Tetroxides, Russell mechanism

An alternative chain-terminating decomposition of the tetroxide, known as the Russell mechanism (29), can occur when there is at least one hydrogen atom in an alpha position the products are a ketone, an alcohol and oxygen (eq. 15). This mechanism is troubling on theoretical grounds (1). Questions about its vaUdity remain (30), but it has received some recent support (31). 

Asymmetric 0-0 bond homolysis of the tetroxide as a first step to product formation has been invoked (Khursan et al. 1990), and the idea of the Russell mechanism replaced by a three-step mechanism [reactions (52)—(54)]. 

Termination reactions. A very common termination reaction, known as the Russel mechanism from its discoverer, is the recombination of two peroxy radicals to form an unstable tetroxide that decomposes through a concerted mechanism to yield a hydroxy moiety and a carbonyl moiety (13) ... 

Several mechanisms have been suggested to produce the energy required to populate an excited carbonyl, which is at least 290-340 kj mol-1 [8]. Direct homolysis of hydroperoxides [9, 10], disproportion of alkoxy radicals [11] and /2-scission of alkoxy radicals [12] are all exothermic enough. However, the most widely accepted mechanism has been the highly exothermic (460 kj mol-1) bimolecular termination of primary or secondary alkyl per-oxyl radicals, i.e. the Russell mechanism (Scheme 2). It proceeds via an intermediate tetroxide to give an excited carbonyl, an alcohol, and oxygen [13, 14]. 

In the bimolecular decay of peroxyl radicals, a short-lived tetroxide is an intermediate. When a hydrogen is present in /3-position to the peroxyl function, a carbonyl compound plus an alcohol and O2 [Russell mechanism, e.g. reaction (42)] or two carbonyl compound plus H2O2 (Bennett mechanism, not shown) may be formed in competition to a decay into two oxyl radicals plus O2 [e.g. reaction (43) for details of peroxyl radical chemistry in aqueous solution, see Refs. 2 and 39]. 

There is considerable controversy over whether and how the Russell Mechanism involving tetroxide intermediates (107) actually occurs in lipids, and whether the oxygen is released as O2. In early work, Ingold proposed that the Russell mechanism (Reaction 69) was the most important termination process for sec... 

It is assumed, what recombination of cumyl peroxo radicals ROO occurs through intermediate formation of tetroxide RO R, decomposition of which is realized on Russell mechanism [34], (through cyclic six-member transient state) and results to formation of decay products CH, a-methyl styrene and 02,on scheme ... 

At atmospheric pressure, termination (k ) occurs first by the combination of peroxyl radicals to an unstable tetroxide intermediate followed rapidly by its decomposition by the Russell mechanism, to yield non-radical products (12),... 

Lipid peroxidation in microsomal fraction initiated by hydroperoxides or iron/ascorbate, as well as lipid peroxidation of isolated hepatocytes under oxygenation, has been studied extensively by Cadenas et al. (1981) and Cadenas and Sies (1982). The available evidence points to 02 formation by the Russell mechanism (Russell 1957, Howard and Ingold 1968). Secondary lipid-peroxy radicals react via a transient tetroxide to yield an alcohol, a carbonyl and oxygen, the carbonyl or oxygen being in an electronically excited state. 

Bimolecular interaction of alkyl-peroxy radicals must initially produce a highly unstable linear tetroxide. Where one of the radicals is secondary, this species rapidly reacts via the so-called Russell mechanism (77) to give a carbonyl and a hydroxy group ... 

The work of Lindsay et at provides good evidence that not all S-RO2 undergo bimolecular self-reaction at ambient temperatures entirely by the Russell mechanism. It would, however, appear from their work that the yield of alkoxy radicals is very dependent on the structure of the peroxy radical. This lead these workers ( 6) to propose that the tetroxide may decompose by multiple bond scission by a concerted but non-eyelie mechanism. 

The difference in termination rate constants between secondary (and primary) alkylperoxys and tertiary alkylperoxys is, therefore, almost entirely due to differences in the rate constants for irreversible tetroxide decay. Thus at 303K 2k- for t-Bu0a = 1.2 X 10 M s and 2k+ for s-BuOa = 10 M" s . That is kt/k-j = 8.3 X 10. Now Ingold (bl) has suggested that AS° for (27) fn-lk.k cal deg mol because of the entropy loss from four hindered internal rotors (i+x3.6 cal deg mol ). Thus A /A and E-b-E-t should be 10 and 9.6 kcal mol , respectively, for t-BuOa and s-BuOa, i.e., E should be ca 1 kcal mol negative. Neither of these predictions are observed experimentally and we are forced to conclude that kinetic data for self-reaction of S-RO2 provides further evidence against the complete acceptance of the Russell mechanism. 

A detailed mechanism is proposed for this recombination process. On the basis of the experimental results obtained, Beutel (for details see 13>) comes to the conclusion that in the case of dimedone autoxidation the triplet triketone D = O cannot be efficiently quenched by ground-state triplet oxygen formed in the decomposition of a Russell tetroxide which in this case should have the formula. 

Russell [179] proposed the following mechanism of chain termination by primary and secondary peroxyl radicals with coordinated decomposition of formed tetroxide to alcohol, ketone, and 02 ... 

This reaction is very exothermic (e.g., AH 405 kJ mol-1 for cyclohexyltetroxide) and supposed to occur rapidly. The values of rate constants for primary and secondary R02 cover the range at 300 K, 2kt = 106 to 108 L mol-1 s-1 (see Table 2.15). According to Russell s mechanism, tetroxide decomposition proceeds via the cyclic transition state and includes the abstraction of the C—H bond ... 

Termination of the autoxidation chain process occurs as peroxyl radicals couple to yield non-radical products. This reaction takes place through an unstable tetroxide intermediate. Primary and secondary tetroxides decompose rapidly by the Russell termination mechanism to yield three non-radical products via a six-membered cyclic transition state (Fig. 95). The decomposition yields the corresponding alcohol, carbonyl compound, and molecular oxygen (often in the higher energy singlet oxygen state) three... 

Russell (10) suggested that the bimolecular self-reaction of S-RO2 involves the concerted decomposition of a cyclic tetroxide formed by combination of the radicals. This mechanism was deduced from a consideration of the results of a kinetic and product study of the autoxidation of ethylbenzene. Thus Russell found that almost one molecule of acetophenone is produced per two kinetic chains and that CeHsCHCCHa)O2 interact to form non-radical products nearly twice as fast as CsHsCDCcHs) O2. The former result is only compatible with (29) if all the alkoxy radicals disproportionate in the solvent cage (30) while the deuterium isotope effect requires a H-atom transfer reaction to be rate controlling, which is unlikely for the radical pathway. 

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