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Published May 7, 2014 | Published + Supplemental Material
Journal Article Open

Carboxylate-Assisted C(sp^3)–H Activation in Olefin Metathesis-Relevant Ruthenium Complexes


The mechanism of C–H activation at metathesis-relevant ruthenium(II) benzylidene complexes was studied both experimentally and computationally. Synthesis of a ruthenium dicarboxylate at a low temperature allowed for direct observation of the C–H activation step, independent of the initial anionic ligand-exchange reactions. A first-order reaction supports an intramolecular concerted metalation–deprotonation mechanism with ΔG^(‡)_(298K) = 22.2 ± 0.1 kcal·mol^(–1) for the parent N-adamantyl-N′-mesityl complex. An experimentally determined ΔS^(‡) = −5.2 ± 2.6 eu supports a highly ordered transition state for carboxylate-assisted C(sp^3)–H activation. Experimental results, including measurement of a large primary kinetic isotope effect (k_(H)/k_(D) = 8.1 ± 1.7), agree closely with a computed six-membered carboxylate-assisted C–H activation mechanism where the deprotonating carboxylate adopts a pseudo-apical geometry, displacing the aryl ether chelate. The rate of cyclometalation was found to be influenced by both the electronics of the assisting carboxylate and the ruthenium ligand environment.

Additional Information

© 2014 American Chemical Society. ACS AuthorChoice. Received: March 3, 2014; Published: April 14, 2014. This work was financially supported by the NIH (NIH R01-GM031332 and NRSA to J.S.C.: F32-GM103002), the NSF (CHE-1212767, CHE-1059084), and the NSF CCI Center for Stereoselective C−H Functionalization (CHE-1205646). NMR spectra were obtained by instruments supported by the NIH (RR027690). Materia, Inc. is acknowledged for the generous donation of complex 4b. D.J.O. thanks Pomona College for supporting a portion of this research. Calculations were performed on the Hoffman2 cluster at UCLA and the Extreme Science and Engineering Discovery Environment (XSEDE), which is supported by the NSF. K. M. Engle, J. Hartung, and Z. K. Wickens are gratefully acknowledged for helpful discussions.

Attached Files

Published - ja5021958.pdf

Supplemental Material - ja5021958_si_001.pdf


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