Transition Metals in Selective Hydrocarbon Oxygenation Catalysis : Kinetic Studies of the Reactions of Cyclohexylhydroperoxide under Gif Conditions

Iron(III) chloride in the presence of excess H2O2 in 10:1 pyridine-acetic acid catalyses the clean conversion of cyclohexylhydroperoxide to cyclohexanone on the same time scale (hours) as observed for the latter stages of the Barton GoAggII system for the oxygenation of cyclohexane. The addition of one equivalent of picolinic acid/Fe-III causes a significant increase (10 fold) in the conversion rate (no change in products) paralleling the rate enhancing effect observed in the Fe-III-picolinate-H2O2 (GoAggIII) oxygenating system. The Fe-III catalysed reaction in the absence of H2O2, however, is totally different with evidence of a linear induction period before the onset of product autocatalysis by the ketone. Here only a very slight rate enhancement is seen on adding picolinic acid and a different mechanism may be relevant. These and other observations have implications for the nature of reactions taking place in typical 'Gif' systems.