Density functional study on highly ortho-selective addition of an aromaticCH bond to olefins catalyzed by a Ru(H)(2)(CO)(PR3)(3) complex

The origin of the high ortho selectivity and the reaction mechanism of the catalytic addition of an aromatic CH bond to olefins by the Ru complex Ru(H )(2)(CO)(PR3)(3) are investigated by means of density functional theory. We assumed the three- and four-coordinate complexes Ru(CO)(PH3)(m) (n = 2, 3) as active species, as suggested by the experimental results, and studied t he reaction of benzaldehyde with ethylene catalyzed by these model complexe s. According to the computational results, in the most favorable path first the formyl oxygen of benzaldehyde coordinates to the Ru atom, and then the cleavage of the closest ortho-CH bond takes place in two steps through an unusual intermediate, 10, a mechanism completely different from the convent ional oxidative addition proceeding in a single step. Before the CH bond br eaking, the RuC bond is formed, being driven by the change in pi bonds of t he conjugated system, to lead to 10, having the RuC bond and a CH agostic i nteraction, and then the hydrogen of the agostic CH bond in 10 is transferr ed to the Ru atom. The high ortho selectivity was ascribed to the existence of the stable, unusual five-coordinated metallacycIe intermediate 10. In t he subsequent reactions, the most favorable path adopts the insertion into the RuH bond of ethylene coordinated to the Ru atom, followed by the CC bon d formation between the resultant ethyl and formylphenyl ligands. In the CC bond formation an intermediate similar to PO plays an important role. The calculations showed that this CC bond formation, requiring an activation en ergy of 27 kcal/mol, is rate-determining.