Nickel Catalysts for the Dehydrative Decarbonylation of Carboxylic Acids to Alkenes

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 Ni^II 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.