported.The results are discussed in terms of several reaction channels following excitation and electron transfer.Radical reductive monodechlorinations of 1 and 3 lead to stereoconvergent radical rearrangements.The syntheses of 2-NHAc and 4-NHAc by Ritter reactions from 2-OAc and 4-OAc respectively are described.

A Mononuclear Non-heme Iron(III)-Peroxo Complex with an Unprecedented High O-O Stretch and Electrophilic Reactivity

A mononuclear non-heme iron(III)-peroxo complex, [Fe(III)(O2)(13-TMC)]+ (1), was synthesized and characterized spectroscopically; the characterization with electron paramagnetic resonance, M?ssbauer, X-ray absorption, and resonance Raman spectroscopies and mass spectrometry supported a high-spin S = 5/2 Fe(III) species binding an O2 unit. A notable observation was an unusually high νO-O at ～1000 cm-1 for the peroxo ligand. With regard to reactivity, 1 showed electrophilic reactivity in H atom abstraction (HAA) and O atom transfer (OAT) reactions. In the HAT reaction, a kinetic isotope effect (KIE) value of 5.8 was obtained in the oxidation of 9,10-dihydroanthracene. In the OAT reaction, a negative ρ value of -0.61 in the Hammett plot was determined in the oxidation of p-X-substituted thioanisoles. Another interesting observation was the electrophilic reactivity of 1 in the oxidation of benzaldehyde derivatives, such as a negative ρ value of -0.77 in the Hammett plot and a KIE value of 2.2. To the best of our knowledge, the present study reports the first example of a mononuclear non-heme iron(III)-peroxo complex with an unusually high νO-O value and unprecedented electrophilic reactivity in oxidation reactions.

Insight into the chemoselective aromatic: Vs. side-chain hydroxylation of alkylaromatics with H2O2catalyzed by a non-heme imine-based iron complex

The oxidation of a series of alkylaromatic compounds with H2O2 catalyzed by an imine-based non-heme iron complex prepared in situ by reaction of 2-picolylaldehyde, 2-picolylamine, and Fe(OTf)2 in a 2?:?2?:?1 ratio leads to a marked chemoselectivity for aromatic ring hydroxylation over side-chain oxidation. This selectivity is herein investigated in detail. Side-chain/ring oxygenated product ratio was found to increase upon decreasing the bond dissociation energy (BDE) of the benzylic C-H bond in line with expectation. Evidence for competitive reactions leading either to aromatic hydroxylation via electrophilic aromatic substitution or side-chain oxidation via benzylic hydrogen atom abstraction, promoted by a metal-based oxidant, has been provided by kinetic isotope effect analysis. This journal is

1,4-Dehydrogenation with a Two-Coordinate Cyclic (Alkyl)(amino)silylene

Cyclic (alkyl)(amino)silylene (CAASi) 1 has been found to successfully dehydrogenate 1,4-dihydroaromatic compounds containing various substituents to afford the corresponding aromatic compounds. The observed high substrate generality proves 1 to be a potential 1,4-dehydrogenation reagent for organic compounds. For the reaction with 9,10-dimethyl-9,10-dihydroanthracene, silylene 1 activated not only benzylic C?H bonds but also aromatic C?H bonds to yield a silaacenaphthene derivative, which is an unprecedented reaction of silylenes. The results of the experimental and computational study of the reaction of CAASi 1 with 9,10-dihydroanthracene and 1,4-cyclohexadiene are consistent with the notion that 1,4-dehydrogenation with CAASi 1 proceeds mainly through a stepwise hydrogen-abstraction mechanism.

Hydrogen atom transfer reactions of imido manganese(V) corroie: One reaction with two mechanistic pathways

Hydrogen atom transfer (HAT) reactions of (tpfc)MnNTs have been investigated (tpfc = 5,10,-15-tris(pentafluorophenyl)corrole and Ts = p-toluenesulfonate). 9,10-Dihydroanthracene and 1,4-dihydrobenzene reduce (tpfc)MnNTs via HAT with second-order rate constants 0.16 ± 0.03 and 0.17 ± 0.01 M-1 s-1, respectively, at 22°C. The products are the respective arenes, TsNH2 and (tpfc)MnIII. Conversion of (tpfc)MnNTs to (tpfc)Mn by reaction with dihydroanthracene exhibits isosbestic behavior, and formation of 9,9′,10,10′- tetrahydrobianthracene is not observed, suggesting that the intermediate anthracene radical rebounds in a second fast step without accumulation of a MnIV intermediate. The imido complex (tpfc)-MnVNTs abstracts a hydrogen atom from phenols as well. For example, 2,6-di-tert-butyl phenol is oxidized to the corresponding phenoxyl radical with a second-order rate constant of 0.32 ± 0.02 M-1 s-1 at 22°C. The other products from imido manganese(V) are TsNH2 and the trivalent manganese corrole. Unlike reaction with dihydroarenes, when phenols are used isosbestic behavior is not observed, and formation of (tpfc)-Mn IV(NHTs) is confirmed by EPR spectroscopy. A Hammett plot for various p-substituted 2,6-di-tert-butyl phenols yields a V-shaped dependence on σ, with electron-donating substituents exhibiting the expected negative ρ while electron-withdrawing substituents fall above the linear fit (i.e., positive ρ). Similarly, a bond dissociation enthalpy (BDE) correlation places electron-withdrawing substituents above the well-defined negative slope found for the electron-donating substituents. Thus two mechanisms are established for HAT reactions in this system, namely, concerted proton - electron transfer and proton-gated electron transfer in which proton transfer is followed by electron transfer.

Oxidation of polycyclic aromatic hydrocarbons catalyzed by iron tetrasulfophthalocyanine FePcS: Inverse isotope effects and oxygen labeling studies

Iron(III) tetrasulfophthalocyanine (FePcS) was shown to catalyze the oxidation of polycyclic aromatic hydrocarbons by H2O2. Benzo[a]pyrene and anthracene were converted to the corresponding quinones while biphenyl-2,2′-dicarboxylic acid was the main product of phenanthrene oxidation. The mechanism of the anthracene oxidation by H2O2 in the presence of FePcS or by KHSO5 with iron(III) mesotetrakis(3,5-disulfonatomesityl)porphyrin (FeTMPS) (see Figure 1 for catalyst structures) has been investigated in details by using kinetic isotope effects (KIEs) and 18O labeling studies. KIEs measured on the substrate consumption in the competitive oxidation of [H10] anthracene and [D10]anthracene by FePcS/H2O2 and FeTMPS/KHSO5 were essentially the same, 0.75 ± 0.02 and 0.76 ± 0.06, respectively. These inverse KIEs on the first oxidation step can be explained by the sp2-to-sp3 hybridization change during the addition of an electrophilic oxoiron complex to the sp2 carbon center of anthracene to form a σ adduct (this inverse KIE being enhanced by stronger slacking interactions between the perdeuterated substrate with the macrocyclic catalyst). Although the first oxidation step seems to be the same, different distribution of the oxidation products of anthracene and very different 18O incorporation into anthrone and anthraquinone in catalytic oxidations performed in the presence of H218O suggested that different active species should be responsible for anthracene oxidation in both catalytic systems. All the results obtained are compatible with an involvement of TMPSFeV=O (or TMPS+FeIV=O), having two redox equivalents above the iron(III) state of the metalloporphyrin precursor, while PcSFeIV=O (one redox equivalent above FeIII state of FePcS) was proposed to be the active species in the metallophthalocyanine-based system.

Hydrogen-Transfer Reactions, 14. - The Dehydrogenation of 1,4-Dihydroarenes by Manganese Oxidants

Hydrogen abstraction from 1,4-dihydroarenes by both MnO2 and the KMnO4-dicyclohexanocrown-6 complex occurs successively.In the first, rate-determining step one hydrogen is transferred.A synchronous transfer (by cyclic or termolecular pathways), a SET mechanism, or the addition of the permanganate ion to the double bond are not in accord with the large isotope effects.A moderate cis-selectivity is found for the abstraction of the second hydrogen by MnO2.