Cyclohexane complexes, with iron

Shul pin and coworkers described the application of a di- and a tetranudear iron complex with triazacyclononane acetate ligands in the oxidation of alkanes and alcohols with H202 [47]. A highly complex tetranudear iron complex with octadentate pyridine carboxylate ligands was described by Gutkina et al. [48]. However, the TONs for cyclohexane oxidation did not exceed 5.0 in the latter case. 

Fe20(0Ac)2Cl2(bipy)2 successfully hydroxylates C6, C3, and C2 alkanes when tert-butyl hydrogen peroxide (TBHP) is used as the oxygen donor ([5] [TBHP]-.[substrate] = 1 150 1100) the observed reactivity is C6 > C3 > C2 (Table IV). This work represents the first report of the oxidation of a small molecular weight alkane (ethane) by a characterized iron model compound. Reactions of this complex with Zn dust and acetic acid under 1 atm of dioxygen with cyclohexane gave rise to only cyclohexanone (turnover number 2.5). The parent tetrameric compound, 6, was reported to be a more efficient catalyst. 

Various alkane oxidations are catalyzed by iron complexes. Such reactions are important in view of the action of non-heme iron enzymes, such as methane monooxygenase, in hydrocarbon oxidations in biological systems. For example, the oxo-bridged complex [Fe2(TPA)2(ju,-0)(ju.-0Ac)]3+ [TPA = tris(2-pyridylmethyl)-amine] catalyzes the oxidation of cyclohexane with Bu OOH. Related complexes with an Fein2(/i-0)(/i-0Ac)2 core oxidize cyclohexane or adamantane to give a mixture of alcohols and ketones.159 Less well defined systems, e.g., FeCl3-6H20/ aldehyde/AcOH/02 are similarly active.160... 

Nakamura et al. successfully synthesized tetramethylporphyrin (TMP) complexes of iron and manganese in NaY zeolite.[161] They investigated the catalytic properties of the composite for oxidation of cyclohexane in the presence of H2O2. The results indicate that the catalytic activity of [Fe(TMP)]-Y and [Mn(TMP)]-Y is enhanced in comparison with the corresponding Fe" and Mnn exchanged Y zeolite, and the catalytic product consists... 

Various iron salts and mononuclear Fe or binuclear Fe complexes with a N,0 environment, biomimetic to methane monooxygenase complexes, have been applied to the oxidation of cyclohexane with various oxidants [6u,v,7a-g], but their catalytic activity is usually modest, with the exception of a hexanuclear Fe(III) compound derived from p-nitrobenzoic acid, which gives the highest total yield to Ol/One of about 30% [7a]. Moreover, most of these complexes are often unstable and very expensive. A hexanuclear heterotrimetallic Fe/Cu/Co complex bearing two Cu(p-0)2Co(p-0)2Fe cores, prepared by self-assembly, oxidizes cyclohexane with aqueous HP, with a maximum yield to Ol/One of 45%, virtually total selectivity to the two compounds, and preferred formation of cyclohexanol [7hj. The remarkable activity of the Fe/Cu/Co cluster was associated with the synergic effect of the three metals. 

Cartoni et al. [88] studied perspective of the use as stationary phases of n-nonyl- -diketonates of metals such as beryllium (m.p. 53°C), aluminium (m.p. 40°C), nickel (m.p. 48°C) and zinc (liquid at room temperature). These stationary phases show selective retention of alcohols. The retention increases from tertiary to primary alcohols. Alcohols are retained strongly on the beryllium and zinc chelates, but the greatest retention occurs on the nickel chelate. The high retention is due to the fact that the alcohols produce complexes with jS-diketonates of the above metals. Similar results were obtained with the use of di-2-ethylhexyl phosphates with zirconium, cobalt and thorium as stationary phases [89]. 6i et al. [153] used optically active copper(II) complexes as stationary phases for the separation of a-hydroxycarboxylic acid ester enantiomers. Schurig and Weber [158] used manganese(ll)—bis (3-heptafiuorobutyryl-li -camphorate) as a selective stationary phase for the resolution of racemic cycUc ethers by complexation GC. Picker and Sievers [157] proposed lanthanide metal chelates as selective complexing sorbents for GC. Suspensions of complexes in the liquid phase can also be used as stationary phases. Pecsok and Vary [90], for example, showed that suspensions of metal phthalocyanines (e.g., of iron) in a silicone fluid are able to react with volatile ligands. They were used for the separation of hexane-cyclohexane-pentanone and pentane-water-methanol mixtures. 

Udenfriend suggested a simple non-enzymatic system as a possible model of monooxygenase [44]. It involves an iron(II) complex with EDTA and ascorbic acid and is capable of hydroxylating aromatic compounds. Later, Hamilton found that this system was able to hydroxylate cyclohexane (however, with very low yield) and epoxidize cyclohexene [45]. Afterwards, a number of similar systems were proposed. For example, Ullrich found two models which are more effective than the Udenfriend system. One of them includes a Sn(II) phosphate complex... 

In oxidation of alkylaromatic compounds and cyclohexane, considerable activity is displayed by iron complexes with terminally functionalized acetyl-acetone 22-25, dipyridyl 26-27 and catechol 28-29 ethylene oxide, the monobutyl... 

These reactions are extremely sensitive to conditions, e.g., temperature, solvent, etc. The best one is the reaction of the palladium complex (XXXVII) with iron pentacarbonyl to give (XIII). At 80° C the yield ranges from 87% in benzene, cyclohexane, or 1,2-dichloroethane, to 17% in tetrahydro-furan-diglyme and is zero in carbon tetrachloride (77). The reaction of (XXXVII) with molybdenum hexacarbonyl only occurs in aromatic hydrocarbon solvents and since the complex (XL) is obtained in the same yield starting from either molybdenum hexacarbonyl or benzenemolyb-denum tricarbonyl, it is reasonable to assume the latter to be an intermediate in this reaction. 

Within the cubane synthesis the initially produced cyclobutadiene moiety (see p. 329) is only stable as an iron(O) complex (M. Avram, 1964 G.F. Emerson, 1965 M.P. Cava, 1967). When this complex is destroyed by oxidation with cerium(lV) in the presence of a dienophilic quinone derivative, the cycloaddition takes place immediately. Irradiation leads to a further cyclobutane ring closure. The cubane synthesis also exemplifies another general approach to cyclobutane derivatives. This starts with cyclopentanone or cyclohexane-dione derivatives which are brominated and treated with strong base. A Favorskii rearrangement then leads to ring contraction (J.C. Barborak, 1966). 

Inspired by Gif or GoAgg type chemistry [77], iron carboxylates were investigated for the oxidation of cyclohexane, recently. For example, Schmid and coworkers showed that a hexanuclear iron /t-nitrobenzoate [Fe603(0H) (p-N02C6H4C00)n(dmf)4] with an unprecedented [Fe6 03(p3-0)(p2-0H)] " core is the most active catalyst [86]. In the oxidation of cyclohexane with only 0.3 mol% of the hexanuclear iron complex, total yields up to 30% of the corresponding alcohol and ketone were achieved with 50% H2O2 (5.5-8 equiv.) as terminal oxidant. The ratio of the obtained products was between 1 1 and 1 1.5 and suggests a Haber-Weiss radical chain mechanism [87, 88] or a cyclohexyl hydroperoxide as primary oxidation product. 

Evaporation of the filtrate and trituration of the oily solid with 10 1 hexanes/cyclohexane affords an additional 2.80 g (mp 85-86°C) for an overall yield of 25.25 g (54%). Yields for this step are typically in the range 45-55%. The filtrate of this second crop and the orange band from the chromatography still contain more of this valuable ligand. Further chromatography does not appreciably improve this material, as the major impurities coelute with the terpyridine. This additional amount of terpyridine can be isolated from the mixture as Fe(terpy)2(PF6)2 and the ligand separated from the complex by the method of Constable.9 A total of 3.40 g of the iron(II) complex (which corresponds to 1.95 g of terpyridine) can be isolated in this manner. Including this product increases the overall yield to 58%. ... 

The porphyrin-iron(III)-peroxo complex [Fe(TPP)02] (163) was prepared by the reaction of K02 with Fen(TPP) in the presence of a crown ether, and characterized by spectroscopic methods [p(0—O) = 806 cm-1]542. This peroxo complex (163) was found to be inactive toward hydrocarbons. However, addition of excess acetic anhydride to (163) dissolved in a benzene-cyclohexane mixture results in the formation of cyclohexanol and cyclohexanone. This reaction is thought to proceed via acylation of the peroxo group, giving iron percarboxylate (164), which decomposes to an Fev-oxo compound (165) capable of hydroxylating alkanes.543 Such a mechanism has been suggested for the hydroxylation of camphor by Pseudomonas cytochrome P-450.544... 

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