| Abstract
| - A ruthenium-based version of Barton's GifIV-type system (Rucat/Zn/O2 in pyridine/acetic acid) for the selectiveoxygenation of cycloalkanes has been studied in detail for the first time using a range of analytical techniques.The system, based on the use of the triruthenium complex [Ru3O(O2CCH3)6(py)3] in the presence of zinc powderin aerated pyridine/acetic acid (10:1 v/v), affords yields of cyclohexanone (main product) and cyclohexanol fromcyclohexane comparable to that of the well-studied iron system based on the use of [Fe3O(O2CCH3)6(py)3]·pybut with a lower selectivity for the ketone product. The time taken for the appearance and distribution of the-one/-ol products is different for the two metals and also depends on the efficiency of stirring of the zinc powder.The differing -one/-ol ratios and their times of appearance have been correlated with competing reactions on theintermediate cyclohexylhydroperoxide, most likely generated via oxygen- and carbon-centered radical chemistry.The appearance of cyclohexanol much earlier in the reaction for the ruthenium-based system has been traced toa slower assembly reaction for ruthenium to form the species responsible for the ketonization step, which allowsproduction of alcohol via zinc reduction of cyclohexylhydroperoxide to compete successfully. Extensiveinvestigations into the nature of the metal species present during turnover, using cyclic voltammetry, 1H NMR,and UV−vis spectroscopy, show that for either system the divalent monomeric complex trans-[M(O2CCH3)2(py)4] (M = Ru or Fe) is the major species present during the appearance of ketone product. Use of trans-[Fe(O2CCH3)2(py)4] as the precursor reagent results in the highest GifIV activity (conversion yield) toward cyclohexaneoxygenation. It is concluded that formation of sec-alkylhydroperoxides in addition to monomeric divalent complexessuch as trans-[M(O2CCH3)2(py)4] (M = Fe or Ru) are key processes central to the mechanism of the Gif oxygenationprocess toward ring hydrocarbons. The combination Fe(II)/ROOH is considered responsible for the formation ofketone (and some alcohol), most likely via Haber−Weiss chemistry, in competition with formation of alcohol viaZn reduction of ROOH.
- In situ monitoring of the oxygenation of cyclohexane by GifIV-type systems (Mcat/Zn/O2), mediated by [M3O(O2CCH3)6(py)3] or trans-[M(O2CCH3)2(py)4] (M = Ru or Fe), indicates that the intermediate cyclohexylhydroperoxide, most likely generated via the action of oxygen-centered radicals, is transformed to the ketone when the appropriate divalent sites are assembled in solution, in competition with reduction by Zn to afford the alcohol.
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