Oxygenation of saturated hydrocarbons

The superacid-catalyzed electrophile oxygenation of saturated hydrocarbons, including methane with hydrogen peroxide (via H302 ) or ozone (via HOs ), allowed the efficient preparation of oxygenated derivatives. 

Biological systems overcome the inherent unreactive character of 02 by means of metalloproteins (enzymes) that activate dioxygen for selective reaction with organic substrates. For example, the cytochrome P-450 proteins (thiolated protoporphyrin IX catalytic centers) facihtate the epoxidation of alkenes, the demethylation of Al-methylamines (via formation of formaldehyde), the oxidative cleavage of a-diols to aldehydes and ketones, and the monooxygenation of aliphatic and aromatic hydrocarbons (RH) (equation 104). The methane monooxygenase proteins (MMO, dinuclear nonheme iron centers) catalyze similar oxygenation of saturated hydrocarbons (equation 105). ... 

The oxygenation of saturated hydrocarbons by methane mono-oxygenases (binuclear nonheme iron proteins, MMO) via the two-electron reduction of... 

The methane mono-oxygenase proteins (MMO, binuclear nonheme iron centers) catalyze similar oxygenation of saturated hydrocarbons - ... 

The activation of dioxygen for the mono-oxygenation of saturated hydrocarbons by the methane mono-oxygenase enzyme systems (MMO hydroxylase/reductase) represents an almost unique biochemical oxygenase, especially for the transformation of methane to methanol. l The basic process involves the insertion of an oxygen atom into the C-H bond of the hydrocarbon via the concerted reduction of O2 by the reductase cofactor... 

Scheme II.6. Mechanisms proposed for the oxygenation of saturated hydrocarbons by dioxirane.

Interest in oxygenation of saturated hydrocarbons is increasing and significant new results can be expected in the near future. 

Work in this laboratory has shown also that the Ru(poip)(0)2 complexes (porp = TMP, TDCPP, and TDCPP-Clg) are practically inactive for thermal 02-oxygenation of saturated hydrocarbons . Some activity data for 0.2 mM Ru solutions in benzene under air at 25°C for optimum substrates such as adamantane and triphenylmethane at 6 mM did show selective formation of 1-adamantol and trityl alcohol, respectively, but with turnover numbers of only -0.2 per day the maximum turnover realized was -15 after 40 days for the TDCPP system Nevertheless, this was a non-radical catalytic processes there was < 10% decomposition of the Ru(TDCPP)(0)2, and a genuine O-atom transfer process was envisaged . Quite remarkably (and as mentioned briefly in Section 3.3), at the much lower concentration of 0.05 mM, Ru(TDCPP-Clg)(0)2 in neat cyclooctene gave effective oxidation. For example, at 90°C under 1 atm O2, an essentially linear oxidation rate over 55 h gave about -70% conversion of the olefin with - 80% selectivity to the epoxide however, the system was completely bleached after - 20 h and, as the activity was completely inhibited by addition of the radical inhibitor BHT, the catalysis is operating by a radical process, but in any case the conversion corresponds to a turnover of 110,000 As in related Fe(porp) systems (Section 3.3, ref. 121), the Ru(porp) species are considered to be very effective catalysts for the decomposition of hydroperoxides (eqs. 

This realization led me to study related possible intermolecular electrophilic reactions of saturated hydrocarbons, Not only protolytic reactions but also a broad scope of reactions with varied electrophiles (alkylation, formylation, nitration, halogenation, oxygenation, etc.) were found to be feasible when using snperacidic, low-nucleophilicity reaction conditions. 

Alkanes undergo combustion reaction with oxygen at high temperatures to produce carbon dioxide and water. This is why alkanes are good fuels. Oxidation of saturated hydrocarbons is the basis for their use as energy sources for heat, e.g. natural gas, liquefied petroleum gas (LPG) and fuel oil, and for power, e.g. gasoline, diesel fuel and aviation fuel. 

The radical mechanism has been proposed to explain the oxidation of saturated hydrocarbons. In the previous mechanisms, the electron density of the double bond or the aromatic ring is considered essential for the attack on the peroxidic oxygen. This condition is absent in saturated hydrocarbons, and considering their inertness, their oxidation probably requires a homolytic mechanism, proceeding through radical intermediates. By analogy with vanadium... 

In the analysis of fatty oils complete expelling of the oxygen by a catalytic high pressure hydrogenation process reduces the problem to the analysis of saturated hydrocarbon mixtures. Such drastic chemical transformations should be executed under strongly controlled conditions only. 

It is possible to measure the heats of combustion for a series of saturated hydrocarbons and thereby determine how much energy is released (—A//comb) when a -CH2-group in a saturated hydrocarbon reacts with oxygen. Thus the heat of combustion of an open-chain methylene group is —157.4 kcal/mol. (The heat of combustion is negative because heat is evolved.)... 

Selective oxidation of saturated hydrocarbons with oxygen in the presence of Fe11 in the so-called Gif reactions63 is also postulated to proceed through an Fev=0 intermediate species. 

The heterobimetallic complexes [N(n-Bu)4] [Os(N)R2(/u.-0)2Cr02] catalyze the selective oxidation of alcohols with molecular oxygen. A mechanism in which alcohol coordinates to the osmium center and is oxidized by B-hydrogen elimination (see -Hydride Elimination) is consistent with the data. The hydroxide adduct of OSO4, [0s(0H)204], with ferric cyanide and other co-oxidants catalyzes the oxidative dehydrogenation of primary aromatic and aliphatic amines to nitriles, the oxidation of primary alcohols to carboxylic acids, and of secondary alcohols to ketones. Osmium derivatives such as OsCb catalyze the effective oxidation of saturated hydrocarbons in acetonitrile through a radical mechanism. ... 

It is interesting to review a general pattern for oxidation of hydrocarbons in flames, as suggested very early by Fristrom and Westenberg [29]. They suggested two essential thermal zones the primary zone, in which the initial hydrocarbons are attacked and reduced to products (CO, H2, H2O) and radicals (H, O, OH), and the secondary zone, in which CO and H2 are completely oxidized. The intermediates are said to form in the primary zone. Initially, then, hydrocarbons of lower order than the initial fuel appear to form in oxygen-rich, saturated hydrocarbon flames according to... 

The catalytic oxidation of alkanes with molecular oxygen under mild conditions is an especially rewarding goal, as the direct functionalization of unactivated C-H bonds of saturated hydrocarbons usually requires drastic conditions such as high temperature. 

Oxygen atom from Cpd I is inserted into the C-H bond of saturated hydrocarbons (Scheme la) by means of hydrogen atom abstraction followed by recombination of the transient hydroxyl with the carbon radical [the so-called oxygen rebound mechanism proposed by Groves in 1976 (8, 10)]. Another possibility can be the concerted oxygen insertion into the C-H bond. Both pathways are rationalized by the two-state mechanism developed by Shaik et al. (6, 9), which describes different reactivities... 

The oxidation of aromatic hydrocarbons to oxygen-containing derivatives is of the same theoretical and practical interest as the oxidation of saturated hydrocarbons. The application of transition-metal compounds in combination with PT catalysts leads to rather interesting results. The hydroxylation of benzene with aqueous H2O2 is catalyzed by an Fe /catechol pair [106]. Hydrophobic 4-substituted catechols are the most effective. Using this method, one can obtain phenol under mild conditions. No reaction occurs in the absence of the one of the catalysts. 

The detailed kinetic studies on the oxidation of saturated hydrocarbons cyclohexane [4,5] and adamantane [26] and epoxidation of unsaturated hydrocarbons [4,5,25] cis-cyclooctene, cyclohexene, styrene and trans-stilbene were done by measuring the rate of reaction with respect to the concentration of each reactant, substrate, catalyst, ascorbic acid, hydrogen ion and molecular oxygen. The dependence of the reaction rate at various initial concentrations of the reactants were determined. While varying the concentration of a particular reactant, the concentrations of other reactants were kept constant under identical physical conditions. 

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