Chiral cationic [Cp′Mo(CO)₂(NCMe)]⁺ species-catalyst precursors for olefin epoxidation with H₂O ₂ and tert-butyl hydroperoxide

A novel cyclopentadienyl ligand bearing a chiral oxazoline pendant group (Cp^ox) has been prepared. Its coordination to molybdenum and tungsten afforded optically pure (R)-Cp^oxM(η³-C ₃H₅)(CO)₂ (M = Mo, W) in which the pendant oxazoline fragment is not coordinated to the metal center. Reaction of (R)-Cp^oxMo(η³-C₃H ₅)(CO)₂ with tetrafluoroboric acid gives the bidentate η⁵-cyclopentadienyloxazoline complex [Cp ^oxMo(CO)₂(NCMe)]BF₄ in which the oxazoline is coordinated through the N-atom to the molybdenum center. Their catalytic performance in the epoxidation of cis-cyclooctene, (R)-limonene, and trans-β-methylstyrene with H₂O₂ and tert-butyl hydroperoxide (TBHP) as oxidants has been studied. (R)-Cp ^oxW(η³-C₃H₅)(CO) ₂ displayed high catalytic activity achieving quantitative conversion of cyclooctene epoxide in 2 h with H₂O₂. [Cp ^oxMo(CO)₂(NCMe)]⁺ has been shown to be an efficient catalyst with TBHP and H₂O₂, reaching quantitative conversions of the corresponding epoxide in 30 min and 11 h, respectively. ESI-MS studies of the reaction of [Cp^oxMo(CO) ₂(NCMe)]⁺ with H₂O₂ and TBHP revealed the in situ formation of the corresponding peroxido [Cp ^oxMo(O₂)O]⁺ and dioxido [Cp ^oxMoO₂]⁺ species, respectively. Further oxidation of these complexes resulted in the loss of the cyclopentadienyloxazoline ligand. Based on spin trap experiments, the involvement of both carbon-and oxygen-centred radicals in the olefin epoxidation catalyzed by [Cp^oxMo(CO)₂(NCMe)]⁺ has been proved.