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-RuCl2(Py)2((R,R)-Norphos)

• As the monomer

– trans-RuCl2(=CHPh)(PCy3)2

– trans-RuCl2(=CHPh)(PCy3)(NHC)• Alkylidenes as catalysts

What did they use?

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

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

A Solution

• Decided to try using cyclooctene as a spacer monomer

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

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 BaSO4

– Chose BaSO4 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 BaSO4) for the catalyst