Heterogeneous oxidation catalysts formed in situ from molybdenum tetracarbonyl complexes and tert-butyl hydroperoxide

The tetracarbonyl complexes cis-[Mo(CO)4{2-[3(5)-pyrazolyl]pyridine}] (1) and cis-[Mo(CO)4{ethyl[3-(2-pyridyl)-1-pyrazolyl]acetate}] (2) were obtained in excellent yields by microwave-assisted heating of a mixture of Mo(CO)6 and the organic ligand in toluene at 110 °C for 30 min. Complexes 1 and 2 are air stable, storable catalyst precursors for the epoxidation of olefins by tert-butyl hydroperoxide (TBHP). In situ oxidative decarbonylation of 1 gives the tetranuclear compound [Mo4O12{2-[3(5)-pyrazolyl]pyridine}4] (3). Elemental analysis and infrared data indicate that the oxidation of 2 gives an octameric species with the molecular formula [Mo8O24{ethyl[3-(2-pyridyl)-1-pyrazolyl]acetate}4] (4). The molybdenum oxide/pyrazolylpyridine compounds 3 and 4, which can be readily prepared in very good yields from the parent carbonyls upon reaction with TBHP in n-decane/CH2Cl2 at room temperature, are active and stable heterogeneous catalysts for the epoxidation of cis-cyclooctene (Cy) by TBHP, giving 1,2-epoxycyclooctane as the only product. The highest activity is found for 4: ca. 90% epoxide yield after 6 h reaction at 55 °C. Further experiments confirmed that 2 is an effective catalyst precursor for (i) the epoxidation of Cy by aqueous TBHP, giving the corresponding epoxide as the only product, (ii) the selective epoxidation of R-(+)-limonene by TBHP (in n-decane), and (iii) the selective oxidation of methyl phenyl sulfide to the corresponding sulfoxide by aqueous TBHP or H2O2 at 35 °C.

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► Efficient microwave-assisted synthesis of tetracarbonyl-pyrazolylpyridine complexes.
► Oxidative decarbonylation gives molybdenum oxide/pyrazolylpyridine catalysts.
► High selectivities in the epoxidation of olefins and the sulfoxidation of sulfides.
► The heterogeneous catalysts are stable and water-tolerant.