Room-temperature tandem conversion of cyclic alkenes into 1,2-diols using molecular oxygen and -MnO₂ heterogeneous catalyst

In the cyclic alkene epoxidation reactions using molecular oxygen as oxidant, aldehyde is generally added as co-oxidant to assist transition metal catalyst to activate the oxygen molecule, oxidizing cyclic alkene and producing the corresponding epoxide; however, the added aldehyde would be converted into byproduct carboxylic acid. Herein, we report the room-temperature tandem conversion of cyclic alkenes into 1,2-diols through the application of byproduct carboxylic acid as a catalyst for epoxide hydration reaction, realizing effective utilization of the byproduct and saving energy consumption. Four kinds of nanosized MnO2 specimens, including alpha-, beta-, delta-, and gamma-MnO2, were hydrothermally synthesized and applied as heterogeneous catalysts for the room-temperature tandem conversion of cyclohexene into 1,2-cyclohexanediol using molecular oxygen as oxidant and isobutyraldehyde as co-oxidant. The characterization results demonstrated that the obtained beta-MnO2 sample had the maximum Mn4+/Mn3+ ratio of 1.42 and the minimum Oads/OLatt ratio of 0.34, consistent with its least oxygen vacancies evidenced by EPR measurement. Accordingly, the beta-MnO2 sample revealed the quickest alkene epoxidation reaction rate and could achieve a 99.2% cyclohexene conversion within 1 h, and meanwhile, a 73.6% 1,2-cycloheanediol yield could be attained within 24 h. A plausible mechanism for the tandem conversion of cyclohexene into 1,2-cyclohexanediol over the beta-MnO2 heterogeneous catalyst was proposed and further verified by quenching experiments and DFT calculations. Furthermore, the synthesized beta-MnO2 catalyst sample could be reused more than ten times, and meanwhile, the tandem conversion way could be expanded to most of the cyclic alkene substrates, which demonstrated that the beta-MnO2 catalyst was an active and stable heterogeneous catalyst for tandem conversion of cyclic alkenes into 1,2-diols.