C-H Bond Activation in Ru-Catalyzed Reactions of Arenes with Olefins: Theoretical Insights into Hydroarylation and Oxidative Coupling Mechanisms

A detailed mechanistic density functional theory (DFT) and coupled cluster study of C-sp2-H activation in benzene and methyl acrylate by the catalyst RuClm(CO)(n) (m=2,3; n=0-4) is presented. We trace the entire reaction pathways from the precursor to the active form of the catalysts followed by catalytic hydroarylation and oxidative coupling reactions. Our results reveal that both computational methods provide very similar qualitative pictures, but only the coupled cluster DLPNO-CCSD(T-1) results are quantitatively consistent with experiment. At the latter level of theory, the Ru(II) and Ru(III) catalyst precursors transform into the same active form of the catalyst, which explains the experimental results. Oxidative addition is extremely endergonic, especially in the presence of CO. Oxidative hydrogen migration (OHM) occurs in complexes with a low Ru coordination number and leads to olefine hydroarylation. Strongly bound carbonyl ligands suppress this interaction. Concerted metalation-deprotonation (CMD) of the aromatic C-H bond and sigma-bond metathesis mechanisms of catalyst regeneration do not involve Ru-H bonding and form the energetically favorable catalytic cycle of the oxidative coupling reaction. The key interaction in CMD mechanisms consists of proton abstraction by an inner-sphere Cl-anion. Carbonyl ligands facilitate CMD by weakening the Ru-Cl bond. An excess of CO slows the interaction by leading to replacement of the reactants in the Ru coordination sphere.