Hydrocarbons homolytic oxidation

Homolytic oxidations involve free radical intermediates and are catalyzed by first-row transition metals characterized by one-electron oxidation-reduction steps, eg. Com/Con, Mnln/Mnn. The hydrocarbon substrate is generally not coordinated to the metal and is oxidized outside the coordination sphere. Consequently, the oxidation is not very selective and does not often preserve the stereochemical configuration of the substrate. 

The oxidation of hydrocarbons by cobalt(lll) acetate has been thoroughly investigated, due to its relevance to industrial homolytic oxidation processes.56 361 547 Radical intermediates are produced from one-electron oxidation of hydrocarbons according to an electron transfer or an electrophilic substitution mechanism previously described in equations (200)-(203). These oxidations are dramatically accelerated by the presence of strong acids or halide salts. 

Saturated hydrocarbons are homolytically oxidized by complexes (205) into alcohols, ketones and t-butyl peroxide products. The hydroxylation reaction occurs at the more nucleophilic C—H... 

For high-temperature homolytical oxidation of hydrocarbons, it should be taken into account that the difference in reactivities of different C—H-bonds in relation to radical attack becomes insignificant, which provides for formation of many different reaction products. 

Peroxides of transition metals are themselves active intermediates in heterolytic and homolytic liquid-phase catalytic oxidation reactions of alkenes, aromatic hydrocarbons and alkanes. Heterolytic oxidations are characterized by a requirement for a free coordination volume near the transition metal atom. Homolytic oxidations proceed via M-O bond cleavage in peroxo complexes. 

Mechanism of the Amoco-MC Process (Sheldon and Kochi 1981 Partenheimer 1995) The discussion that follows is based on an exhaustive review by Partenheimer (1995) of metal-Zbromide-promoted auto-oxidation of hydrocarbons. The homolytic oxidation process has three steps commonly encountered in free radical-assisted reactions. Considering toluene as the simplest methylbenzene and I as a radical initiating species, the steps involved have been listed as follows ... 

The hydrocarbon with a tertiary C—H bond is oxidized to stable tertiary hydroperoxide. This hydroperoxide is decomposed homolytically with the formation of alcohol [82] ... 

In the oxidized hydrocarbon, hydroperoxides break down via three routes. First, they undergo homolytic reactions with the hydrocarbon and the products of its oxidation to form free radicals. When the oxidation of RH is chain-like, these reactions do not decrease [ROOH]. Second, the hydroperoxides interact with the radicals R , RO , and R02. In this case, ROOH is consumed by a chain mechanism. Third, hydroperoxides can heterolytically react with the products of hydrocarbon oxidation. Let us consider two of the most typical kinetic schemes of the hydroperoxide behavior in the oxidized hydrocarbon. The description of 17 different schemes of chain oxidation with different mechanisms of chain termination and intermediate product decomposition can be found in a monograph by Emanuel et al. [3]. 

Hence, the copper surface catalyzes the following reactions (a) decomposition of hydroperoxide to free radicals, (b) generation of free radicals by dioxygen, (c) reaction of hydroperoxide with amine, and (d) heterogeneous reaction of dioxygen with amine with free radical formation. All these reactions occur homolytically [13]. The products of amines oxidation additionally retard the oxidation of hydrocarbons after induction period. The kinetic characteristics of these reactions (T-6, T = 398 K, [13]) are presented below. 

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... 

Homolytic liquid-phase processes are generally well suited to the synthesis of carboxylic acids, viz. acetic, benzoic or terephthalic acids which are resistant to further oxidation. These processes operate at high temperature (150-250°C) and generally use soluble cobalt or manganese salts as the main catalyst components. High conversions and selectivities are usually obtained with methyl-substituted aromatic hydrocarbons such as toluene and xylenes.95,96 The cobalt-catalyzed oxidation of cyclohexane by air to a cyclohexanol-cyclohexanone mixture is a very important industrial process since these products are intermediates in the manufacture of adipic acid (for nylon 6,6) and caprolactam (nylon 6). However, the conversion is limited to ca. 10% in order to prevent consecutive oxidations, with roughly 70% selectivity.97... 

All these data are in favor of a homolytic mode of oxygen transfer from Vv alkyl peroxides to hydrocarbons, and the mechanism suggested in Scheme 4, based on that of oxidation by Vv-peroxo complexes (Scheme 2), was tentatively attributed to a biradical V17 — OR—O species which can add to arenes and abstract hydrogen atoms from alkanes. It is probable that the absence of a releasable coordination site adjacent to the triangular alkyl peroxide group in (22) precludes the possibility of the alkene coordination to the metal and therefore prevents its heterolytic epoxi-dation. 

Catalytic oxidations by copper compounds are mainly homolytic in nature. Copper salts have been extensively used in conjunction with molecular oxygen, peroxides and persulfate for the oxidation of a variety of alkenic and aromatic hydrocarbons.56,584... 

The reaction chemistry of simple organic molecules in supercritical (SC) water can be described by heterolytic (ionic) mechanisms when the ion product 1 of the SC water exceeds 10" and by homolytic (free radical) mechanisms when <<10 1 . For example, in SC water with Kw>10-11 ethanol undergoes rapid dehydration to ethylene in the presence of dilute Arrhenius acids, such as 0.01M sulfuric acid and 1.0M acetic acid. Similarly, 1,3 dioxolane undergoes very rapid and selective hydration in SC water, producing ethylene glycol and formaldehyde without catalysts. In SC methanol the decomposition of 1,3 dioxolane yields 2 methoxyethanol, il lustrating the role of the solvent medium in the heterolytic reaction mechanism. Under conditions where K klO"11 the dehydration of ethanol to ethylene is not catalyzed by Arrhenius acids. Instead, the decomposition products include a variety of hydrocarbons and carbon oxides. 

Homolytic autoxidations of hydrocarbons often give complex mixtures of products-the autoxidation of olefins is a prime example. There is a great incentive, therefore, to search for catalysts that can promote the selective oxidation of olefins by essentially nonradical mechanisms. For example, there is no method available for carrying out the selective epoxidation or oxidative cleavage of olefins (see Section III.C) by molecular oxygen. In order to be successful, any heterolytic pathway for the metal-catalyzed oxidation of a substrate must, of course, be considerably faster than the ubiquitous homolytic processes for autoxidation. Thus, the metal catalysts discussed in the following sections, in addition to being able to promote heterolytic oxidations, are also able to catalyze homolytic processes. 

Oxidation of hydrocarbons with dioxygen is more facile when the C-H bond is activated through aromatic or vinylic groups adjacent to it. The homolytic C-H bond dissociation energy decreases from ca. 100 kcal mol-1 (alkyl C-H) to ca. 85 kcal mol-1 (allylic and benzylic C-H), which makes a number of autoxidation processes feasible. The relative oxidizability is further increased by the presence of alkyl substituents on the benzylic carbon (see Table 4.6). The autoxidation of isopropylbenzene (Hock process, Fig. 4.49) accounts for the majority of the world production of phenol [131] ... 

In homolytic reactions the hydrocarbon to be oxidized is generally not coordinated to the metal and is oxidized outside the coordination sphere via a radical chain. These processes are common and constitute the basis for several very important industrial applications (Sections 2.2 and 2.3). However radical chains are difficult to control, they do not often preserve the configuration of the substrate and typically lead to the formation of a wide variety of products. Consequently, reactions involving one-electron processes with dioxygen as the oxidant generally show only moderate to low selectivities towards the desired product. 

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