title: 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 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(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.
authors.affiliation: Arnold and Mabel Beckman Laboratory of Chemical Synthesis, Division of Chemistry and Chemical Engineering, California Institute of Technology , Pasadena, California 91125, United States.
authors.forename: Jeffrey S
authors.initials: JS
authors.lastname: Cannon
authors.forename: Lufeng
authors.initials: L
authors.lastname: Zou
authors.forename: Peng
authors.initials: P
authors.lastname: Liu
authors.forename: Yu
authors.initials: Y
authors.lastname: Lan
authors.forename: Daniel J
authors.initials: DJ
authors.lastname: O'Leary
authors.forename: K N
authors.initials: KN
authors.lastname: Houk
authors.forename: Robert H
authors.initials: RH
authors.lastname: Grubbs
mesh_terms: D000475:Alkenes / Q000737:chemistry*; D002264:Carboxylic Acids / Q000737:chemistry*; D017975:Ruthenium Compounds / Q000737:chemistry*
chemical_list: D000475:Alkenes; D002264:Carboxylic Acids; D017975:Ruthenium Compounds
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