Peroxy radicals rearrangements

Other evidence that peroxy radical rearrangements are essentially high temperature phenomena is the formation of methyl ethyl ketone in cool flames where the carbon atom of the carbonyl group was originally tertiary in the alkane (17). Since cool flame temperatures are around 500°C., heterogeneous processes are unlikely, and all products must presumably arise homogeneously. 

Figure 3. The formation of hydroperoxy cyclic peroxides and prostaglandin-like endoperoxides from the 13S-hydroperoxide of linolenic acid by peroxy radical rearrangement. Structures are...

It was again observed that rearrangement pathways comprise a substantial portion of the oxidation routes for alkylated aromatics.Since this phenomenon is mainly due to peroxy radical reactivity rather than to identity of the parent compound, it is clear that comparable rearrangements would be factors for PAHs, as well as for nitrogen-, oxygen-, and sulfur-containing heteroaromatic rings and their alkylated derivatives. 

ROS and other radical intermediates dictate the oxidative decomposition of fuels. We have noted that peroxy radical intermediates provide an enormous amount of flexibility in the combustion of a given compound, specifically in the unimolecular steps available to that compound. In an instructive display of the interaction of experimental and theoretical techniques, rearrangement pathways of the peroxy radicals have been modeled computationally and provide justification for several unexpected products. 

Mention should be made of studies of slow, controlled combustion of alkanes, where formation of oxetanes can be detected. For example, oxetane is observed during combustion of propane, while 2-f-butyl-3-methyloxetane and 2-isopropyl-3,3-dimethyloxetane are observed from combustion of isooctane. While the yields are extremely low, the presence of these compounds, along with the other products found, have provided evidence for the mechanism of combustion. The oxetanes are believed to result from rearrangement of peroxy radicals in the radical chain process (equation 114) (70MI51300,73MI51301). 

C. Walling Rearrangements particularly of peroxy radicals have been mentioned recurrently in todays discussion of high temperature gas-phase autoxidations, and I will summarize briefly what is known about such processes in liquid-phase reactions run near room temperature. 

The asymmetric rearrangement of peroxy radical (5) has recently been used as the key step in the asymmetric synthesis of Plakorin (Scheme l).17 The thermal isomerization of buta-1,2- to buta-1,3-diene has been studied using ab initio calculations and the mechanism concluded to proceed stepwise via radical intermediates.18 The competition between cyclopropyl formation and the homoallyl-homoallyl radical rearrangement has been studied in the radical (6) and found to give the 3-exo cyclization product (7) and the rearranged product (8) in a 1 5 ratio, respectively, under the conditions shown (Scheme 2).19... 

Molecular oxygen reacts with the free-radical, giving a peroxy radical of atrazine as shown in Equation (6.137). This is reduced by Fe2+ to form a hydroperoxide. The rearrangement of the hydroperoxide forms amide through oxidation of secondary C with loss of a water molecule, as shown in Equation (6.139). 

Bauer G (2000) Reactive oxygen and nitrogen species efficient, selective and interactive signals during intercellular induction of apoptosis. Anticancer Res 20 4115-4140 Beckwith AU, Davies AG, Davison IGE, Maccoll A, Mruzek MH (1989) The mechanisms of the rearrangements of allylic hydroperoxides 5a-hydroperoxy-3p-hydrocholest-6-ene and 7a-hydro-peroxy-3(1-hydroxycholest-5-ene. J Chem Soc Perkin Trans 2 815-824 Behar D, Czapski G, Rabani J, Dorfman LM, Schwarz HA (1970) The acid dissociation constant and decay kinetics of the perhydroxyl radical. J Phys Chem 74 3209-3213 Benjan EV, Font-Sanchis E, Scaiano JC (2001) Lactone-derived carbon-centered radicals formation and reactivity with oxygen. Org Lett 3 4059-4062 Bennett JE, Summers R (1974) Product studies of the mutual termination reactions of sec- alkylper-oxy radicals Evidence for non-cyclic termination. Can J Chem 52 1377-1379 Bennett JE, Brown DM, Mile B (1970) Studies by electron spin resonance of the reactions of alkyl-peroxy radicals, part 2. Equilibrium between alkylperoxy radicals and tetroxide molecules. Trans Faraday Soc 66 397-405... 

After an initial tentative of rearrangement (diene formation) a further reaction with 02 generates a peroxy radical (LOO ). 

The rate constant for the reaction of ethyl radicals and oxygen increases with the increasing concentration of inert helium [57]. This indicates the possible participation of the molecules in the process of stabilization of peroxy radicals formed. The excited alkyl peroxy radicals do not fragmentate only to original reactants but rearrange to products through separate pathways [58]. 

One of the termination steps that lead to the simultaneous formation of both cyclohexanol and cyclohexanone deserves attention. The peroxy radical dimerizes to give the intermediate 8.12. The dimer as shown by reaction 8.20, undergoes intramolecular rearrangement to give the products. 

This rearrangement proceeds via a 2,3-peroxy radical mechanism4,5 24. Radicals are probable intermediates as these reactions are initiated or accelerated by light or free radical sources (benzyl peroxides, tert-butyl hyponitrite) 2 6, are inhibited by radical scavengers (4-methyl-2,6-di-/ert-butyl phenol)6,7, and display ESR signals of allyl peroxy radicals7-9. 

---