Functional aspects of Gif-type oxidation of hydrocarbons mediated by iron picolinate H2O2-dependent systems:: Evidence for the generation of carbon- and oxygen-centered radicals

The present investigation explores the functional features of several novel and other previously ill-defined ferrous and ferric complexes of the picolinic acid anion (Pic), which are used to mediate Gif-type oxidation of hydrocarbons by H2O2 Complexes [Fe(Pic)(2)(py)(2)], [Fe(Pic)(3)].0.5py, [Fe2O(Pic)(4)(py)(2)], [Fe-2(mu-OH)(2)(Pic)(4)], and FeCl3 have been employed in oxygenations of adamantane by H2O2 mostly in py/AcOK to reveal that tert- and sec-adamantyl radicals are generated in Gif solutions. The alleged absence of sec-adamantyl radicals from Gif product profiles has been previously interpreted as compelling evidence in support of a non-radical mechanism for the activation of secondary C-H sites in Gif chemistry. The product profile is entirely dictated by trapping of the diffusively free adamantyl radicals, since authentic tert- and sec-adamantyl radicals are shown to partition between dioxygen and protonated pyridine at 4% O-2 (in N-2), or between dioxygen and TEMPO at 100% O-2, in a manner analogous to that observed in Gif oxygenations of adamantane. The low tert/sec selectivity (2.2-4.5) obtained, increasing with increasing dioxygen partial pressure, and the small intramolecular kinetic isotope effect values revealed by employing adamantane-1,3-d(2) (1.06(6) (Ar); 1.73(2) (4% O-2 in N-2)), indicate the presence of an indiscriminate oxidant under inert atmosphere, Coupled to a more selective oxidant at higher partial pressures of dioxygen. Gif oxygenation of DMSO by H2O2 mediated by [Fe(Pic)(2)(py)(2)] provides pyridine-trapped methyl radicals under argon, as expected for the addition reaction of hydroxyl radicals to DMSO. The reaction is progressively inhibited by increasing amounts of EtOH, generating pyridine-captured CH3. CHOH and . CH2CH2OH radicals. Quantification of the DMSO- versus EtOH-derived alkyl radicals affords an estimate of k(EtOH)/k(DMSO) equal to 0.34(3), in reasonable agreement with the kinetics of radiolytically produced hydroxyl radicals (k(EtOH)/k(DMSO) = 0.29). The formation df methyl radicals in Gif oxygenation of DMSO is also supported by the quantitative generation of tert-adamantyl radicals in the presence of 1-iodoadamantane. These results are consistent with the action of hydroxyl radicals in Gif oxygenations by Fe-II/III/H2O2 (Ar), most likely coupled to substrate-centered alkoxyl radicals under O-2. The oxygen-centered radicals perform H-atom abstractions from Gif substrates to generate diffusively free carbon-centered radicals, in accord with previously reported findings.