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
To 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.