Alkane hydroperoxidation

The mechanism was substantiated by independent treatment of alkane hydroperoxides with Magic Acid.603 Similarly, Baeyer-Villiger oxidation of several ketones in the presence of H2O2 and superacids gave similar product compositions. 

Scheme IX, 10. Selective aerobic photochemical alkane hydroperoxidation catalyzed by the system quincme-copper n) acetate .

Hydroperoxides have been obtained from the autoxidation of alkanes, aralkanes, alkenes, ketones, enols, hydrazones, aromatic amines, amides, ethers, acetals, alcohols, and organomineral compounds, eg, Grignard reagents (10,45). In autoxidations involving hydrazones, double-bond migration occurs with the formation of hydroperoxy—azo compounds via free-radical chain processes (10,59) (eq. 20). 

Reaction conditions depend on the reactants and usually involve acid or base catalysis. Examples of X include sulfate, acid sulfate, alkane- or arenesulfonate, chloride, bromide, hydroxyl, alkoxide, perchlorate, etc. RX can also be an alkyl orthoformate or alkyl carboxylate. The reaction of cycHc alkylating agents, eg, epoxides and a2iridines, with sodium or potassium salts of alkyl hydroperoxides also promotes formation of dialkyl peroxides (44,66). Olefinic alkylating agents include acycHc and cycHc olefinic hydrocarbons, vinyl and isopropenyl ethers, enamines, A[-vinylamides, vinyl sulfonates, divinyl sulfone, and a, P-unsaturated compounds, eg, methyl acrylate, mesityl oxide, acrylamide, and acrylonitrile (44,66). 

Autoxidation of alkanes generally promotes the formation of alkyl hydroperoxides, but d4-tert-huty peroxide has been obtained in >30% yield by the bromine-catalyzed oxidation of isobutane (66). In the presence of iodine, styrene also has been oxidized to the corresponding peroxide (44). 

Alkane oxidation via a hydroperoxide was suggested many years ago, and seems to be operative in Acinetobacter sp. strain M-1 that has, in addition, a rather unusual range of substrates that include both n-alkanes and -alkenes. The purified enzyme contains FAD and requires copper for activity (Maeng et al. 1996). 

In yet another version of adopting a biphase system, oxidation of alkanes with rert-butyl hydroperoxide has been conducted with an aqueous phase. Launay et al. (1998) have developed an efficient and highly selective conversion of cyclo-octane to cyclo-octanone using Ru colloidial particles formed in situ from RuCli. 5H2O. The aqueous phase can be recycled. 

This test is used for both in vitro and in vivo determinations. It involves reacting thiobarbituric acid (TBA) with malondialdehyde (MDA), produced by lipid hydroperoxide decomposition, to form a red chromophore with peak absorbance at 532 nm (Fig. 10.1). The TBARS reaction is not specific. Many other substances, including other alkanals, proteins, sucrose, or urea, may react with TBA to form colored species that can interfere with this assay. 

To perform the dissociation of the hydrocarbon to alkyl radicals with C—C bond scission, a hydrocarbon molecule should absorb light with the wavelength 270-370 nm. However, alkanes do not absorb light with such wavelength. Therefore, photosensitizers are used for free radical initiation in hydrocarbons. Mercury vapor has been used as a sensitizer for the generation of free radicals in the oxidized hydrocarbon [206-212], Nalbandyan [212-214] was the first to study the photooxidation of methane, ethane, and propane using Hg vapor as photosensitizer. Hydroperoxide was isolated as the product of propane oxidation at room temperature. The quantum yield of hydroperoxide was found to be >2, that is, oxidation occurs with short chains. The following scheme of propane photoxidation was proposed [117] ... 

These data appeared to be very useful for the estimation of the relative O H bond dissociation energies in hydroperoxides formed from peroxyl radicals of oxidized ethers. All reactions of the type R02 + RH (RH is hydrocarbon) are reactions of the same class (see Chapter 6). All these reactions are divided into three groups RO + R (alkane, parameter bre = 13.62 (kJ moC1)172, R02 + R2H (olefin, bre = 15.21 (kJ mob1)1 2, and R02 + R3H (akylaromatic hydrocarbon), hrc 14.32 (kJ mol )12 [71], Only one factor, namely reaction enthalpy, determines the activation energy of the reaction inside one group of reactions. Also,... 

The peculiarities of the oxidation of PP, whose molecules have alternating tertiary C—H bonds in the (3-position, are of special interest. Such branched alkanes are oxidized with the formation of polyatomic hydroperoxides produced by the intramolecular isomerization of the peroxyl radical [88],... 

We emphasize that the above mechanism is strictly valid only for H202 and alkyl hydroperoxide epoxidations of alkenes catalyzed by TS-1 and Ti-MCM-41. In view of the observation of similar titanium oxo species when H2 + 02 are brought in contact with TS-1 or Ti-MCM-41 (54), similar conclusions may be drawn for that system as well. A radical mechanism involving the Ti=0 groups had been proposed earlier by Khouw et al. (221) for the hydroxylation of alkanes. No spectroscopic investigation of the TS-l/H202/alkane has yet been reported. 

The calcined iron-grafted materials exhibit high selectivity as catalysts for oxidations of alkanes, alkenes and arenes with H2O2 as the oxidants [13a]. A similar method has been used by Tilley et al. to prepare a pseudotetrahedral (Co(II) [Co(4,4 -di Bu-bipy) OSi(0 Bu)3 2]) complex grafted onto the SBA-15 surface and subsequently use it in catalytic oxidation of alkylaromatic substrates with tert-butyl hydroperoxide [14]. Unfortunately, neither iron nor cobalt surface organometaUic compounds have been tested in the recycled catalytic system. 

The involvement of transition metal peroxo species in the oxidative functionalization of alkanes and arenes has been postulated for several metals with both hydrogen peroxide and alkyl hydroperoxides. 

Analogously, alkanes have been oxidized with anhydrous hydrogen peroxide and catalytic amounts of MTO in CHsCN . With these substrates, the main products are alkyl hydroperoxides together with alcohols and ketones. 

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