A radical reappraisal of Gif reactions

Abstract

Selective alkane functionalisation is achievable by a variety of natural enzymic systems. Perhaps the most extensively studied of these are the cytochrome P-450 enzymes,] in which the active site contains an iron atom coordinated with a tetradentate porphyrin ligand. The mechanism of alkane functionalisation catalysed by P-450 enzymes has widely been accepted as involving the so-called 'oxygen rebound' mechanism,'" in which an iron (IV)-OXO intermediate* is generated which removes hydrogen from the hydrocarbon substrate [eqn. ( l ) ] ; the resulting alkyl radical immediately removes hydroxyl from iron to form alcohol and regenerate a lowvalent iron species [eqn. (2)l. Very recent studies,'( depending upon the use of cyclopropylcarbinyl radical 'clocks'2 have attempted to 'time' the rebound process.* Since, in the iron(iv) species, the porphyrin ligand is oxidised to a radical cation, the whole has sometimes been represented as incorporating iron(v) * Iron(iii) is also a plausible candidate for the oxidant in Scheme 4 (but see footnoteThis diminution in yield is very close to that calculated taking known rate constants for reaction of secondary alkyl radicals with oxygen and with PhSH,and making a reasonable assumption regarding the oxygen concentration in the reaction mixture (M. Newcomb, personal communication).* At present, the possibility that a small amount of radical fragmentation might redirect the overall process cannot be dismissed.The interesting question arises here as to what intramolec ulur isotope effect would be expected tor a diftusm controlled hydrogen abstraction in a mobile solvent e , g by hydroxyl radicals from C,H,D, Apparently experimental data do not exist The value of unity noted earlier was obtained by companng liquid phase rate data for C,H,, and for C,DI, This was appropriate tor comparison with the Gif experiments There are however gas phase data for hydrogen abstraction from alkanes by HO which not only give a kinetic isotope effect ot ((I 2 hui ulto 3hou ( I rhreejold prejerence (ut 2Y4 K ) for uh5truc tion of tertruri ruthrr than tei ondur\ h\dro,gen -lv

The Fenton Reaction in Aerated Aqueous Solutions Revisited

“…Therefore, also in these media the mechanisms of the Fenton process are probably considerably more complicated than proposed. [6,[49][50][51][52] The results point out that the yield of Fe III depends on the nature of the peroxide and the nature of all the organic compounds present in the solution. Thus, the use of the FOX assay for the determination of the total peroxides in biological samples is difficult if at all feasible.…”

Lewis Acids as Catalysts in Oxidation Reactions: From Homogeneous to Heterogeneous Systems

“…276,277,[545][546][547][548][549][550][551] It seems that alkane activation on a solid surface requires the presence of strong acid or basic sites. 481,552 In the following section, we will present some recent progress in the application of solid Lewis acids as catalysts using oxygen as oxidizing reagent.…”

Distinguishing Biomimetic Oxidations from Oxidations Mediated by Freely Diffusing Radicals

“…This is, of course, a free radical mechanism. 24 The radicals are produced by the reaction of the hydroperoxide with the iron. In this simple mechanism, the hydrogen atom abstracting species is an oxygen-centered radical (HO* or RO*) and free carbon-centered radicals are produced rather than an Fe v -R intermediate.…”