Carboxylate-Assisted C(sp3)–H Activation in Olefin Metathesis-Relevant Ruthenium Complexes
2014
Cannon, Jeffrey S. | Zou, Lufeng | Liu, Peng | Lan, Yu | O’Leary, Daniel J. | Houk, K. N. | Grubbs, Robert H.
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(sp³)–H activation. Experimental results, including measurement of a large primary kinetic isotope effect (kH/kD = 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.
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