The use of monodentate ligands results in structural fluxionality of metal complexes, and has been shown to be the key to success in asymmetric alkene metathesis reactions
Ring-closing metathesis (RCM) has been phenomenally successful in the field of organic synthesis in recent years. The reaction allows the construction of a wide range of ring sizes and is compatible with many functional groups. There are many reactions that could be achieved by RCM, but for which no effective catalysts have been reported. Two areas in which this is most notable are in asymmetric RCM and in the reaction of sterically hindered alkenes.
Now, Amir Hoveyda and co-workers from Boston College, in collaboration with Richard Schrock from MIT, have synthesized1 a series of molybdenum alkylidene catalysts that can boast success in both these challenging areas. Hoveyda and co-workers reasoned that structural fluxionality in the catalyst would be a key factor in achieving an effective reaction, allowing the catalyst to easily adopt certain geometries required by the mechanism of the reaction. Focusing their efforts on the use of molybdenum catalysts bearing two different monodentate ligands (only one of which is chiral) resulted in the identification of a series of stereogenic-at-metal catalysts.
The new series of catalysts was shown to be far more reactive than previously reported enantioselective RCM catalysts, providing high conversion rates in hours rather than days. Furthermore, the enantioselectivities were as good as or better than previously reported. Finally, one of the catalysts was shown to be particularly effective in an RCM reaction that is the key synthetic step in the asymmetric synthesis of the natural product quebrachamine.
Malcolmson, S. J., Meek, S. J., Sattely, E. S., Schrock, R. R. & Hoveyda, A. H. Highly efficient molybdenum-based catalysts for enantioselective alkene metathesis. Nature 10.1038/nature07594 (2008).
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Davey, S. Monodentate magic. Nature Chem (2008). https://doi.org/10.1038/nchem.91