Nickel-catalyzed hydrocarboxylation of alkynes with formic acid†

Acrylic acid is an important commodity chemical and is widely used as a base material for the industrial production of paints, plastics, superabsorbent polymers, rubber, and so on. Currently acrylic acid is produced by partial oxidation of propene, which is refined from petroleum. Catalytic hydrocarboxylation of acetylene with CO is another way to synthesize acrylic acid with high atom-economy. However, this nonpetroleum process uses toxic CO gas at high pressure. Formic acid is a renewable, nontoxic liquid and is an ideal replacement for CO gas for the hydrocarboxylation of alkenes. Recently, we developed a method for palladium-catalyzed hydrocarboxylation of acetylene with formic acid, providing acrylic acid with a TON of up to 350. As part of our search for low-cost, more-efficient catalysts for the hydrocarboxylation of acetylene, we studied catalysts based on first-row metals and found that nickel complexes with diphosphine ligands are efficient catalysts. Although organonickel chemistry has been thoroughly explored, only limited progress in the nickel-catalyzed hydrocarboxylation of alkynes has been made. Herein, we report the first nickel-catalyzed hydrocarboxylation of acetylene with formic acid as the CO surrogate (Scheme 1). A nickel catalyst bearing a di(tert-butylmethylphosphino)-benzene ligand produced acrylic acid with a TON of up to 7700, which is much higher than the TONs of previously reported palladium catalysts. The hydrocarboxylation of acetylene with formic acid was initially carried out at 100 °C in THF with Ni(acac)2 as a catalyst precursor and acetic anhydride as a promoter. The nature of the ligand markedly affected the reaction (see Fig. 1 and the ESI†). Only bidentate phosphine ligands with a certain bite angle and substituents on the phosphorus atom afforded the desired acrylic acid product, with 3c giving the highest TON for acrylic acid (130). Scheme 1 Hydrocarboxylation of acetylene with formic acid.