Solvent-free allylic oxidation of alkenes with O2 mediated by Fe- and Cr-MIL-101

Abstract

Catalytic properties of Fe-MIL-101 and Cr-MIL-101 metal–organic frameworks in the solvent-free oxidation of cyclohexene and α-pinene with molecular oxygen have been explored. Both catalysts allow alkene oxidation under mild conditions (1bar O2, 40–60°C) and afford allylic oxidation products. The nature of catalysis and the product distribution strongly depend on the nature of the transition metal. Cr-MIL-101 behaves as truly heterogeneous catalyst to give predominantly α,β-unsaturated ketones. Catalysis over Fe-MIL-101 has true heterogeneous nature only at 40°C, producing mainly 2-cyclohexene-1-ol. At 50–60°C, iron leaching into solution occurs, leading to cyclohexenyl hydroperoxide as the major product. Under optimal conditions, both catalysts can be reused several times without suffering a loss of the catalytic properties. Rate-retarding and rate-accelerating effects of inhibitors and initiators, respectively, indicate radical chain mechanism. Different pathways for transformation of hydroperoxide have been suggested to rationalize the observed differences in the reaction selectivities over Cr- and Fe-MIL-101.