Chiral Cationic [Cp′Mo(CO)2(NCMe)]+ Species – Catalyst Precursors for Olefin Epoxidation with H2O2 and tert-Butyl Hydroperoxide
✍ Scribed by Patrícia M. Reis; Carla A. Gamelas; José A. Brito; Nathalie Saffon; Montserrat Gómez; Beatriz Royo
- Publisher
- John Wiley and Sons
- Year
- 2011
- Tongue
- English
- Weight
- 423 KB
- Volume
- 2011
- Category
- Article
- ISSN
- 1434-1948
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✦ Synopsis
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)‐Cp^ox^M(η^3^‐C~3~H~5~)(CO)~2~ (M = Mo, W) in which the pendant oxazoline fragment is not coordinated to the metal center. Reaction of (R)‐Cp^ox^Mo(η^3^‐C~3~H~5~)(CO)~2~ with tetrafluoroboric acid gives the bidentate η^5^‐cyclopentadienyloxazoline complex [Cp^ox^Mo(CO)~2~(NCMe)]BF~4~ 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 H~2~O~2~ and tert‐butyl hydroperoxide (TBHP) as oxidants has been studied. (R)‐Cp^ox^W(η^3^‐C~3~H~5~)(CO)~2~ displayed high catalytic activity achieving quantitative conversion of cyclooctene epoxide in 2 h with H~2~O~2~. [Cp^ox^Mo(CO)~2~(NCMe)]^+^ has been shown to be an efficient catalyst with TBHP and H~2~O~2~, reaching quantitative conversions of the corresponding epoxide in 30 min and 11 h, respectively. ESI‐MS studies of the reaction of [Cp^ox^Mo(CO)~2~(NCMe)]^+^ with H~2~O~2~ and TBHP revealed the in situ formation of the corresponding peroxido [Cp^ox^Mo(O~2~)O]^+^ and dioxido [Cp^ox^MoO~2~]^+^ 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^ox^Mo(CO)~2~(NCMe)]^+^ has been proved.