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Published August 2014 | public
Conference Paper

Carboxylate-assisted C(sp^3)-H activation in olefin metathesis-relevant ruthenium complexes

Abstract

Ruthenium-based olefin metathesis catalysts that are selective for the synthesis of exclusively Z-olefins have advanced the utility and understanding of olefin metathesis reactions. These catalysts are synthesized with a key carboxylate-assisted C-H activation step. The study of this cyclometalation step would allow for the accelerated development of this important class of catalysts and would expand the understanding of C-H functionalization reactions in general. The mechanism of this C-H activation at metathesis-relevant ruthenium(II) benzylidene complexes was studied both exptl. and computationally. Synthesis of a ruthenium dicarboxylate at low temp. allowed for direct observation of the C-H activation step independent of initial anionic ligand exchange reactions. A first-order reaction order supports an intramol. concerted metalation-deprotonation (CMD) mechanism with ΔG‡298K = 22.2 ± 0.1 kcal×mol^(-1) for the parent N-adamantyl-N'-mesityl complex. An exptl. detd. ΔS‡ of -5.2 ± 2.6 eu supports a highly ordered transition state for carboxylate-assisted C(sp^3)-H activation. Exptl. results, including measurement of a large primary kinetic isotope effect (k_H/k_D = 8.1 ± 1.7), agree closely with a computed 6-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 both by the electronics of the assisting carboxylate, and the ruthenium ligand environment.

Additional Information

© 2014 American Chemical Society.

Additional details

Created:
August 20, 2023
Modified:
October 17, 2023