RingOpening Metathesis A Reusable Polymeric Asymmetric Hydrogenation Catalyst
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 – 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
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
What did they use? • The Precursors – trans-Ru. Cl 2(Py)2((R, R)-Norphos) • As the monomer – trans-Ru. Cl 2(=CHPh)(PCy 3)2 – trans-Ru. Cl 2(=CHPh)(PCy 3)(NHC) • Alkylidenes as catalysts
What did they use?
Why did they use them? • trans-Ru. Cl 2(Py)2((R, R)-Norphos) – Easily prepared – Contains no accessible donor atoms that may deactivate catalysts • trans-Ru. Cl 2(=CHPh)(PCy 3)2 and trans. Ru. Cl 2(=CHPh)(PCy 3)(NHC) – Known catalysts developed by Grubbs
What did they try? • Reacted the monomer with 5 mol % of each catalyst for 24 h (22 o. C, CH 2 Cl 2) – Failed to produce a polymer – Models showed sever crowding would exist between adjacent active sites • Needed to reduce the crowding – Too bulky
A Solution • Decided to try using cyclooctene as a spacer monomer
Eureka! • Added 1 equivalent of COE to solution – 33% complete after 3 h • 1 H 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
The Cycle
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 Ba. SO 4 – Chose Ba. SO 4 as a support because it is inert and helps improve the mechanical stability
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
Results • ee obtained from using the original monomer was 48% S • ee obtained from new catalyst was 83% S – Much better
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 Ba. SO 4) for the catalyst
- Slides: 14