Carboxylate-assisted C(sp3)-H activation in olefin metathesis-relevant ruthenium complexes

Abstract

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 ‡298K = 22.2 ± 0.1 kcal·mol-1 for the parent N-adamantyl-N-mesityl complex. An experimentally determined ‡ =-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/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. © 2014 American Chemical Society.