Examining Polytopal Rearrangements around Ruthenium

by John Phillips on May 31, 2014

Torker, S.; Khan, R.K.M.; Hoveyda, A.M. “The Influence of Anionic Ligands on Stereoisomerism of Ru Carbenes and Their Importance to Efficiency and Selectivity of Catalytic Olefin Metathesis Reactions” J. Am. Chem. Soc. 2014, 136, 3439-3455.

For the past 15 years ruthenium catalysts have been a workhorse for olefin metathesis. Chemists have been able to tune ruthenium complexes to exhibit a variety of properties ranging from slow/fast initiation, selectivity in cross metathesis and stereochemical control. Recent work by Hoveyda and co-workers analyze the mechanism of stereochemical inversion of stereogenic-at-Ru complexes that proceed through both non-olefin-metathesis (non-OM) and olefin-metathesis (OM) based polytopal rearrangement pathways (Figure 1). A deeper understanding of the factors governing these polytopal rearrangements is important because of the implications it has in understanding the stereochemcial control of different catalysts. Their work is focused in two areas: computational studies of the various polytopal rearrangements followed by experimental analysis and conclusions of the calculations.

Figure 1. Polytopal rearrangements

fig_1_rrTo begin with, Hoveyda and coworkers use computational calculations to analyze the mechanism of both non-OM-based and OM-based polytopal rearrangements. Initially the mechanism of non-OM-based polytopal rearrangements were examined both under thermally and acid-catalyzed conditions. These computational studies are then built on with further calculations studying the driving force for polytopal rearrangements. Interestingly, Hoveyda and coworkers postulate that the basis for the rearrangements are two-fold and are related to donor-donor interactions and the dipole effects that occur from anionic ligands. They then use the foundation of their calculations to build a mechanistic understanding of the implications anionic ligands can have toward the outcome of olefin metathesis reactions.

Hoveyda and co-workers conclude with a couple examples of catalysts designed based on their calculations. In the first example, a tertiary allylic alcohol is used to demonstrate how both hydrogen bonding and ligand interactions can cause what would generally be deemed a slow reaction to proceed quickly. In the second example, a dianionic ligand is used to control the stereochemistry of olefin metathesis yielding a product with high cis selectivity (Figure 2). The selectivity can be attributed to the small ring restriction of the dianionic ligand which disfavors a conformation where both charged groups are syn to the NHC ligand thereby providing the cis stereochemical control. This resulting distorted geometry disallows addition of the alkene anti to the NHC. Both of these examples elegantly showcase the insight gained from their earlier calculations.

Figure 2. Newly designed catalyst with dianionic ligand


These studies by Hoveyda and co-workers are a great example of how calculations can be used to gain a deeper understanding of the reactions at work. Furthermore the examples of catalyst systems based on the calculation synergistically strengthen the conclusions. This effort demonstrates that the development of metathesis catalysts is not stagnant and there is definitely much more to be discovered in the near future.


Metathesis Featured at Modern Synthetic Methods & Chiral USA

May 1, 2014

Modern Synthetic Methods & Chiral USA Resort Orlando-Celebration Hotel, Florida USA May 19–21, 2014 Scientific Update has created a 3-day conference and exhibition to bridge a gap between industry and academia. Since industry rarely devises new synthetic methods to perform each step in the synthetic route, the event will provide a forum where industry and […]

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Cooperation between a Organometallic Complex and a Metalloenzyme

April 28, 2014

Denard, C. A.; Huang; H.; Bartlett, M. J., Lu, L.; Tan, Y.; Zhao, H.; and Hartwig, J. F. Angew. Chem. Int. Ed. 2014, 53, 465. Recently, the Hartwig group reported a tandem reaction in which a metalloenzyme and a metathesis catalyst worked cooperatively to yield products which cannot be generated in comparable yield by sequential […]

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Olefin Metathesis at the 247th ACS Meeting

February 27, 2014

The 247th ACS National Meeting and Exposition is less than a month away. The late winter/early spring event is set for Dallas, Texas from March 16-20, 2014. Olefin metathesis has no problem fitting into the meeting’s theme of “Chemistry and Materials for Energy” and keeping with tradition, we here at All Things Metathesis will provide […]

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Ruthenium Initiators Deliver Syndiotactic Polymers

December 16, 2013

Keitz, B.K.; Fedorov, A.; Grubbs, R.H. “Cis-Selective Ring-Opening Metathesis Polymerization with Ruthenium Catalysts” J. Am. Chem. Soc. 2012, 134, 2040-2043. Rosebrugh, L.E.; Marx, V.M.; Keitz, B.K.; Grubbs, R.H. “Synthesis of Highly Cis, Syndiotactic Polymers via Ring-Opening Metathesis Polymerization Using Ruthenium Metathesis Catalysts” J. Am. Chem. Soc. 2013, 135, 10032-10035. Controlling the stereochemistry of ring opening […]

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New Developments in Cis-Selective Ruthenium Metathesis Catalysts

November 22, 2013

Since Daryl Allen wrote his post in 2011 on the development of the first cis-selective ruthenium catalysts with chelating NHC ligands, a lot of activity and exciting new developments occurred in the area which warrants an update. After the pivalate complex 1 was published1, it was found that the nitrate derivative 2 is more stable and […]

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Major Achievement in Olefin Metathesis: First World-Scale Bio-Refinery

October 3, 2013

July 18, 2013 was a very exciting day for the olefin metathesis community. The first press release from Elevance Renewable Sciences and Wilmar International Limited reported that their joint venture, a $40 million olefin metathesis-based bio-refinery located in Gresik, Indonesia was operational, leading to the first shipments of commercial products to customers.1 The Gresik facility […]

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