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 picol inic acid anion (Pic), which are used to mediate Gif-type oxidation of hydr ocarbons by H2O2 Complexes [Fe(Pic)(2)(py)(2)], [Fe(Pic)(3)].0.5py, [Fe2O(P ic)(4)(py)(2)], [Fe-2(mu-OH)(2)(Pic)(4)], and FeCl3 have been employed in o xygenations of adamantane by H2O2 mostly in py/AcOK to reveal that tert- an d sec-adamantyl radicals are generated in Gif solutions. The alleged absenc e of sec-adamantyl radicals from Gif product profiles has been previously i nterpreted as compelling evidence in support of a non-radical mechanism for the activation of secondary C-H sites in Gif chemistry. The product profil e is entirely dictated by trapping of the diffusively free adamantyl radica ls, 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 di oxygen 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) obtaine d, increasing with increasing dioxygen partial pressure, and the small intr amolecular 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 a n indiscriminate oxidant under inert atmosphere, Coupled to a more selectiv e oxidant at higher partial pressures of dioxygen. Gif oxygenation of DMSO by H2O2 mediated by [Fe(Pic)(2)(py)(2)] provides pyridine-trapped methyl ra dicals under argon, as expected for the addition reaction of hydroxyl radic als to DMSO. The reaction is progressively inhibited by increasing amounts of EtOH, generating pyridine-captured CH3. CHOH and . CH2CH2OH radicals. Qu antification of the DMSO- versus EtOH-derived alkyl radicals affords an est imate 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 s upported by the quantitative generation of tert-adamantyl radicals in the p resence of 1-iodoadamantane. These results are consistent with the action o f hydroxyl radicals in Gif oxygenations by Fe-II/III/H2O2 (Ar), most likely coupled to substrate-centered alkoxyl radicals under O-2. The oxygen-cente red radicals perform H-atom abstractions from Gif substrates to generate di ffusively free carbon-centered radicals, in accord with previously reported findings.