Hydroperoxide radical oxygen atom transfer

Both oxidizing equivalents of the hydroperoxide are incorporated into compound I, through an oxygen-atom transfer process ". A free radical is generated elsewhere in the molecule on amino acid residue(s) in the case of yeast cytochrome c peroxidase" and at a site strongly coupled to the iron in horseradish peroxidase . (Compound I of yeast cytochrome c peroxidase is called complex ES in earlier literature.) EPR results on horseradish peroxidase are interpreted in terms of a porphyrin rr-cation radical for compound I . Thus, EPR data prove that one oxidizing equivalent obtained from the hydroperoxide is a free radical species"" ... 

TT-cation radical complexes.In the reactions, oxoiron(IV) porphyrin rr-cation radicals of electron-rich porphyrins reacted fast with ROOH (i.e., catalase and peroxidase type of chemistry one-electron oxidation of ROOH) (Scheme 2, pathway A). On the other hand, oxoiron(IV) porphyrin rr-cation radicals of electron-deficient porphyrins reacted fast with olefins to yield epoxide products (i.e., cytochrome P450 type of chemistry oxygen atom transfer) (Scheme 2, pathway B). These results demonstrated that electron-deficient iron porphyrin complexes are better catalysts in hydrocarbon oxygenations by hydroperoxides, since these complexes can avoid the facile decomposition of oxoiron intermediates by ROOH (Scheme 2, pathway A). Indeed, highly electron-deficient iron(III) porphyrin complexes efficiently catalyze alkane hydroxylations by H2O2 in aprotic solvents. 

Work in this laboratory has shown also that the Ru(poip)(0)2 complexes (porp = TMP, TDCPP, and TDCPP-Clg) are practically inactive for thermal 02-oxygenation of saturated hydrocarbons . Some activity data for 0.2 mM Ru solutions in benzene under air at 25°C for optimum substrates such as adamantane and triphenylmethane at 6 mM did show selective formation of 1-adamantol and trityl alcohol, respectively, but with turnover numbers of only -0.2 per day the maximum turnover realized was -15 after 40 days for the TDCPP system Nevertheless, this was a non-radical catalytic processes there was < 10% decomposition of the Ru(TDCPP)(0)2, and a genuine O-atom transfer process was envisaged . Quite remarkably (and as mentioned briefly in Section 3.3), at the much lower concentration of 0.05 mM, Ru(TDCPP-Clg)(0)2 in neat cyclooctene gave effective oxidation. For example, at 90°C under 1 atm O2, an essentially linear oxidation rate over 55 h gave about -70% conversion of the olefin with - 80% selectivity to the epoxide however, the system was completely bleached after - 20 h and, as the activity was completely inhibited by addition of the radical inhibitor BHT, the catalysis is operating by a radical process, but in any case the conversion corresponds to a turnover of 110,000 As in related Fe(porp) systems (Section 3.3, ref. 121), the Ru(porp) species are considered to be very effective catalysts for the decomposition of hydroperoxides (eqs. 

Termination of hydrocarbon radicals is not the only recombination reaction. Other possibilities (depending on the structure of the radicals) are (i) the termination by disproportionation, where hydrogen is eliminated, which yields an olefin (R-CHj-CHj R-CH=CH2-I-H ), (ii) termination by transfer (e.g. in reaction with antioxidants), (ill) termination by recombination with a hydroxyl radical, which leads to an alcohol (R-CHj-CHj -I- HO R-CH -GHj-OH) and (iv) further oxidation of the terminal hydroperoxide (R-CHj-CHj -I- Oj -> R-CHj-CHj-O-O -> R-CHj-CHj-O-OH). In the terminal hydroperoxide, the link between oxygen atoms is then cleaved, which gives an alkanal (R-CHj-CHj-O-OH R-CH2-CH=0- -H20). Alternatively, the link between carbon atoms can be cleaved, which results in a shorter alkyl radical (R-CHj ). Some of these reactions are discussed in relation to the secondary reactions of hydroperoxides. 

R00, is formed in the propagation step by rapid reaction with oxygen. In the absence of an efficient antioxidant, the peroxy radical is converted to the hydroperoxide, ROOH, by hydrogen-atom abstraction from the polymer chain giving rise to another polymeric radical, R , and peroxy radical R00. Propagation via chain transfer is also promoted by homolysis of ROOH to RO and 0H in the absence or depletion of the antioxidant AH. 

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