Free radicals autoxidations

Type I involves the excited triplet state of the photosensitiser reacting with a substrate, by electron transfer or hydrogen abstraction, giving a radical. This radical reacts with triplet oxygen producing hydroperoxides, which initiate free radical autoxidation. 

Benzoic Acid. The industrial-scale oxidation of toluene to benzoic acid is carried out with cobalt catalysts.973,978,982 983 The process, a free-radical autoxidation, is significantly promoted by bromide ions.984 Under these conditions bromine atoms rather than alkylperoxy radicals serve as a regenerable source of chain-initiating free radicals. Substantial rate increase can be achieved by the addition of manga-nese(II) ions.984... 

In contrast to catalytic hydrogenation, where no reaction takes place in the absence of a catalyst, catalytic oxidations with molecular oxygen are complicated by the fact that oxygen reacts with organic substrates even in the absence of a catalyst. This involves the so-called free radical autoxidation mechanism with the following as key steps ... 

Free-radical autoxidation of aldehydes with 02 is facile and affords the corresponding peradds, which are used as oxidants for carbonyl compounds. The peracid can transfer an oxygen atom to a substrate such as an olefin or ketone, resulting in the formation of one equivalent of epoxide or ester and add as a co-produd in the absence of metal catalysts [59]. Kaneda and coworkers have developed several HT materials that are active for heterogeneous Baeyer-Villiger reactions with 02/aldehyde [60]. Combination with Lewis addic metals improved the reaction by allowing coordination of the peracid and the intermediate. 

This result is important in modeling wet S0x-N0x flue-gas scrubbers. It shows that whereas N02 will oxygenate SO 2 " rapidly, it will not initiate the free-radical autoxidation of sulfite (1 7) Further, since the slightly soluble gas NO is the product, this reaction could reduce the efficiency of N0X removal (unless an NO scavenger is added). 

Chromium is useful as an oxidation catalyst, especially with t-BuOOH or O2 as the oxidant. When a Cr precursor [e.g., a Cr(VI) compound] is used, alcohols can be oxidized to ketones with t-BuOOH. Moreover, CH2 groups with relatively weak C-H bonds, for instance, in allylic or benzylic positions, are easily converted to carbonyl groups in the presence of Cr and r-BuOOH or 62. Often these reactions are free radical autoxidations, in which alkylhydroperoxide and alcohol products react further to form ketones (2, 4, 6). Relatively little is known about the active Cr species in these reactions, but it is plausible that high valent, neutral Cr compounds such as alkylchromates(VI) are involved. 

Surprisingly, alkanes containing tertiary C—H bonds showed poor reactivity in these reactions.2943 b 29Sa d Thus, isobutane was less reactive than n-butane, and methylcyclohexane less reactive than cyclohexane (cf., lower reactivity of cumene to toluene). In the series of normal alkanes, n-butane reacted faster than n-pentane. n-Undecane was unreactive. These results are inconsistent with a normal free radical autoxidation. The authors used the analogy with arene oxidations to postulate that formation of radical cations by electron transfer from the alkane to Co(III) was a critical factor ... 

Therefore, it appears that the redox properties of the metallo-porphyrin are required only for the initiation step in these free-radical autoxidations and that the porphyrin is not a stoichiometrically significant catalyst (21, 22, 23). The failure of these simple approaches to a reaction iron-porphine oxide or its equivalent could indicate that the ligation state of iron in the protein, presumably including an axial thiolate, is crucial to the oxygen-transfer properties of P 450. Support... 

Few examples involve the use of dioxygen alone as the primary oxidant. The use of a Ru(III) ethylenediaminetetraacetate complex has been described [28] but this almost certainly involves a free-radical autoxidation pathway and offers no advantages. Following the initial report by Neumann et al. [29] on the use of [WZnRu2(0H)(H20)(ZnW9034)2]11 attention has been focused on the use of ruthenium-containing polyoxometalates (POMs) as catalysts for the aerobic... 

The results were rationalized by assuming that the corresponding percar-boxylic acid is formed by cobalt-mediated free-radical autoxidation of the aldehyde. Subsequent reaction of ruthenium(III) with the peracid affords oxo-ruthenium(V) carboxylate, which is the active oxidant. Compared with the aerobic oxidations discussed earlier the method suffers from the drawback that 1 eq of a carboxylic acid is formed as a coproduct. 

Ruthenium-catalysed oxidations with dioxygen or hypochlorite are currently methods of choice for the oxidation of alcohol, ethers, amines and amides. In hydrocarbon oxidations, in contrast, ruthenium has not yet lived up to expectations. The proof of principle with regard to direct oxidation of, for example, olefins, with dioxygen via a nonradical, Mars-van Krevelen pathway has been demonstrated but this has, as yet, not led to practically viable systems with broad scope. The problem is one of rate although feasible the heterolytic oxygen-transfer pathway cannot compete effectively with the ubiquitous free-radical autoxidation. 

In the cyclohexane oxidation route cyclohexane is oxidized with air at 125-126°C and 8-15 bar in the liquid phase using Co or Mn naphthanates as the catalyst. This affords a mixture of cyclohexanol and cyclohexanone via a classical free radical autoxidation mechanism. Cyclohexane conversion is limited to 10-12% in order to minimize by-product formation via further oxidation. The selectivity to cyclohexanol/cyclohexanone is 80-85%. 

Figure 13. (a, b) Schematic representation of the oxidation pathways using redox molecular sieves (a) homolytic free radical autoxidation and (b) heterolytic oxygen transfer, (c) Oxidation of styrene to styrene oxide and transformation to 2-phenylacetaldehyde using a bifunctional Ti-silicalite catalyst. 

Figure 9. The products of free radical autoxidation of cholesterol (1), 25-hydroxycholesterol ...

Our hypothesis is that marine humic substances are formed from the free radical autoxidative cross-linking of unsaturated lipids released into the water by plankton. This hypothesis was arrived at for several reasons which are discussed in more detail in Harvey et al. (1983). Briefly, autoxidations are accelerated by light and catalyzed by transition metals, both of which are abundant in surface seawater. 

Free-Radical Chain Mechanisms. A frequently postulated initiation step for the free-radical autoxidation of S(IV) catalyzed by Fe(III) is as follows ... 

The overwhelming majority of petrochemical oxidation processes involve either free radical autoxidation in the liquid phase or gas phase oxidation over solid catalysts. The former type of process displays rather indiscriminate reactivity and... 

In the gas phase a Mars-van-Krevelen type mechanism can compete successfully with free radical autoxidation because if free radicals are formed they are not surrounded by substrate molecules (RH) as in the liquid phase, i.e. free radical chains are very short. Conversely, a Mars-van-Krevelen type mechanism is difficult to achieve with 02 in the liquid phase (but see later) due to competing facile autoxidation. The key to designing selective catalysts for liquid phase oxidations is to create a gas phase environment in the liquid phase. 

The term free-radical autoxidation describes a reaction pathway in which dioxygen reacts with an organic substrate to give an oxygenated product in a free-radical chain process that requires an initiator in order to get the chain reaction started. (A free-radical initiator is a compound that yields free radicals readily upon thermal or photochemical decomposition.) The mechanism of free radical autoxidation is as shown in Reactions (5.16) to (5.21). 

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