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Article

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

Supporting Info

Salma Kiani,^† Amy Tapper,^† Richard J. Staples,^‡ and Pericles Stavropoulos*^†

Contribution from the Departments of Chemistry, Boston University, Boston, Massachusetts 02215, and Harvard University, Cambridge, Massachusetts 02138

J. Am. Chem. Soc., 2000, 122 (31), pp 7503–7517

DOI: 10.1021/ja000063h

Publication Date (Web): July 22, 2000

†

Boston University.

‡

Harvard University.

In papers with more than one author, the asterisk indicates the name of the author to whom inquiries about the paper should be addressed.

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 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 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 Fe^II/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.