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Ring-Opening Metathesis

Ring-Opening Metathesis. “A Reusable Polymeric Asymmetric Hydrogenation Catalyst Made by Ring-Opening Olfein Metathesis Polymerization”. By Corbin K. Ralph, Okwado M. Akotsi, and Steven H. Bergens. What did they do?. Made the first polymeric asymmetric hydrogenation catalyst via ROMP

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Ring-Opening Metathesis

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  1. Ring-Opening Metathesis “A Reusable Polymeric Asymmetric Hydrogenation Catalyst Made by Ring-Opening Olfein Metathesis Polymerization” By Corbin K. Ralph, Okwado M. Akotsi, and Steven H. Bergens

  2. What did they do? • Made the first polymeric asymmetric hydrogenation catalyst via ROMP • These types of catalysts are of interest because they are thought to have favorable characteristics of recovery and reuse • Usually synthesized using other types of reactions besides ROMP • Metal center often interferes with the reaction

  3. How did they do it? • First noticed what catalysts other people were creating with ROMP • Predominantly organic-based, but some metal-based polymeric catalysts • Ru, Mo, Pd, and Fe • Decided to focus on Ru

  4. What did they use? • The Precursors • trans-RuCl2(Py)2((R,R)-Norphos) • As the monomer • trans-RuCl2(=CHPh)(PCy3)2 • trans-RuCl2(=CHPh)(PCy3)(NHC) • Alkylidenes as catalysts

  5. What did they use?

  6. Why did they use them? • trans-RuCl2(Py)2((R,R)-Norphos) • Easily prepared • Contains no accessible donor atoms that may deactivate catalysts • trans-RuCl2(=CHPh)(PCy3)2 and trans-RuCl2(=CHPh)(PCy3)(NHC) • Known catalysts developed by Grubbs

  7. What did they try? • Reacted the monomer with 5 mol % of each catalyst for 24 h (22 oC, CH2Cl2) • Failed to produce a polymer • Models showed sever crowding would exist between adjacent active sites • Needed to reduce the crowding • Too bulky

  8. A Solution • Decided to try using cyclooctene as a spacer monomer

  9. Eureka! • Added 1 equivalent of COE to solution • 33% complete after 3 h • 1H NMR after 66% had reacted showed the degree of alternating growth was high • As they increased the ratio of COE to original monomer, the rate increased • 4:1 ratio, 12 times as fast

  10. The Cycle

  11. What did they do with it? • Wanted to create a chiral hydrogenation catalyst • Cross-linked the ends of the long-chain polymer using dicyclopentadiene • Coated the catalyst as a thin film over BaSO4 • Chose BaSO4 as a support because it is inert and helps improve the mechanical stability

  12. Results • Used this catalyst to hydrogenate 1’-acetonaphthone • Ran for 2 h and compared to homogeneous run using the original monomer • Rate was ~40% the rate of the original monomer • Showed low mass transport losses • Isolated the catalyst via filtration and reused it 10 more times • No significant drops in enantiomeric excess (ee) or rate

  13. Results • ee obtained from using the original monomer was 48% S • ee obtained from new catalyst was 83% S • Much better

  14. In Conclusion… • Important finding because it is a reusable catalyst and has good yields • Synthesized more directly than other catalysts • To further explore this topic… • Figure out a monomer than doesn’t require a spacer • Try different backbones (instead of BaSO4) for the catalyst

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