Selective Oxygenation of Cyclohexene by Dioxygen via an Iron(V)-Oxo Complex-Autocatalyzed Reaction

Abstract

An iron complex with a tetraamido macrocyclic ligand, [(TAML)Fe-III](-), was found to be an efficient and selective catalyst for allylic oxidation of cyclohexene by dioxygen (O-2); cyclohex-2-enone was obtained as the major product along with cyclohexene oxide as the minor product. An iron(V)-oxo complex, [(TAML)Fe-V(O)](-), which was formed by activating O-2 in the presence of cyclohexene, initiated the autoxidation of cyclohexene with O-2 to produce cyclohexenyl hydroperoxide, which reacted with [(TAML)-Fe-III](-) to produce [(TAML)Fe-V(O)](-) by autocatalysis. Then, [(TAML)Fe-V(O)](-) reacted rapidly with [(TAML)-Fe-III](-) to produce a mu-oxo dimer, [(TAML)Fe-IV(O)Fe-IV(TAML)](2-), which was ultimately converted to [(TAML)Fe-V(O)](-) when [(TAML)Fe-III](-) was not present in the reaction solution. An induction period was observed in the autocatalytic production of [(TAML)Fe-V(O)](-). The induction period was shortened with increasing catalytic amounts of [(TAML)Fe-V(O)](-) and cyclohexenyl hydroperoxide, whereas the induction period was prolonged by adding catalytic amounts of a spin trapping reagent such as 5,5-dimethyl-1-pyrroline N-oxide (DMPO). The allylic oxidation of cycloalkenes was also found to depend on the allylic CH bond dissociation energies, suggesting that the hydrogen atom abstraction from the allylic CH bonds of cycloalkenes is the rate-determining radical chain initiation step. In this study, we have shown that an iron(III) complex with a tetraamido macrocyclic ligand is an efficient catalyst for the allylic oxidation of cyclohexene via an autocatalytic radical chain mechanism and that [(TAML)Fe-V(O)](-) acts as a reactive intermediate for the selective oxygenation of cyclohexene with O-2 to produce cyclohex-2-enone predominantly.