Carboxylate-Assisted C(sp3)–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(sp3)–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.