Alkane Oxidations with Dioxygen

During the past several decades, a number of catalytic systems have been developed for the oxidation of alkanes with dioxygen in the presence of reducing agents, for 

NHPI Species (30 pmol) Co(acac)2 (3 pmoD Mn(OAc)2 (0.3 imol) 

Catalytic aerobic oxidation of ethane to acetic acid was successfully performed through a catalytic radical process using NHPl derivatives combined with a Co(II) salt in acetonitrile or propanoic acid. Among the catalysts examined, N,N-dihydroxypyr-omellitimide (NDHPI) was found to be the best, for instance, when a mixture of ethane (20 atm) and air (20 atm) in acetonitrile was allowed to react in the presence of NDHPI (100 xmol) and Co(OAc)2 (30 gmol) at 150 °C for 15 h, 830 gmol of acetic acid was obtained, and the turnover number (TON) of NDHPI reached 8.3 (Eq. (6.2)). In this reaction, other products such as ethanol or acetaldehyde were not detected at all. 

In the oxidation of ethane using NHPI as a catalyst under these conditions, the amount of NHPI used was twice that of NDHPI, but the yield of acetic acid and the TON of the catalyst were 530 [xmol and 2.7, respectively. The highest TON (15.3) was obtained when the reaction was carried out using NDPHI combined with C0CI2 in propanoic acid [35]. 

The autoxidation of isobutane is now mainly carried out to obtain terf-butyl hydroperoxide [36]. Halogenated metalloporphyrin complexes are reported to be efficient catalysts for the aerobic oxidation of isobutane [18,37]. It was found that the oxidation of isobutane by air (lOatm) catalyzed by NHPI and Co(OAc)2 in benzoni-trile at 100 °C produced tert-butyl alcohol in high yield (81%) along with acetone (14%) (Eq. (6.3)) [38]. 2-Methylbutane was converted into the carbon arbon bond-deaved products, acetone and acetic acid, rather than the alcohols, as prindpal products. These cleaved products seem to be formed via P-sdssion of an alkoxy radical derived from the decomposition of a hydroperoxide by Co ions. The extent of the P Scission is known to depend on the stability of the radicals released from the alkoxy radicals [39]. It is thought that the 3-scission of a terf-butoxy radical to acetone and a methyl radical occurs with more difficulty than that of a 2-methylbutoxy radical to acetone and an ethyl radical. As a result, isobutane produces terf-butyl alcohol as the principal product, while 2-methylbutane affords mainly acetone and acetic acid. 

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An important goal is, therefore, to develop effective methods for catalytic oxidations with dioxygen, under mild conditions in the liquid phase. Two substrates which are often chosen as models for alkane oxidations are cyclohexane and adamantane. Cyclohexane is of immense industrial importance as its oxidation products - cyclohexanone and adipic acid - are the raw materials for the manufacture of nylon-6 and nylon-6,6. Adamantane is an interesting substrate as the ratio of oxidation at the secondary versus the tertiary C-H bonds is used as a measure of radical versus nonradical oxidation pathways. Industrial processes for the oxidation of cyclohexane, to a mixture of cyclohexanol and cyclohexanone, generally involve low conversions (under 10%). Even at such low conversions, selectivities are modest (70-80%) and substantial amounts of overoxidation products, mostly dicarboxylic acids, are formed. 

Several interesting variations on the above radical chemistry have been described recently. One such system is copper salt catalyzed alkane oxidation by dioxygen in the presence of an aldehyde [17]. The proposed mechanism involves the initial autoxidation of the aldehyde to the corresponding peracid, which is the real oxidant for the Cu"-mediated oxidation of the alkane (eqs. (3)-(5)). The ratio of alkane oxidized to aldehyde converted is relatively low because much of the peracid formed reacts with the aldehyde to form two molecules of carboxylic acid. 

Instead of the dioxygen-reductant pair, one can employ oxo componds containing an oxygen atom which is already partly reduced H2O2 [68], ROOH [69], PhIO [70], NaOCl [71], KHSO. [72], amine Af-oxides [73], and magnesium monoperoxyphtalate [74] (see also Chapter X). One of the most efficient (in terms of reaction rate and turnover number) systems is the combination of ruthenium porphyrin and 2,6-dichloropyridine A-oxide [73]. A simplified mechanism of alkane oxidation with iodosylbenzene catalyzed by iron porphyrinate is demonstrated in Scheme XI. 17. 

This enzyme [EC 1.14.15.3], also known as alkane 1-monooxygenase, lauric acid ca-hydroxylase, fatty acid hydroxylate fatty acids in the [Pg.47]

Using the analogy of model reactions of alkane oxidation in mixtures ofFe(II) and dioxygen in solvents, a mechanism invoking the formation of intermediate with an iron-carbon bond followed by interaction with soxygen was proposed (Waller and Fimscomb, 1996 Shilov, 1997). 

Recently, Sen has reported two catalytic systems, one heterogeneous and the other homogeneous, which simultaneously activate dioxygen and alkane C-H bonds, resulting in direct oxidations of alkanes. In the first system, metallic palladium was found to catalyze the oxidation of methane and ethane by dioxygen in aqueous medium at 70-110 °C in the presence of carbon monoxide [40]. In aqueous medium, formic acid was the observed oxidation product from methane while acetic acid, together with some formic acid, was formed from ethane [40 a]. No alkane oxidation was observed in the absence of added carbon monoxide. The essential role of carbon monoxide in achieving difficult alkane oxidation was shown by a competition experiment between ethane and ethanol, both in the presence and absence of carbon monoxide. In the absence of added carbon monoxide, only ethanol was oxidized. When carbon monoxide was added, almost half of the products were derived from ethane. Thus, the more inert ethane was oxidized only in the presence of added carbon monoxide. 

There are many reviews that cover various aspects of oxidation. These include ones on alkane activation,166 catalytic selective oxidation,167 metal complexes of dioxygen,168 metal-catalyzed oxidation,169 biomimetic oxidations,170 oxidation with peroxides,171 catalytic oxidations with peroxides,172 catalytic oxidations with oxygen,173 oxidations with dioxiranes,174 and oxidation of pollutants.175... 

The general reaction path operative in metal ion (complex) catalyzed alkane (alkyl group) oxidation consists in the generation of free radicals, which subsequently react with dioxygen to produce hydroperoxy radicals and, via H-atom abstraction, hydroperoxides. The latter are often stable products but also important intermediates in further oxidation. Hydroperoxides can decompose thermally to an alcohol and 0 ... 

Aerobic oxidation of alkanes is also possible, using dioxygen as the terminal oxidant. In these cases, Ru-porphyrin and RuCla systems have been shown to oxidize cyclohexane to cyclohexanone in the presence of acetaldehyde, with a fairly high turnover number (TON = 14,100 moles/(mole catalyst-h)). The mechanism for alkane oxidation remains largely unexplored but is suspected to be similar to the oxo-transfer mechanism that governs epoxidation of alkenes (44). 

Oxidation. Since some metalloenzymes such as cytochrome P-450 and methane mono-oxygenase can selectively oxidize alkanes with dioxygen, there are many approaches that mimic the reactivity of these metalloenzymes using various artificial metal complexes (1,23,24). The first approach is to combine dioxygen and reducing agents in the presence of metal complexes (eq. (12)). 

M. Lin, T. Hogan, A. Sen, A highly catalytic bimetallic system for the low-temperature selective oxidation of methane and lower alkanes with dioxygen as the oxidant, J. Am. Chem. Soc. 119 (1997) 6048-6053. 

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