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Displacement of a cis-Olefin from a trans-Olefin Complex:  CpRu(CO)2(trans-olefin)+

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journal contribution
posted on 03.04.2020, 16:26 by Kevin M. McWilliams, Robert J. Angelici
Ruthenium(II)−olefin complexes CpRu(CO)22-trans-olefin)+ (Cp = η5-C5H5; olefin = trans-3-hexene, trans-2-pentene, trans-3-octene, trans-4-octene, trans-5-decene) have been synthesized and characterized by IR, 1H NMR, and 13C NMR spectroscopies. The reactions of these complexes with a wide range of ligands (L) result in the formation of CpRu(CO)2(L)+ and the release of both cis- and trans-olefins:  [CpRu(CO)2(trans-olefin)]BF4 + L → [CpRu(CO)2(L)]BF4 + cis/trans-olefin. The relative amounts of cis- and trans-olefin released are controlled by several factors:  identity and amount of the incoming ligand L, identity of the olefin, temperature, and solvent. For 4-substituted pyridines, the cis/trans ratio increases as the electron-donating ability of the 4-substituent increases:  F3C (18/82) < H (67/33) < CH3 (74/26) < CH3O (76/24). Increases in temperature, solvent polarity, and olefin side-chain length reduce the cis/trans ratio. A mechanism is proposed to account for the isomerization of trans-olefin ligands to their cis isomers during the substitution process.