Chiral Cationic Cp ' Mo(CO)(2)(NCMe) (+) Species - Catalyst Precursors for Olefin Epoxidation with H2O2 and tert-Butyl Hydroperoxide

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Abstract

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)(CpM)-M-ox(eta(3)-C3H5)(CO2)(2) (M = Mo, W) in which the pendant oxazoline fragment is not coordinated to the metal center. Reaction of (R)-(CpMo)-Mo-ox(eta(3)-C3H5)(CO)(2) with tetrafluoroboric acid gives the bidentate eta(5)-cyclopentadienyloxazoline complex [(CpMo)-Mo-ox(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-beta-methylstyrene with H2O2 and tertbutyl hydroperoxide (TBHP) as oxidants has been studied. (R)-(CpW)-W-ox(eta(3)-C3H5)(CO)(2) displayed high catalytic activity achieving quantitative conversion of cyclooctene epoxide in 2 h with H2O2. [(CpMo)-Mo-ox(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 [(CpMo)-Mo-ox(CO)(2)(NCMe)](+) with H2O2 and TBHP revealed the in situ formation of the corresponding peroxido [(CpMo)-Mo-ox(O-2)O](+) and dioxido [(CpMoO2)-Mo-ox](+) 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 [(CpMo)-Mo-ox(CO)(2)(NCMe)](+) has been proved.
Original languageUnknown
Pages (from-to)666-673
JournalEuropean Journal of Inorganic Chemistry
Volumen.d.
Issue number5
DOIs
Publication statusPublished - 1 Jan 2011

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