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 Delta G(298K)double dagger = 22.2 +/- 0.1 kcal.mol(-1) for the parent N-adamantyl-N'-mesityl complex. An experimentally determined Delta S double dagger = 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.