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

Abstract

A novel cyclopentadienyl ligand bearing a chiral oxazoline pendant group (Cpox) has been prepared. Its coordination to molybdenum and tungsten afforded optically pure (R)-CpoxM(η3-C 3H5)(CO)2 (M = Mo, W) in which the pendant oxazoline fragment is not coordinated to the metal center. Reaction of (R)-CpoxMo(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 [CpoxMo(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 [CpoxMo(CO)2(NCMe)]+ has been proved. Copyright © 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.