Comparison Between Iron and Ruthenium-reagents Mediating Gif(IV)-type Oxygenation of Cyclohexane(section)
A ruthenium-based version of Barton's Gif(IV)-type system (Ru(cat)/Zn/O2 in pyridine/acetic acid) for the selective oxygenation 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 powder in aerated pyridine/acetic acid (10:1 v/v), affords yields of cyclohexanone (main product) and cyclohexanol from cyclohexane comparable to that of the well-studied iron system based on the use of [Fe3O(O2CCH3)6(py)3]·py but 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 effficiency of stirring of the zinc powder. The differing -one/-ol ratios and their times of appearance have been correlated with competing reactions on the intermediate 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 to a slower assembly reaction for ruthenium to form the species responsible for the ketonization step, which allows production of alcohol via zinc reduction of cyclohexylhydroperoxide to compete successfully. Extensive investigations 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 Gif(IV) activity (conversion yield) toward cyclohexane oxygenation. It is concluded that formation of sec-alkylhydroperoxides m addition to monomeric divalent complexes such as trans-[M(O2CCH3)2(py)4] (M = Fe or Ru) are key processes central to the mechanism of the Gif oxygenation process toward ring hydrocarbons. The combination Fe(II)/ROOH is considered responsible for the formation of ketone (and some alcohol), most likely via Haber-Weiss chemistry, in competition with formation of alcohol via Zn reduction of ROOH.
S. B. Marr et al., "Comparison Between Iron and Ruthenium-reagents Mediating Gif(IV)-type Oxygenation of Cyclohexane(section)," Inorganic Chemistry, American Chemical Society (ACS), Jan 2000.
The definitive version is available at http://dx.doi.org/10.1021/ic0001560
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