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.