α,β-C—C—C Agostic Bonding Interactions in Ruthenacyclobutane and π-Complex Assisted Olefin Metathesis Catalyzed by Ruthenium-Alkylidene Complexes

Computational aspects of concerted [2+2] oxidative-retrocycloaddition-cycloreversion reaction through ruthenium alkylidene π-complexes and ruthenacyclobutane with α,β-(C—C—C) agostic bonding interactions in olefin metathesis are presented. d⁶-Ruthenium carbene complexes, with ruthenium in the oxidation state +2, undergo successive [2+2] cycloaddition and cycloreversion steps, through associative, dissociative, or interchange mechanisms. This process involves coordination of the olefin to 16-electron Ru complex followed by phosphine dissociation, or first phosphine dissociation then coordination of the olefin to the 14-electron Ru complex with rearrangement to a ruthenacyclobutane intermediate, followed by symmetrical reverse steps. Donation of σ-electron density from the two C—C σ-bonds to the metal center leads to α,β-(C—C—C) agostic bonds, which stabilized metallacyclobutane as a formally 16-electron complex, with lower energy than the corresponding π-complex. In the transformation from π-complex to ruthenacyclobutane the ruthenium atom is formally oxidized to Ru(IV). The most efficient ligands are those that stabilize the high-oxidation state metallacyclobutane (IV) intermediate relative to the ruthenium carbene.