Gif Chemistry Research Articles

Quantum Chemical Studies on Dioxygen Activation and Methane Hydroxylation by Diiron and Dicopper Species as well as Related Metal–Oxo Species

Abstract Quantum chemical studies by the author and co-workers on dioxygen activation and methane hydroxylation by diiron and dicopper enzyme species as well as related metal–oxo species are reviewed. The activation of the O–O bond of dioxygen at the mono- and dimetal sites of metalloenzymes is an essential process in the initial catalytic stages of naturally occurring oxygenation reactions. A way of orbital thinking about dioxygen activation at diiron and dicopper enzymes is developed at the extended Hückel level of theory. Two different reaction pathways that lead from dimetal peroxo complexes with µ-η1:η1-O2 and µ-η2:η2-O2 modes to the corresponding dioxo complexes are discussed in detail. In considering the mechanism of methane hydroxylation, special attention to FeO+-mediated methane hydroxylation that occurs under ion cyclotron resonance (ICR) conditions is given. Mechanistic aspects about methane hydroxylation by the bare transition-metal oxide ions ScO+, TiO+, VO+, CrO+, MnO+, FeO+, CoO+, NiO+, and CuO+ are systematically analyzed on the basis of density functional theory (DFT) calculations. An important feature in the reaction is the spin crossover between the high-spin and low-spin potential energy surfaces in particular in the C–H activation process. The spin inversion from the high-spin state to the low-spin state effectively decreases the barrier height of C–H activation. Another feature in the reaction is that no radical species is involved in the course of the hydroxylation because the methyl species formed as a result of the C–H bond cleavage can directly coordinate to the metal active site. The coupling of the OH and CH3 ligands occurs to produce methanol at the metal active center. The reaction profiles obtained from DFT calculations are similar to those of the Gif chemistry proposed by D. H. R. Barton, in particular the involvement of the HO–M–CH3 species as an intermediate in the hydroxylation of methane. The nonradical mechanism is extended to methane hydroxylation by the diiron and dicopper species of methane monooxygenase (MMO). This mechanism is possible when the metal active center is coordinatively unsaturated to have a space for the coordination of the OH and CH3 groups as ligands. Although compelling discussion to support the involvement of oxygen- and carbon-centered radicals is provided, the nonradical mechanism still seems to be applicable for methane hydroxylation from the viewpoint of reaction selectivity. Kinetic isotope effects (KIEs) in the C–H activation process by the bare FeO+ complex and diiron and dicopper enzyme models are compared with respect to the radical and nonradical mechanisms by using transition state theory.

ChemInform Abstract: Comments on an Article by Francesco Minisci and Francesca Fontana.

Abstract Evidence is summarized to show that the latest aspects of Gif chemistry do not involve Fenton-type radical chemistry.

Gif Chemistry for Oxidation of Activated Methylenes to Ketones.

AbstractFor Abstract see ChemInform Abstract in Full Text.

Gif Chemistry for Oxidation of Activated Methylenes to Ketones

CN (5 mL), stirred for30 min and the substrate (10 mmol) was slowly added. 30%aqueous t-BuOOH (30 mmol) was then added dropwise withsyringe pump for 30 min and the reaction went on for theappropriate period. Sodium oxalate (3.73 mmol) was addedto eliminate Fe(III) ion. The filterate was evaporated todryness that was dissolved in small amount CH

The gif paradox.

This Account summarizes research work on the structural aspects and functional features encountered in all major branches of the Gif family of hydrocarbon-oxidizing reagents. Despite assertions by the inventor of Gif chemistry, D. H. R. Barton, to the effect that nonradical pathways could better explain the behavior of Gif systems, detailed experimental investigations provide compelling evidence to support the preponderance of oxygen- and carbon-centered radical chemistry.

Gif chemistry: new evidence for a non-radical process

When performed under a nitrogen monoxide atmosphere, Gif oxidation of cyclohexane with hydrogen peroxide gives no changes in activity or selectivity, showing that triplet oxygen is neither formed nor required in the oxidation. Gif oxidation of cis- and trans-decalin occurs preferentially at secondary positions giving high yields of ketones. Oxidation of tertiary positions is as low as approximately 5% and probably involves free radicals. Oxidation of cyclohexane in the presence of mannitol gives no significant loss of activity, showing that OH radicals are not involved in Gif oxidation. Based on these results the mechanism of Gif oxidation is discussed.

Functional Aspects of Gif-Type Oxidation of Hydrocarbons Mediated by Iron Picolinate H2O2-Dependent Systems: Evidence for the Generation of Carbon- and Oxygen-Centered Radicals

The present investigation explores the functional features of several novel and other previously ill-defined ferrous and ferric complexes of the picolinic acid anion (Pic), which are used to mediate Gif-type oxidation of hydrocarbons by H2O2. Complexes [Fe(Pic)2(py)2], [Fe(Pic)3]·0.5py, [Fe2O(Pic)4(py)2], [Fe2(μ-OH)2(Pic)4], and FeCl3 have been employed in oxygenations of adamantane by H2O2 mostly in py/AcOH to reveal that tert- and sec-adamantyl radicals are generated in Gif solutions. The alleged absence of sec-adamantyl radicals from Gif product profiles has been previously interpreted as compelling evidence in support of a non-radical mechanism for the activation of secondary C−H sites in Gif chemistry. The product profile is entirely dictated by trapping of the diffusively free adamantyl radicals, since authentic tert- and sec-adamantyl radicals are shown to partition between dioxygen and protonated pyridine at 4% O2 (in N2), or between dioxygen and TEMPO at 100% O2, in a manner analogous to that observed in Gif oxygenations of adamantane. The low tert/sec selectivity (2.2−4.5) obtained, increasing with increasing dioxygen partial pressure, and the small intramolecular kinetic isotope effect values revealed by employing adamantane-1,3-d2 (1.06(6) (Ar); 1.73(2) (4% O2 in N2)), indicate the presence of an indiscriminate oxidant under inert atmosphere, coupled to a more selective oxidant at higher partial pressures of dioxygen. Gif oxygenation of DMSO by H2O2 mediated by [Fe(Pic)2(py)2] provides pyridine-trapped methyl radicals under argon, as expected for the addition reaction of hydroxyl radicals to DMSO. The reaction is progressively inhibited by increasing amounts of EtOH, generating pyridine-captured CH3•CHOH and •CH2CH2OH radicals. Quantification of the DMSO- versus EtOH-derived alkyl radicals affords an estimate of kEtOH/kDMSO equal to 0.34(3), in reasonable agreement with the kinetics of radiolytically produced hydroxyl radicals (kEtOH/kDMSO = 0.29). The formation of methyl radicals in Gif oxygenation of DMSO is also supported by the quantitative generation of tert-adamantyl radicals in the presence of 1-iodoadamantane. These results are consistent with the action of hydroxyl radicals in Gif oxygenations by FeII/III/H2O2 (Ar), most likely coupled to substrate-centered alkoxyl radicals under O2. The oxygen-centered radicals perform H-atom abstractions from Gif substrates to generate diffusively free carbon-centered radicals, in accord with previously reported findings.

ChemInform Abstract: Distinction Between Radical Chemistry and Gif Chemistry. Competitive Oxidation of Alkanes and sec‐Alcohols.

AbstractChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 100 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select a “Full Text” option. The original article is trackable via the “References” option.

On the distinction between radical chemistry and Gif chemistry competitive oxidation of alkanes and sec.-alcohols

By comparing the reactivities of alcohols and alkanes towards TBHP radical oxidation and non-radical Gif oxidation, we bring additional evidence regarding the non-radical nature of Gif chemistry.

Gif chemistry: The present situation

Gif chemistry: The present situation

The selective functionalization of saturated hydrocarbons. Part 44. Measurement of size of reagent by variation of steric demands of competing substrates using Gif chemistry

Cyclohexane has been co-oxidized with a number of aromatic compounds of varying steric requirements. The oxidant was hydrogen peroxide with Fe III-picolinic acid in a Gif type solvent of pyridine and acetonitrile. Both the cyclohexane and the aromatic derivatives were reacting with the same iron species. From the steric requirements for the hydroxylation of the aromatic partner, it can be deduced that the iron species is of significant size. For this reason, and also because of known pulse radiolysis results, hydroxyl radicals are not involved.

Evidence for a higher oxidation state of manganese in the reaction of dinuclear manganese complexes with oxidants. Comparison with iron based Gif chemistry

Binuclear manganese complexes mimic the catalase enzyme by converting hydrogen peroxide rapidly and efficiently to oxygen and water. The complex (1) may be activated by either periodic acid or Oxone ® and can oxidize selected organic substrates. Potassium manganate gave similar oxidation products suggesting that the manganese is transformed to a higher oxidation state. Kinetic studies with the Mn IV-Mn IV complex show an induction period indicating that it is not the active catalyst. Further studies suggested that the actual catalytic species is a Mn III-Mn IV complex. These complexes show similar properties to the activation of FeCl 3 with hydrogen peroxide. This is particularly evident by the formation of a new and unusual peroxide from ergosterol acetate.

ChemInform Abstract: The Selective Functionalization of Saturated Hydrocarbons: Recent Developments in Gif Chemistry

AbstractChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 100 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select a “Full Text” option. The original article is trackable via the “References” option.

On the distinction between radical chemistry and Gif chemistry

Normalized data for cyclooctane and for cyclododecane are compared with cyclohexane as unity. In radical chemistry cyclooctane is 2–3 times more reactive per hydrogen, whereas in Gif chemistry cyclohexane is more reactive than cyclooctane or cyclododecane. Thus by simple competitive experiments a distinction between radical chemistry and Gif chemistry can be made.

ChemInform Abstract: The Functionalization of Saturated Hydrocarbons. Part 39. Further Evidence for the Role of the Iron‐Carbon Bond in Gif Chemistry.

AbstractChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 100 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select a “Full Text” option. The original article is trackable via the “References” option.

Synthesis, Reactivity, and Catalytic Behavior of Iron/Zinc-Containing Species Involved in Oxidation of Hydrocarbons under Gif-Type Conditions

The present study explores the nature and reactivity of iron- and zinc-containing species generated in hydrocarbon-oxidizing GifIV-type solutions (Fe catalyst/Zn/O2 in pyridine/acetic acid (10:1 v/v). The ultimate goal of this investigation is to unravel the role of metal sites in mediating dioxygen-dependent C−H activation, which in the case of Gif chemistry demonstrates an enhanced selectivity for the ketonization of secondary carbons. Reaction of [Fe3O(O2CCH3)6(py)3]·py (1) with zinc powder in CH3CN/CH3COOH or CH2Cl2/CH3COOH affords the trinuclear compound [Zn2FeII(O2CCH3)6(py)2] (2). Single-crystal X-ray analysis confirms that one monodentate and two bidentate acetate groups bridge adjacent pairs of metals with the iron atom occupying a centrosymmetric position. The analogous reduction of 1 in py/CH3COOH (10:1, 5:1, 2:1 v/v) yields [FeII(O2CCH3)2(py)4] (3), [FeII2(O2CCH3)4(py)3]n (4), and [Zn(O2CCH3)2(py)2] (5) depending on the isolation procedure employed. Compound 3 possesses a distorted octahedral geometry, featuring a C2 axis bisecting the equatorial, pyridine-occupied plane, whereas the two acetate groups reside along the perpendicular axis. Compound 4 is a one-dimensional solid constructed by asymmetric diferrous units. Two bidentate and one monodentate acetate groups bridge the two iron sites, with the monodentate bridge also acting as a chelator to one ferrous center. The two iron centers exhibit weak antiferromagnetic coupling. Compounds 3 and 4 are also accessible from the reduction of 1 with iron powder or treatment with H2/Pd. Solutions of 3 and 4 in pyridine or py/CH3COOH react with pure dioxygen or air to eventually regenerate 1 in a concentration-dependent manner. Oxidation of 2 in py/CH3COOH with pure dioxygen or air yields [Fe2.22(2)Zn0.78(2)O(O2CCH3)6(py)3]·py (1‘) and [Zn2(O2CCH3)4(py)2] (6). Compound 1‘ is isostructural to 1, exhibiting rhombohedral symmetry at 223 K. The filtrate of the reduction of 1 with zinc in neat pyridine, when exposed to dioxygen, affords dichroic red−green crystals of monoclinic [Fe2ZnO(O2CCH3)6(py)3]·py (1‘‘). Species 1‘‘ yields products identical with those provided by 1 under reducing conditions. Compounds 2−6 are related by pyridine-dependent equilibria, as demonstrated by mutual interconversions and electronic absorption data in pyridine and py/CH3COOH solutions. In non-pyridine solutions, Zn-containing species 5 and 6 rearrange to the crystallographically characterized species [Zn(O2CCH3)2(py)]n (7) and [Zn3(O2CCH3)6(py)2] (8). Compound 7 is a one-dimensional solid featuring a chain of Zn sites linked by a bidentate acetate group while additionally coordinated by a chelating acetate. Compound 8 is isostructural to 2. Further perturbations of the described structures are apparent in ionic iron-containing species, such as the pseudo-seven-coordinate iron in [Ph3PNPPh3][FeII(O2CCH3)3(py)] (9), which is obtained from the reaction of 3 with [PPN][O2CCH3], and the water-coordinated iron in [FeII(H2O)4(trans-py)2][O2CCH3]2 (10), which reveals an extensive two-dimensional network of hydrogen-bonding interactions. The pyridine-free species [FeII3(O2CCH3)6(OS(CD3)2)2]n (11) is isolable upon extensive incubation of 3 in (CD3)2SO. Compound 11 exhibits a remarkable one-dimensional structure, featuring four different types of acetate groups. Catalytic oxidations of adamantane, isopentane, benzene, toluene, cis-stilbene, and pyridine mediated by the system 1 (or 2−4)/Zn/O2 in py/AcOH (10:1) afford product profiles which are not fully compatible with the reported outcome of analogous oxidations by hydroxyl radicals or biologically relevant high-valent iron−oxo species alone. The intermolecular deuterium kinetic isotope effect for the oxidation of adamantane to adamantanone is small (kH/kD = 2.01(12)) by comparison to values obtained for oxidation of hydrocarbons by biological oxygenases. Employment of hydrogen peroxide, t-BuOOH, or peracetic acid as potential oxo donors does not provide viable shunt pathways in the catalytic oxygenation of adamantane. The nature of active oxidant in GifIV-type oxidation is discussed in light of these structural and functional findings.

The functionalization of saturated hydrocarbons. Part 39. Further evidence for the role of the iron-carbon bond in Gif chemistry

The functionalization of saturated hydrocarbons utilizing the Gif oxidation system is considered to occur via an intermediate containing an FeC bond. Iodine and iodide ion have been found to capture efficiently the FeC bond to give the corresponding alkyl iodide in both the FeII-FeIV and FeIII-FeV manifolds. This trapping of the FeC bond in both manifolds is shown to be non-radical in nature.

On the Distinction between Gif Chemistry and TBHP Based Radical Chemistry

On the Distinction between Gif Chemistry and TBHP Based Radical Chemistry

ChemInform Abstract: The Functionalization of Saturated Hydrocarbons by Gif Chemistry. Part 1. Use of Superoxide and of Hydrogen Peroxide. Part 2. Use of t‐ Butylhydroperoxide (TBHP)

AbstractChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 100 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select a “Full Text” option. The original article is trackable via the “References” option.

The selective functionalization of saturated hydrocarbons: Recent developments in Gif chemistry

Abstract