Aerobic oxidation of alkanes

The supported Co2+-substituted Wells-Dawson POM, Cs6H2[P2W17061Co(OH2)], on silica was stable up to 773 K and catalyzed the heterogeneous oxidation of various aldehydes to the corresponding carboxylic acids with 02 as a sole oxidant [116], The H5PV2Mo10O40 POM, impregnated onto meso-porous MCM-41, catalyzed the aerobic oxidation of alkanes and alkenes using isobutyraldehyde as a... 

We prepared Ru(TPFPP)(CO) (33) complex for the first time, and showed it to be an efficient catalyst for the aerobic oxidation of alkanes using acetaldehyde [156]. Thus, the 33-catalyzed oxidation of cyclohexane with molecular oxygen in the presence of acetaldehyde gave cyclohexanone and cydohexanol in 62% yields based on acetaldehyde with high turnover numbers of 14 000 (Eq. 3.100). 

Very few methods have been reported for direct aerobic oxidation of alkanes using a perfluorinated ruthenium catalyst [Ru30(0C0CF2CF2CF3)6(Et20)3] [120c] and a ruthenium-substituted polyoxometalate [WZnRu2(0H)(H20)(ZnW9034)2] (Eqs. 3.102 and 3.103) [159, 160[. 

Aerobic oxidation of alkanes.1 Various metal complexes arc known to catalyze air oxidation of unactivatcd C-—H bonds. Murahashi et al. have found that both ruthenium and iron complexes arc useful catalysts for aerobic oxidation in combination with an aldehyde and an acid. Iron powder is the most effective catalyst, but FeCl3 6H2(), RuCI3 H20, and RuCI2[P(C6H5)3]3 can be used. Useful aldehydes arc hcptanal, 2-mcthylpropanal, and even acetaldehyde. A weak acid is suitable thus acetic acid is preferred to chloroacetic acid. By using the most satisfactory conditions, cyclohexane... 

Murahashi and coworkers have studied the aerobic oxidation of alkanes catalyzed by poly-fluorinated metalloporphyrins including Ru" (TPFPP)(CO) in the presence of acetaldehyde (Scheme 1.13) . ... 

Aerobic oxidation of alkanes requires dioxygen, hence it was originated only after plant photosynthesis thereby producing most part of the dioxygen on earth. Meanwhile methane is produced in anaerobic methanogenesis, therefore apparently this biological process existed before when life on earth was... 

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). 

Here we show a novel methodology for the functionalizations of hydrocarbons, including oxygenation, nitration, sulfoxidation, epoxidation, carboxylation, and oxyalkylation through generation of the catalytic carbon radical. In particular, the NHPI-catalyzed aerobic oxidations of alkanes, which are very important in industry worldwide, are described in detail. 

Co-containing POMs have been found to be among the most efficient catalysts for homogeneous aerobic oxidation and co-oxidation processes [91-93]. This prompted many researchers to design solid Co-POM-containing materials [78,94-100]. Thus, various Co-POMs have been deposited on cotton cloth [94] and silica [100], datively [95] or electrostatically [96,97] bonded to NH2-modified silica surfaces (vide infra) as well as intercalated in LDHs [78,98,99]. The resulting materials were successfully used for aerobic oxidation of aldehydes, alkenes, alkanes, alcohols and some other organic substrates. 

It is appropriate and very instructive to briefly discuss a relatively new and very successful approach, namely, the development of catalysts with designed and atomically engineered active centers. Thomas and coworkers used micro- and meso-porous solids and carried out delicate structural and compositional variations to prepare specific catalysts capable of promoting regioselective, shape-selective, and enantioselective conversions.183-185 This strategy resulted in the development of framework-substituted CoALPO-18 and MnALPO-18 molecular sieves for the selective aerobic oxidation of linear alkanes to the corresponding monocarboxylic acids,186 and that of hexane to adipic acid.187 Framework-substituted MALPO-36... 

Gold NPs deposited on carbons are active and selective for mild oxidations in liquid phase although they exhibit almost no catalytic activity in the gas phase. Examples are aerobic oxidation of mono-alcohols, diols, glycerol, glucose, alkenes and alkanes. 

Aerobic hydrocarbon metabolism has been well studied and reviewed by an number of authors (1-6L Two general pathways are shown in Figures 1 and 2. The initial oxidation of alkanes commonly occurs on the terminal carbon yielding an alcohol (Figure 1). This is further oxidized to an aldehyde and then to a carboxylic acid which undergoes a series of (5-oxidations. Each p-oxidation yields acetic acid and a carboxylic acid with two fewer carbon atoms than the previous acid. 

Recently the Co/Mn/N-hydroxyphthalimide (NHPI) systems of Ishii have been added to the list of aerobic oxidations of hydrocarbons, including both aromatic side chains and alkanes. For example, toluene was oxidized to benzoic acid at 25°C [125] and cyclohexane afforded adipic acid in 73% selectivity at 73% conversion [126], see Fig. 4.46. A related system, employing N-hydroxysac-charine, instead of NHPI was reported for the selective oxidation of large ring cycloalkanes [127]. 

Co(acac)3 in combination with N-hydroxyphthalimide (NHPI) as cocatalyst mediates the aerobic oxidation of primary and secondary alcohols, to the corresponding carboxylic acids and ketones, respectively, e.g. Fig. 4.71 [205]. By analogy with other oxidations mediated by the Co/NHPI catalyst studied by Ishii and coworkers [206, 207], Fig. 4.71 probably involves a free radical mechanism. We attribute the promoting effect of NHPI to its ability to efficiently scavenge alkylperoxy radicals, suppressing the rate of termination by combination of al-kylperoxy radicals (see above for alkane oxidation). 

In the past, acetic acid was produced by aerobic oxidation of n-butane (and also of light naphtha, which is mainly a mixture of liquid -alkanes up to C9H2o). The process catalyzed by cobalt(II) acetate, closely resembled the one-step... 

Table 16.4 Aerobic partial oxidation of alkanes by metal oxide catalysts. 

Nonporphyrin-based oxo-metal species can be generated by the reaction of a low-valent ruthenium complex with molecular oxygen in the presence of an aldehyde [141]. Thus, the ruthenium-catalyzed oxidation of alkanes with molecular oxygen in the presence of acetaldehyde gives alcohols and ketones efficiently [155]. These aerobic oxidations can be rationalized by assuming the sequence shown in Scheme 3.10. 

Recently, we found that a copper catalyst - as well as ruthenium - is effective for the oxidation of alkanes with molecular oxygen in the presence of acetaldehyde [157]. The catalytic system CUCI2 and 18-crown-6 has proved to be efficient [157cj. Furthermore, we found that specific copper complexes derived from copper salts and acetonitrile are convenient and highly useful catalysts for the aerobic oxidation of unactivated hydrocarbons [158], For example, oxidation of cyclohexane with molecular oxygen (1 atm of O2 diluted with 8 atm of N2) in the presence of acetaldehyde and Cu(OAc)2 catalyst (0.0025 mol%) in CH3CN/CH2CI2 (3 2) at 70°C in an... 

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