Lewis Acid Induced β‑Elimination from a Nickelalactone: Efforts toward Acrylate Production from CO₂ and Ethylene

The Lewis acid tris­(pentafluorophenyl)­borane was found to rapidly promote ring-opening β-hydride elimination in a 1,1′-bis­(diphenylphosphino)­ferrocene (dppf) nickelalactone complex under ambient conditions. The thermodynamic product of nickelalactone ring-opening was characterized as (dppf)­Ni­(CH­(CH3)­CO2BArf3), the result of β-hydride elimination and subsequent 2,1-insertion from a transient nickel­(II) acrylate hydride intermediate. Treatment of (dppf)­Ni­(CH­(CH3)­CO2BArf3) with a nitrogen-containing base afforded a diphosphine nickel(0) η2-acryl borate adduct. Formation of the diphosphine nickel(0) η2-acryl borate adduct completes a net conversion of nickelalactone to acrylate species, a significant obstacle to catalytic acrylate production from CO2 and ethylene. Displacement of the η2-acrylate fragment from the nickel center was accomplished by addition of ethylene to yield a free acrylate salt and (dppf)­Ni­(CH2CH2).研究发现，路易斯酸（Lewis acid）三(五氟苯基)硼烷可在环境条件下，快速促进1,1'-双(二苯基膦基)二茂铁（dppf）配位的镍内酯配合物发生开环β-氢消除反应。镍内酯开环的热力学产物被表征为(dppf)·Ni(CH(CH3)·CO2BArf3)，该产物源自瞬态镍(II)丙烯酸氢化物中间体的β-氢消除及后续2,1-插入反应。将(dppf)·Ni(CH(CH3)·CO2BArf3)与含氮碱作用，可得到二膦合镍(0) η2-丙烯酸硼酸酯加合物。二膦合镍(0) η2-丙烯酸硼酸酯加合物的生成，完成了镍内酯向丙烯酸酯物种的净转化——这是由二氧化碳与乙烯催化制备丙烯酸酯过程中的显著阻碍。通过向体系中加入乙烯，可将镍中心上的η2-丙烯酸酯片段置换下来，得到游离丙烯酸酯盐与(dppf)·Ni(CH2=CH2)。