The Invention of Chemical Reactions

Chemical and 13 C- and 2 H-N.M.R. spectroscopic evidence is presented, proving that secondary alkyl hydroperoxides are reaction intermediates in the oxidation of saturated hydrocarbons under GoAggli 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 benzeneselenol) on the Gif dV oxidation 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. Benzeneselenol lowers the ketone to alcohol ratio, with formation of phenylsekno-derivatives. Triphenylphosphine (up to 3.5 mmol) shows the same effect on the ketone to alcohol ratio, but the total amount of activation remains approximately constant and no by-product is formed. The mechanism of formation of the alkyl hydroperoxide intermediate has been studied using 18 O 2 . The results indicate that the oxygen atoms in the alkyl hydroperoxide do not arise directly from hydrogen peroxide but from O 2 , 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 H 2 18 O, combined with those from the above mentioned experiments under an 18 O 2 atmosphere, proved that the oxygen atom in the alcohol does not arise from an 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 O 2 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 2 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-hydroxy-pyridine-2-thione derivative of an alkyl-carboxylic acid. This paradox can only be explained if intermediate A is not a carbon centered radical. Finally, studies with trimethylphosphite, which affords with cyclohexane cyclohexyl dimethyl phosphate, show that the 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.