Synthesis and Catalytic Applications of ansa Compounds with Cycloalkyl Moieties as Bridging Units: A Comparative Study

Abstract

The molybdenum and tungsten compounds (Mo{η⁵-C₅H₄[CH(CH₂)_n]-η¹-CH}(CO)₃) and (W{η⁵-C₅H₄[CH(CH₂)₃]-η¹-CH}(CO)₃) (2a, 3an=3; 2b, 3bn=4; 2c, 3cn=5; 2d, 3dn=6) were synthesized by reacting the spiro-bicyclic compounds 1a–d with the complex [M(CO)₃(tach)] (M=molybdenum, tungsten; tach=1,3,5-trimethylhexahydro-1,3,5-triazine). NMR spectroscopy, as well as X-ray diffraction studies, confirm the formation of an intramolecular ansa bridge. The complexes display a good stability in the solid state (stable up to 180 °C under air, as determined by thermogravimetric studies) and are highly active catalysts at room temperature (molybdenum compounds) or above (tungsten compounds) in olefin epoxidation. In the case of cyclooctene as substrate, TOFs up to ca. 11800 h⁻¹ are obtained. Moreover, most of the catalysts described here display a high selectivity in the epoxidation of cis- and trans-stilbene. In addition, the novel complexes were compared with previously synthesized related compounds, at least matching their catalytic performances.