Formation of divalent ruthenacycles via oxidative cyclization of 1,3,5-cyclooctatriene with maleic anhydride or maleimides: An intermediate for the transition metal-mediated [6+2] cycloaddition of 1,3,5-trienes and alkenes

Reactions of a zerovalent ruthenium complex, Ru(eta(4)-cod)(eta(6)-cot) (3; cod = 1,5-cyclooctadiene, cot = 1,3,5-cyclooctatriene), with maleic anhydride or maleimides under mild reaction conditions afforded a series of novel divalent ruthenacycles 4 with an eta(5)-cyclooctadienyl moiety via oxidative cyclization between the carbon-carbon double bonds of cot and the electron-deficient alkene. The solid-state structure clearly showed a newly constructed ruthenacycle skeleton, which was formed in a trans manner. Complex 4 was further reacted with H-2 and HCl to give hydrogenated and protonated succinimides with a C-8-ring, respectively. When 4 was heated in toluene, a [ 6 + 2] cycloadduct of cot and a maleimide was obtained via reductive elimination, which shows that a ruthenium-mediated stepwise [ 6 + 2] cycloaddition was achieved. The addition of PPh3 to complex 4d promoted the reductive elimination, while bidentate phosphines such as 1,2-bis(diphenylphosphino) ethane did not give the cycloadduct and stable ruthenacycles 11 were formed instead. Reactions of 4 with CO gave novel tricarbonyl ruthenacycles 12 along with dissociation of the cod ligand, where neither reductive elimination nor CO insertion took place. The results of a theoretical study were consistent with the idea that the energy barriers for reductive elimination from ruthenacycles bearing cod or monophosphine ligands were lower than those from a ruthenacycle bearing a diphosphine ligand. The formation of 12 was found to be more energetically favorable than reductive elimination.