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 experimentally and computationally. Synthesis of a ruthenium dicarboxylate at low temperature allowed for direct observation of the C�H activation step independent of initial anionic ligand exchange reactions. A first-order reaction order supports an intramolecular concerted metalation-deprotonation (CMD) mechanism with ?G�298K = 22.2 � 0.1 kcal�mol�1 for the parent N-adamantyl-N'-mesityl complex. An experimentally determined ?S� of �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 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.

Carboxylate-assisted C(sp<sup>3</sup>)�H activation in olefin metathesis-relevant ruthenium complexes

248th National Meeting (2014)

The official archive and repository for all ACS Meetings & Expositions abstracts and select posters or presentations.

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

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