Nickel Catalysts for the Dehydrative Decarbonylation
of Carboxylic Acids to Alkenes
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Abstract
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<sup>II</sup> or Ni<sup>0</sup> precursors
(NiI<sub>2</sub>, Ni(PPh<sub>3</sub>)<sub>4</sub>) 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(PPh<sub>3</sub>)<sub>4</sub> 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)(PPh<sub>3</sub>)<sub>3</sub>, 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
