Dioxygen radical pathways

As noted above, dioxygen reacts with organic molecules, e.g. hydrocarbons, via a free radical pathway. The corresponding hydroperoxide is formed in a free radical chain process (Fig. 4.3). The reaction is autocatalytic, i.e. the alkyl hydroperoxide accelerates the reaction by undergoing homolysis to chain initiating radicals, and such processes are referred to as autoxidations [1]. 

There is an amazing report in literature claiming that CuCliyPcY in presence of dioxygen [29] is able to oxidize primary C atoms in a selective way, excluding the occurrence of a free radical pathway, normally expected for a Cu-catalyzcd autoxidation [30]. This important observation (see table) urgently needs experimental confirmation. 

Cobalt complexes with pyridine ligands, for example, catalyzed the oxidation of neat ethylbenzene to acetophenone in 70% conversion and 90% selectivity [35]. Mn porphyrin complex catalyzes the ethylbenzene oxidation with dioxygen to 3 14 mixture of methylphenylcarbinole and acetophenone in the presence of acetaldehyde [36]. The system CUCI2-crown ether in the presence of acetaldehyde is efficient as catalyst of oxidation of ethylbenzene, indane, and tetralin by dioxygen (70°C) into the corresponding alcohols and ketones with high TON [37]. The oxidations were established to occur via a radical pathway and not by a metal-oxo... 

Since metal dioxygen complexes are usually diamagnetic, it was of interest to determine whether the allylic hydroperoxide formed in autoxidation reactions catalyzed by [RhCl(Ph3P)3] arose via an ene addition pathway, equation (275), or via conventional radical pathways. 

Holland and Milner [474] re-examined this reaction recently in some detail. It was found that reaction of [(C8Hi4)2RhQ]2 with dioxygen in benzene at 74 gave equimolar amounts of 2-cyclooctene-l-one, cyclooctanone and water, equation (279). Reaction proceeds via a pathway which is independent of radical chains and does not involve a Wacker cycle. By contrast, cyclooctene was autoxidized by cobalt naphthenate by a radical pathway to give mainly cyclooctene oxide and this reaction was completely suppressed by the radical inhibitor 2,6-di-f-butyl-p-cresol.. This inhibitor had no effect on reaction (279). Labehng experiments with added H20 showed that water did not take part in this reaction, ruling out a Wacker process. 

There is much interest at present in the reactivity of excited states of dioxygen and various oxygen species such as superoxide ion and the perhydroxyl radical, and hydroxyl radicals. The photooxidation of acetone azine proceeds via a free radical pathway initiated by electron transfer taking place from the azine to singlet oxygen ( 02). This generates superoxide and azine cation radical,... 

Radical initiated polymerization of vinylpyridinium ions has been recognized since 1964 when Shyluk reported 1,2-addition polymerization of 1,2-dimethyl-5-vinylpyridinium methylsulfate initiated by potassium peroxydisulf-ate [39]. Later, Ringsdorf proposed a radical mechanism for the spontaneous polymerization of 1 (R = H) at concentrations greater than 1.0 M in water [37, 42]. The postulated mechanism was supported by experiments in which inhibition of polymerization was demonstrated by radical inhibitors such as dioxygen, copper (II) ions, and 5-butylcatechol. Kinetic analysis of these reactions was also consistent with the proposed radical pathway. 

The recent quantum-chemical analysis of the reaction of dioxygen with ethyl radical in the gas phase provided evidence for two pathways of interaction [83] ... 

Whereas several transient species have been observed for dioxygen activation by MMOH, no intermediates were found by rapid-mixing spectroscopic methods for the actual methane hydroxylation step. Mechanistic probes, i.e. certain non-natural substrates that are transformed into rearranged products only if the reaction proceeds via a specific intermediate such as a radical or a cation, give ambivalent results Some studies show that products according to a pathway via cationic intermediates are obtained in sMMO hydroxylations and at least one study suggests the presence of a radical intermediate [40]. Computational analyses of the reaction of MMOHq with methane suggest a so-called radical recoil/rebound mechanism in which MMOHq... 

The riboflavin triplet reacts with dGMP acid by ET (k = 6.6 x 109 dm3 mol-1 s 1), and evidence for the formation of the (deprotonated) Gua radical cation has been obtained by laser flash photolysis (Lu et al. 2000). The photosensitized reactions of dGuo by TRP is thought to follow two pathways, the formation of Z has been attributed to an ET reaction (Type I), and the reaction of singlet dioxygen [ChCAg) Type II] leads to 4-OH-8-oxo-G and 8-oxo-G (Ravanat et al. 1998). The effect of D20 and azide on the 4-OH-8-oxo-G yields shows that this... 

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