Allylic oxidation of olefins with a manganese-based metal-organic framework

Selective oxidation of olefins to alpha,beta-unsaturated ketones under mild reaction conditions has attracted considerable interest, since alpha,beta-unsaturated ketones can serve as synthetic precursors for various downstream chemical products. The major inherent challenges with this chemical oxidation are chemo- and regio-selectivity as well as environmental concerns, i.e. catalyst recycle, safety and cost. Using atmospheric oxygen as an environmentally friendly oxidant, we found that a metal-organic framework (MOF) constructed with Mn and a tetrazolate ligand (CPF-5) showed good activity and selectivity for the allylic oxidation of olefins to alpha,beta-unsaturated ketones. Under the optimized conditions, we could achieve 98% conversion of cyclohexene and 87% selectivity toward cyclohexanone. The combination of a substoichiometric amount of TBHP (tert-butylhydroperoxide) and oxygen not only provides a cost effective oxidation system but significantly enhances the selectivity to alpha,beta-unsaturated ketones, outperforming most reported oxidation methods. This catalytic system is heterogeneous in nature, and CPF-5 could be reused at least five times without a significant decrease in its catalytic activity and selectivity.