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

Abstract

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.