Microwave-assisted molybdenum-catalysed epoxidation of olefins

The performance of CpMo(CO)3CH3 as a catalyst precursor in the epoxidation of cis-cyclooctene and R-(+)-limonene with tert-butylhydroperoxide (TBHP), at 55 °C, was investigated for the first time using microwave (MW)-assisted heating, and the results were compared with those obtained using conventional heating with a thermostated oil bath (OB). The reactions were carried out under monophasic (no co-solvent, or with 1,2-dichloroethane, toluene, hexane) or biphasic liquid–liquid (using the ionic liquid 1-butyl-3-methylimidazolium tetrafluoroborate, [BMIM][BF4]) conditions. The heating methods did not affect product selectivity: 100% to cyclooctene oxide, and 93% to limonene oxide (66–72% diastereomeric excess, trans-1,2-(+)-limonene oxide) at 80% limonene conversion. However, the initial reaction rates strongly depend on the applied heating method. The favourable MW effect is essentially due to the faster heating up of the reaction mixture increasing the overall epoxidation rate in the early stage of the heating process, which is supported experimentally by a comparison of (i) the measured temperature vs time curves, (ii) the kinetic data for the MW method with that for the OB method where the reagents and reactor walls are pre-heated (OB-preH method), and (iii) the estimated apparent activation energies and pre-exponential factors for the two olefins using the MW, OB, and OB-preH methods. The crystal structure of the molybdenum tricarbonyl catalyst precursor was unveiled from single-crystal X-ray diffraction studies at 150 K. The complex crystallises as a racemic twin in the monoclinic P21 space group.

This work presents a comparison between microwave-assisted heating and conventional oil bath heating for liquid-phase olefin epoxidation using CpMo(CO)3CH3 as catalyst precursor and tert-butylhydroperoxide as oxidant. Results showed that the microwave-assisted method benefits the initial reaction rate and the recyclability of the system, without affecting selectivities to the epoxides.Figure optionsDownload high-quality image (111 K)Download as PowerPoint slide