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

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 (O₂); 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 O₂ in the presence of cyclohexene, initiated the autoxidation of cyclohexene with O₂ 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 μ-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 C-H bond dissociation energies, suggesting that the hydrogen atom abstraction from the allylic C-H 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 O₂ to produce cyclohex-2-enone predominantly.