Hydroxylation homolytic

The high selectivity in the a positions which is illustrated in the formation of compounds 49 and 50 has been tentatively explained50 by a homolytic hydroxylation, a process well known for phenols50 and also for other heterocyclic compounds.60 This hypothesis is supported by studies on oxidation of unsubstituted pyrrole with hydrogen peroxide in conditions suitable for the generation of hydroxy radicals heating, light, or ferrous ions.50... 

In broad terms, the following types of reactions are mediated by the homolytic fission products of water (formally, hydrogen, and hydroxyl radicals), and by molecular oxygen including its excited states—hydrolysis, elimination, oxidation, reduction, and cyclization. 

The reaction endothermicity establishes a minimum for the activation energy whereas abstraction of a hydrogen atom from carbon is a feasible step in a chain process, abstraction of a hydrogen atom from a hydroxyl group is unlikely. Homolytic cleavage of an O-H bond is likely only if the resulting oxygen radical is stabilized, such as in phenoxy radicals formed from phenols. 

Fig. 5. Possible mechanisms for the MMO hydroxylation step. Pathway A insertion of the oxygen atom of Q into the C-H bond B concerted addition of the C-H bond to Q followed by reductive elimination C, D homolytic attack of Q on the C-H bond E reaction of the peroxo species with substrate.

For the lability of alkoxy-type radicals, see Ando, W. (ed.). (1992). Organic Peroxides. Wiley, New York. In the same way, olefin epoxidation with peracids can be simply viewed as an electron transfer, followed by mesolytic cleavage of the peracid anion radical to carboxylate and hydroxyl radical, followed by homolytic coupling and proton loss. See also Nugent, W.A., Bertini, F. and Kochi, J.K. (1974). J. Am. Chem. Soc. 96,4945... 

Hydroxy radicals are intermediates in the reaction of Ti3+ and H2O2 (175). This system is also capable of hydroxylation of aromatics and alkanes but, in contrast to reactions with Fenton s reagent (Fe2+ + H202, reductive, homolytic cleavage, Eq. (11)), only non-chain processes are possible, because Ti4+ is not usually an oxidant. Hence, relatively high selectivities are feasible. 

In the field of enzyme catalysis, heme-proteins such as cytochrome P450, for example, exhibit both types of 0-0 bond cleavages in organic hydroperoxides and peroxy acids (178). Heterolytic cleavage of HOOH/ROOH yields H20 or the corresponding alcohol, ROH and a ferryl-oxo intermediate (Scheme 4). Homolytic 0-0 bond cleavage results in the formation of a hydroxyl (HO ) or an alkoxyl (RO ) radical and an iron-bound hydroxyl radical. 

It has been known for many years that homolytic fusion of the 0-0 bond in H2O2 will yield the hydroxyl free radical OH. This may be produced by exposure of H2O2 solutions to heat or ionising radiation and hence may be formed, for example, after accidental or therapeutic exposure to radiation (e.g. during cancer therapy) ... 

If an aqueous solution saturated with oxygen is sonicated hydrogen peroxide formation occurs. This is due to hydroxyl ( OH) and hydroperoxyl ( OOH) radical recombination outside the cavitation bubble (Scheme 4.1). These radicals result from HjO and O2 homolytic cleavage inside the bubble and have been observed by spin trapping experiments (Scheme 4.1) [19]. 

Peroxynitrous acid is a powerful oxidizing agent with estimated one- and two-electron reduction potentials of ° (ONOOH, H+/"N02, HjO) = 1.6-1.7 V and ° (ONOOH, H /N02 , H2O) = 1.3-1.4 V, respectively . In addition, it was reported that, upon protonation, ONOO can undergo decomposition via homolytic 0—0 cleavage to generate nitrogen dioxide radical ("NO2) and hydroxyl radical ( OH) in approximately 30% yields... 

At odds with other similar peroxo metal complexes, V0(02)pic(H20)2, 36, performs non-selective epoxidation reactions. On this occasion Mimoun proposed a mechanism where a homolytic rupture of one metal-peroxo oxygen bond produces the active oxidant (Scheme 14). When aromatic substrates are allowed to react with 36, hydroxylation reaction takes place by way of the same active species as indicated in Scheme 15. 

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