Nickel Catalysts for the Dehydrative Decarbonylation of Carboxylic Acids to Alkenes
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Combining high-throughput experimentation with conventional experiments expedited discovery of new first-row nickel catalysts for the dehydrative decarbonylation of the bioderived substrates hydrocinnamic acid and fatty acids to their corresponding alkenes. Conventional experiments using a continuous distillation process (180 °C) revealed that catalysts composed of NiII or Ni0 precursors (NiI2, Ni(PPh3)4) and simple aryl phosphine ligands were the most active. In the reactions with fatty acids, the nature of the added phosphine influenced the selectivity for α-alkene, which reached a maximum value of 94%. Mechanistic studies of the hydrocinnamic reaction using Ni(PPh3)4 as catalyst implicate a facile first turnover to produce styrene at room temperature, but deactivation of the Ni(0) catalyst by CO poisoning occurs subsequently, as evidenced by the formation of Ni(CO)(PPh3)3, which was isolated and structurally characterized. Styrene dimerization is a major side reaction. Analysis of the reaction mechanism using density functional theory supported catalyst regeneration along with alkene formation as the most energetically demanding reaction steps.
