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Published October 12, 2011 | Accepted Version + Supplemental Material
Journal Article Open

Probing the Origin of Degenerate Metathesis Selectivity via Characterization and Dynamics of Ruthenacyclobutanes Containing Variable NHCs


The preparation of new phosphonium alkylidene ruthenium metathesis catalysts containing N-heterocyclic carbenes (NHCs) that result in a preference for degenerate metathesis is described. The reaction of the catalysts with ethylene or substrates relevant to ring-closing metathesis (RCM) produced ruthenacyclobutanes that could be characterized by cryogenic NMR spectroscopy. The rate of α/β methylene exchange in ethylene-only ruthenacycles was found to vary widely between ruthenacycles, in some cases being as low as 3.97 s^(–1) at −30 °C, suggesting that the NHC plays an important role in degenerative metathesis reactions. Attempts to generate RCM-relevant ruthenacycles resulted in the low-yielding formation of a previously unobserved species, which we assign to be a β-alkyl-substituted ruthenacycle. Kinetic investigations of the RCM-relevant ruthenacycles in the presence of excess ethylene revealed a large increase in the kinetic barrier of the rate-limiting dissociation of the cyclopentene RCM product compared with previously investigated catalysts. Taken together, these results shed light on the degenerate/productive selectivity differences observed for different metathesis catalysts.

Additional Information

© 2011 American Chemical Society. Received: August 2, 2011. Publication Date (Web): September 15, 2011. We thank Ms. Taylor Lenton, Ms. Rachel Klet, Mr. Ian Tonks, Dr. Edward Weintrob, and Prof. John Bercaw for assisting with high-vacuum Schlenk techniques and quantitative gas transfers. Dr. Jay Labinger and Prof. Anna Wenzel are thanked for helpful discussions. Dr. David VanderVelde is acknowledged for assisting with NMR experimentation and analysis. This work was financially supported by the NIH (NIH 5R01GM031332-27), the NSF (CHE-1048404), and the NDSEG (fellowship to B.K.K.). Instrumentation facilities on which this work was carried out were supported by NIH RR027690. Materia, Inc., is thanked for its donation of metathesis catalysts.

Attached Files

Accepted Version - nihms325912.pdf

Supplemental Material - ja207252r_si_001.pdf


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