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

• Published in: Green chemistry : an international journal and green chemistry resource : GC Vol. 25; no. 22; pp. 9262 - 9271
• Main Authors: Su, Shihao, Lv, Guojun, Zou, Xuyang, Wang, Jiangzhang, Zhou, Chaoyi, Chen, Yan, Shen, Jialing, Shen, Yangbin, Liu, Zhongmin
• Format: Journal Article
• Language: English
• Published: 13.11.2023
• ISSN: 1463-9262; 1463-9270
• DOI: 10.1039/D3GC02863A

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 MnO 2 specimens, including α-, β-, δ-, and γ-MnO 2 , 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 β-MnO 2 sample had the maximum Mn 4+ /Mn 3+ ratio of 1.42 and the minimum O ads /O Latt ratio of 0.34, consistent with its least oxygen vacancies evidenced by EPR measurement. Accordingly, the β-MnO 2 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 β-MnO 2 heterogeneous catalyst was proposed and further verified by quenching experiments and DFT calculations. Furthermore, the synthesized β-MnO 2 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 β-MnO 2 catalyst was an active and stable heterogeneous catalyst for tandem conversion of cyclic alkenes into 1,2-diols.