Ring-Opening Metathesis

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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
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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

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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

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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

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What did they use?
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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)(P
  Cy3)(NHC)
• Known catalysts developed by Grubbs

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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

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A Solution

• Decided to try using cyclooctene as a spacer
  monomer

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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
• 41 ratio, 12 times as fast

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The Cycle
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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

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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

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Results

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

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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 doesnt require a
  spacer
• Try different backbones (instead of BaSO4) for
  the catalyst