A Comparison Between Iron and Ruthenium Complexes Mediating Gif ^IV-type Oxygenations on Cyclohexane

A ruthenium-based version of Barton's Gif(IV)-type system (Ru-cat/Zn/O-2 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 efficiency 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, H-1 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 in 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.