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

Abstract

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], [Fe2(μ-OH)2(Pic)4], and FeCl3 have been employed in oxygenations of adamantane by H2O2 mostly in py/AcOH 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% O2 (in N2), or between dioxygen and TEMPO at 100% O2, 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-d2 (1.06(6) (Ar); 1.73(2) (4% O2 in N2)), 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 kEtOH/kDMSO equal to 0.34(3), in reasonable agreement with the kinetics of radiolytically produced hydroxyl radicals (kEtOH/kDMSO = 0.29). The formation of 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 FeII/III/H2O2 (Ar), most likely coupled to substrate-centered alkoxyl radicals under O2. The oxygen-centered radicals perform H-atom abstractions from Gif substrates to generate diffusively free carbon-centered radicals, in accord with previously reported findings.