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

• Autor(a) principal: Reis, Patrícia
• Data de Publicação: 2011
• Outros Autores: Gamelas ou Carla A. Gamelas, Carla, Brito, José, Saffon, Nathalie, Goméz, Montserrat, Royo, Beatriz
• Tipo de documento: Artigo
• Idioma: eng
• Título da fonte: Repositório Científico de Acesso Aberto de Portugal (Repositórios Cientìficos)
• Texto Completo: http://hdl.handle.net/10400.26/21979

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-C3H5)(CO)2 (M = Mo, W) in which the pendant oxazoline fragment is not coordinated to the metal center. Reaction of (R)-CpoxMo(η3-C3H5)(CO)2 with tetrafluoroboric acid gives the bidentate η5-cyclopentadienyloxazoline complex [CpoxMo(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)-CpoxW(η3-C3H5)(CO)2 displayed high catalytic activity achieving quantitative conversion of cyclooctene epoxide in 2 h with H2O2. [CpoxMo(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 [CpoxMo(O2)O]+ and dioxido [CpoxMoO2]+ 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.