Radical autoxidation pathway

The API methoxamine hydrochloride, which contains a benzyl hydroxyl, was found to decompose in aqueous solution to the primary degradation product 2,5-dimethoxybenzaldehyde, presumably via a benzylic radical autoxidation pathway (Fig. 100) (142). 

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... 

Spontaneous oxidation of amines by one-electron transfer has been reported as a key process in polar solvents (35). It is not easy to distinguish the spontaneous and initiated mechanisms, because these pathways have a common intermediate (XI, Fig. 9). Thus, potassium hexacyanoferrate (III), a one-electron oxidant, gives electron transfer oxidation of amines (56) yielding the classical radical autoxidation products. 

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. 

A schematic autoxidation pathway involves initiation (equation 10), propagation (equation 11-12) and termination (equation 13). Since hydroperoxides easily undergo metal-catalyzed decomposition, the addition of a metal can both speed up an air oxidation and avoid the formation of a hydroperoxide, which is rarely the desired final product. This is perhaps the simplest type of metal catalysis. The metal is normally capable of le redox behavior, hence typically a 3d transition metal. As shown in equations (14 -15), it can decompose the RO2H to give the highly reactive RO- radical in a manner resembling equation (3 -4). RO- radical can abstract... 

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. 

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). 

As previously noted, HA behaves as a substituted o-aminophenol, and the overall process has been studied in depth [17, 35, 50-55]. The autoxi-dative process then starts as in (3), but the superoxide produced is not efficiently consumed by reactions such as (5) and (6) and its spontaneous dismutation is comparatively slow, so it could accumulate to a certain extent. The addition of SOD to the reaction mixture therefore accelerates HA autoxidation 4-fold, probably by preventing back reactions between superoxide and the anthranilyl radical B [34, 56]. This could be a case where the reducing power of superoxide becomes evident [37 and therein]. It is worth noting that anthranilyl radical can react with another oxygen molecule to produce another superoxide [17, 34], Note also that the tricyclic lactone H, Fig (3) has been isolated from autoxidised solutions of HA, so giving further proof of the intermediacy of anthranilyl radicals B on the autoxidation pathway [35],... 

The mechanism of hydroquinone autoxidation likely proceeds by a radical chain pathway. Kinetic studies carried out under relevant reaction conditions support a second-order rate law for the reaction, rate =Ar[QH2] [Oj], with an apparent activation energy of = 15 kcal/mol [21]. Based on these kinetic findings, as well as DFT studies [22], anthrahydroquinone autoxidation has been proposed to occur through initial, rate-limiting, direct H-atom abstraction from the hydroquinone species by O2 (Eq. (14.2)). The semiquinone species then react readily with triplet O2 (Eq. (14.3)), and hydroperoxy radical, HO2, has been proposed to act as a radical chain carrier (Eq. (14.4)). 

There are two basic pathways for obtaining photodegradable polymers, either by chemically modifying the polymer main chains with the insertion of a light responsive photodegradable chromophore entity, such as polyolefins or carbonyls [18], or by blending them with specific additives able of initiating photochemical decomposition processes (typically radical autoxidation reactions) within the polymer [6, 19, 20]. 

As a reasonable biogenetie pathway for the enzymatic conversion of the polyunsaturated fatty acid 3 into the bicyclic peroxide 4, the free radical mechanism in Equation 3 was postulated 9). That such a free radical process is a viable mechanism has been indicated by model studies in which prostaglandin-like products were obtained from the autoxidation of methyl linolenate 10> and from the treatment of unsaturated lipid hydroperoxides with free radical initiators U). 

The same pathway of activation has been postulated in the formation of quinones, although the putative 6-hydroxyBP precursor has never been isolated (19,20). In this mechanism, formation of quinones would proceed by autoxidation of 6-hydroxyBP (20). However, substantial evidence indicates that the first step in formation of quinones does not involve the typical attack of the electrophilic active oxygen to yield 6-hydroxyBP, but instead consists of the loss of one electron from BP to produce the radical cation. 

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