The functionalization of saturated hydrocarbons. Part 20.+ Alkyl hydroperoxides: Reaction intermediates in the oxidation of saturated hydrocarbons by gif-type reactions and mechanistic studies on their formation

Article abstract of DOI:10.1021/ja00032a032

Chemical and ¹³C and ²H NMR spectroscopic evidence is presented, proving that secondary alkyl hydroperoxides are reaction intermediates in the oxidation of saturated hydrocarbons under GoAgg^II conditions (ferric chloride, hydrogen peroxide in pyridine - acetic acid solution). Isolation of cyclohexyl hydroperoxide from a GoAgg^III oxidation of cyclohexane (GoAgg^II + picolinic acid as catalyst) and the effect of reducing agents (such as thiophenol, benzeneselenol, diphenyl disulfide, and diphenyl diselenide) on the Gif^IVoxidation of cyclohexane (ferrous chloride, zinc powder, oxygen gas, in pyridine - acetic acid solution) permit a generalization of the alkyl hydroperoxide intermediacy to the whole family of Gif systems. All those reducing agents lower the ketone/alcohol ratio, with either a decrease in the total amount of hydrocarbon activation or formation of phenylseleno derivatives. Triphenylphosphine (up to 3.5 mmol) shows the same effect on the ketone/alcohol ratio, but the total amount of activation remains approximately constant and no byproduct is formed. The mechanism of formation of the alkyl hydroperoxide intermediate has been studied using ¹⁸O₂- The results indicate that the oxygen atoms in the alkyl hydroperoxide do not arise directly from hydrogen peroxide but from O₂, formed in situ by the well-known iron(III)-catalyzed decomposition of hydrogen peroxide. The mechanism of formation of alcohols under these conditions has been addressed as well. The results from experiments in the presence of H₂¹⁸O, combined with those from the above-mentioned experiments under an ¹⁸O₂ atmosphere, proved that the oxygen atom in the alcohol does not arise from a hydroxy species but from reduction of the intermediate alkyl hydroperoxide. The participation of another reaction intermediate (prior to the alkyl hydroperoxide) has been secured. If O₂ is eliminated from the reaction mixture (running the reaction at reduced pressure) this first intermediate A can be trapped with Tempo. However, when the reaction was carried out at reduced pressure and in the absence of Tempo, no traces of alkylpyridine coupled products were detected. These alkylpyridines have been demonstrated to be formed when carbon radicals are generated under the same reaction conditions by photolysis of the corresponding N-hydroxypyridine-2-thione derivative of an alkyl carboxylic acid. This paradox can only be explained if intermediate A is not a carbon-centered radical. In a competitive experiment a ratio cyclopentyl-Tempo adduct/cyclohexyl-Tempo adduct of 0.66 was obtained. This figure differs from the values reported for typical carbon-radical competitive experiments, where cyclopentane is more reactive than cyclohexane. This further supports the nonradical nature of reaction intermediate A. Finally, studies with trimethyl phosphite, which affords with cyclohexane cyclohexyl dimethyl phosphate, show that cyclohexyl peroxyl radical is not an intermediate in the formation of the hydroperoxide as it would be if the cyclohexyl radical was reacting with oxygen.