Mechanism of the selective functionalization of saturated hydrocarbons by Gif systems: relationship with methane monooxygenase.

Abstract

GifIII, GifIV, and Go. In GoAggI these two reagents are naturally present. In GoAggII and GoAggIII, Fei"' plus H202 gives the same iron species. This formalism (Scheme I) is the same as that used in arguments about the mechanism of P450 oxidation (2,8).There are two well-known mechanisms for the selective oxidation of saturated hydrocarbons. The first is the Fenton reaction of H202 with Fe"l, which leads to the production of hydroxyl radicals (1). The second mechanism is the ironbased porphyrin system of the P450 enzymes. In porphyrin models, based on this system, alkoxy radical-like chemistry is clearly seen (2). Thus, saturated hydrocarbons are attacked with a selectivity normal to radical reactions (tertiary> secondary>primary), sulfides are oxidized to sulfoxides, and olefins are epoxidized (3, 4).The family of Gif systems for the selective substitution of saturated hydrocarbons has very different reactivity from any prior process (5, 6). We first define (7) the systems: GifIII consists of iron powder suspended in pyridine-acetic acid with oxygen as oxidant. GiflV is similar but has an Fe"l catalyst with suspended zinc dust as a source ofelectrons and again oxygen is the oxidant. The Gif-Orsay (Go) electrochemical system replaces the zinc dust in GiflV by the cathode of an electrochemical cell. This system has a good coulombic yield (up to 50%). More recently, three new systems have been added: GoAggI, GoAggII, and GoAggIII. The GoAggI system consists of pyridine-acetic acid with stoichiometric Fell and K02 under argon. The GoAggII system is the same except that the oxidant is hydrogen peroxide and the catalyst is FeCl3 or other Fe"' salt.As will be described in more detail later, the addition of picolinic acid and its congeners has a major effect on the rate of oxidation of saturated hydrocarbons while preserving the characteristic Gif selectivity. The system using Fe"' plus H202 in the presence of picolinic acid or other special ligand is now referred to as GoAggIII. The names are geographical: G is for Gif-sur-Yvette, 0 is for Orsay, and Agg is for "Aggieland," where the last three systems were developed.Fe" +°j - In the Go, GoAggI, GoAggII, and GoAggIII systems it is clear that pyridine is not being reduced to dipyridyls or other pyridine-derived species. The role of the pyridine is to act as a ligand and to prevent hydroxyl radical-induced autooxidation chains.With certain hydrocarbons (adamantane, cis-and transdecalins, and the steroid side chain) radicals have been detected but only at tertiary positions, not at secondary positions. In adamantane, tertiary radicals can be detected by competitive attack on oxygen and on pyridine. The reduction of oxygen pressure increases the amount of attack of the tertiary radicals on the pyridine. However, the selectivity (C2/C3, where C2 = total of secondary oxidation products and C3 = total oxidation and pyridine-coupled products) is always constant. Coupling at the secondary position is not seen. When the secondary coupled product is prepared by a rational synt...

Regio- and Stereoselectivity of Particulate Methane Monooxygenase from Methylococcus capsulatus (Bath)

“…18 Several mechanisms, which have been suggested to account for the observations, invoke alkyl free radical and/or carbocation structures as potential intermediate species 13,[18][19][20] or a concerted process involving direct oxygen insertion into a substrate carbon-iron bond. 21,22 Unfortunately, while pMMO warrants similar consideration due to its pervasive presence in methanotrophs, analogous studies using pMMO have been hindered by difficulty in maintaining the activity of the purified pMMO, as well as the limited substrate range of the enzyme. 1 However, the hydroxylation mechanisms of sMMO from Methylosinus trichosporium (OB3b) and pMMO from M. capsulatus (Bath) have recently been investigated by the use of cryptically chiral ethanes.…”

“…Determining the mechanism of C−H bond activation in biological systems is of great interest currently. , To this end, the mechanism of sMMO-mediated hydroxylation has been studied with radical spin-traps as well as radical clock substrate probes . Several mechanisms, which have been suggested to account for the observations, invoke alkyl free radical and/or carbocation structures as potential intermediate species 13,18-20 or a concerted process involving direct oxygen insertion into a substrate carbon−iron bond. , Unfortunately, while pMMO warrants similar consideration due to its pervasive presence in methanotrophs, analogous studies using pMMO have been hindered by difficulty in maintaining the activity of the purified pMMO, as well as the limited substrate range of the enzyme ,, The determination of the stereoselectivity in ethanol production using either ( R )- or ( S )-[1,1- 3 H 1 , 2 H 1 ] ethane demonstrated essentially 100% stereoselection for pMMO and retention accompanied by substantial inversion of configuration for sMMO.…”

Alkane oxidation by polynuclear non-haem iron complexes—an imidazole effect

“…Previous work has included studies using [Fe30(0A~)~]-HOAc-pyridine-0~,3 [Fe20(OA~)~(bipy)2Cl~]-tbhp~ and [ {Fe(HBpz3)(hfcac))20J-Zn-02.5 On the other hand, a (y-oxo)diiron(m) intermediate has been proposed as the active species during the selective oxidation of saturated hydrocarbons to ketones by Gif-type systems. 6 Here we report that simple binuclear and trinuclear 0x0-bridged complexes are good catalysts for oxidation of cyclohexane to cyclohexanol and cyclohexanone by tbhp and that addition of imidazole improves the efficiency and the selectivity of the reaction.…”

Catalytic oxidation of cyclohexane by a binuclear Fe(III) complex biomimetic to methane monooxygenase

“…Other iron catalytic systems have been published in the literature, as exemplified by the Gif systems studied by Barton and coworkers [31][32][33].…”