Recently, I wrote a post describing a series of Ru-olefin metathesis catalysts developed in the Grubbs’ laboratory, containing one sterically demanding X-type ligand as a replacement for the standard chloride ligand of the classic Grubbs-type catalyst. These species displayed greater cis-selectivity in a standard cross-metathesis assay1 compared to the traditional 2nd generation catalysts; however, they still provided the trans isomer as the major product. This E/Z selectivity is a difficult problem in olefin metathesis, owing to the thermodynamic nature of the process and the preference for the formation of the favored trans isomer.
Now, Koji Endo and Bob Grubbs have described the first report of a Ru-based catalyst that favors the formation of cis olefins, employing a novel chelated NHC catalyst architecture in conjunction with a pivalate ligand. The catalyst synthesis involves treatment of a standard 2nd generation Grubbs-Hoveyda type catalyst with silver pivalate, which leads to a subsequent intramolecular C-H activation of the ortho-CH3 group of the symmetrical mesityl NHC ligand, or of the CH2 group of the 1-adamantyl substituent in the unsymmetrical adamantyl/mesityl NHC ligand. This marks the first report of such C-H activated chelates providing metathesis active complexes. Generally, this type of NHC ligand derived C-H activation occurs from coordinatively unsaturated species during a metathesis reaction, leading to catalyst decomposition.
Employing the standard cross-metathesis assay previously described, the chelated-adamantyl catalyst provides the heterocoupled product in 87% Z-selectivity at 64% reaction conversion. Interestingly, allylbenzene also undergoes self-metathesis during the above cross-metathesis assay, providing the homocoupled product in >95% Z-selectivity. Also noteworthy is the fact that the best experimental conditions involve performing the reaction in a 1:1 mixture of THF:H2O, thus, dry solvents are not a requirement. However, it is noted that strict exclusion of oxygen is required.
The above work has led to a follow-up communication from the Grubbs’ group describing the use of the chelated-adamantyl catalyst for the cis-selective homodimerization of terminal olefins.2 The catalyst was effective for a variety of terminal olefin substrates, supplying the corresponding homocoupled products with high levels of Z-selectivity. The catalyst was also shown to be tolerant to a wide array of reaction solvents and temperatures (25 °C – 70 °C). Notably, these results compare favorably to those previously reported from the research groups of Professors Schrock and Hoveyda.3
Finally, as we can see from the concluding remark from this communication:
“However, despite the recent success of ruthenium and Group VI systems, new catalysts, which undergo more turnovers and function under practical experimental conditions, are clearly needed to tackle more advanced olefin substrates and metathesis reactions.”
There are still significant challenges to address when dealing with this difficult problem, and personally, I look forward to seeing what new research is spurred in this area from the Grubbs’ lab and the various other skilled laboratories that have active research programs in the field of olefin metathesis. Novel catalyst developments will ultimately provide new opportunities and applications for this versatile technology.
1 Ritter, T.; Hejl, A.; Wenzel, A. G.; Funk, T. W.; Grubbs, R. H. “A Standard System of Characterization for Olefin Metathesis Catalysts.” Organometallics, 2006, 25, 5740-5745.
2 Keitz, B. K.; Endo, K.; Herbert, M. B.; Grubbs, R. H. “Z-Selective Homodimerization of Terminal Olefins with a Ruthenium Metathesis Catalyst.” J. Am. Chem. Soc., 2011, 133, 9686-9688.
3 (a) Jiang, A. J.; Zhao, Y.; Schrock, R. R.; Hoveyda, A. M. “Highly Z-Selective Metathesis Homocoupling of Terminal Olefins.” J. Am. Chem. Soc., 2009, 131, 16630-16631. (b) Marinescu, S. C.; Schrock, R. R.; Müller, P.; Takase, M. K.; Hoveyda, A. M. “Room-Temperature Z-selective Homocoupling of ?-Olefins by Tungsten Catalysts.” Organometallics, 2011, 30, 1780-1782.
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