Radical substitution autoxidation

This paper presents the results of an investigation of the oxidation of substituted olefins in the presence of hydrocarbon-soluble transition metal complexes. Results indicate that the initial interaction of oxygen with the olefin probably does not occur within the coordination sphere of the metal. The best interpretation appears to be autoxidation of the olefin, initiated either by the metal or by metal catalyzed decomposition of peroxidic impurities. The initial product of an olefin having allylic hydrogens is an allylic hydroperoxide species this is usually the case in radical initiated autoxidations. Nonetheless, with some metal complexes the product profile differs markedly from that observed when radical initiators are used. In the presence of several complexes, oxidation is... 

Radical Substitution Reactions at the Saturated C Atom 1.8 Autoxidations... 

The free-radical substitution of H for OOH in alkanes is called autoxidation. ( Autoxidation is a misnomer, because the substrate is not oxidizing itself O2 is oxidizing the substrate ) Autoxidation proceeds by a free-radical chain mechanism. Note that the mechanism for oxidation includes a very rare radical-radical combination step in the propagation part. The radical-radical combination step doesn t terminate the chain in this particular reaction because O2 is a... 

The reactions were inhibited by hydroquinone which is consistent with a free radical initiated autoxidation. The reaction of tetramethylethylene was more rapid than was oxidation of less substituted olefins in the presence of the Rh(I) and Ir(I) complexes suggesting that initial coordinative interaction between the olefin and the metal center is not an important factor. 

The propagation step, Eq. (4), is much slower than Eq. (3) as an example, its rate constant kp is 0.18 M 1 sec-1 for cumene at 303K. Values of kp can vary considerably for different substrates, as shown by the oxidation rates of substituted toluenes (8). With respect to toluene, taken as 1.0, the reactivity of 4-nitrotoluene toward ROO is 0.33 and that of / -xylene is 1.6. A homolytic process like the fission of the C-H bond should be essentially apolar, but data for substituted toluenes correctly suggest that the hydrogen radical abstraction is favored by electron-donor substituents and that in the transition state the carbon atom involved has a partial positive charge. The difference in kp between different molecules or different groups of the same molecule is the reason of the selectivity of autoxidation. 

To be effective as autoxidation inhibitors radical scavengers must react quickly with peroxyl or alkyl radicals and lead thereby to the formation of unreactive products. Phenols substituted with electron-donating substituents have relatively low O-H bond dissociation enthalpies (Table 3.1 even lower than arene-bound isopropyl groups [68]), and yield, on hydrogen abstraction, stable phenoxyl radicals which no longer sustain the radical chain reaction. The phenols should not be too electron-rich, however, because this could lead to excessive air-sensitivity of the phenol, i.e. to rapid oxidation of the phenol via SET to oxygen (see next section). Scheme 3.17 shows a selection of radical scavengers which have proved suitable for inhibition of autoxidation processes (and radical-mediated polymerization). 

In addition to the usual reactions of the catalyst with intermediate hydroperoxides, the second type of reaction undoubtedly involves direct reaction of the metal catalyst with the hydrocarbon substrate and/or with secondary autoxidation products. Two possible pathways can be visualized for the production of radicals via direct interaction of metal oxidants with hydrocarbon substrates, namely, electrophilic substitution and electron transfer. Both processes are depicted below for the reaction of a metal triacetate with a hydrocarbon. 

Contrary to our results, other workers (4, 9, 20, 36) state that in the stabilization of carotene, paraffin wax, and lard the activity of pyrocatechol is favorably affected by substitution at position 4, not only by normal but by tertiary alkyl groups as well. Disparate influences of substitution are not surprising when comparing the activity in different substrates owing to the possibility of directive influences in the process of inhibited oxidation. The participation of phenolic antioxidants in the inhibition of autoxidation can be demonstrated (1, 2, 3) simply as a reaction between the molecule of antioxidant AH and the alkylperoxy radical ROO formed duririg the autoxidation of the substrate RH. During this process, an aryloxy radical (A ) is first generated. 

In Type la antioxidants substituted by normal or tertiary alkyl groups, the reaction with active radicals formed in the autoxidized sub-... 

One mole of a hindered phenolic is theoretically capable of eliminating two active radicals in the autoxidation sequence. Of course, the position and type of group or groups substituted on the phenol will control to a great extent the activity of the particular phenolic used. However, a discussion of these effects is beyond the scope of this paper and will not be treated here. 

B Reactions of compounds with oxygen with the development of flames are called combustions. In addition, flameless reactions of organic compounds with oxygen are known. They are referred to as autoxidations. Of the autoxidations, only those that take place via sufficiently stable radical intermediates can deliver pure compounds and at the same time appealing yields. Preparatively valuable autoxidations are therefore limited to substitution reactions of hydrogen atoms that are bound to tertiary, allylic, or ben-zylic carbon atoms. An example can be found in Figure 1.27. Unintentional autoxidations can unfortunately occur at the O—Cprtm—H of ethers such as diethyl ether or tetrahydrofuran (THF) (Figure 1.28). 

Aryl-substituted enolizable keto compounds initiate the copolymerization of unsaturated polyesters with styrene. Gel times of the same order as those obtained with conventional peroxide initiators can be attained exotherms, however, are considerably lower, this latter effect being of technological interest—e.g., casting resins. Since a radical mechanism has been proved, it is postulated that radicals result from keto hydroperoxides which have been formed from the aryl-substituted enols via autoxidation. Steric effects and resonance may partly account for differences in the catalytic activity of some and for the inhibiting effect of other ketones and enols. NMR spectroscopy indicates further that cis-trans isomerism may influence the catalytic effectiveness of pure enols. 

Aromatically substituted enols are easily autoxidized to the keto-hydroperoxide form (4, 5, 6, 10, 11, 12, 16), which, we postulate, would then initiate a radical polymerization. A radical mechanism is proved by the inhibiting effect of quinone and a,polymer yield is directly proportional to the square root of the initiator concentration (7). 

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