Reppe‐Carbonylation of Alkenes with Carboxylic Acids: A Catalytic and Mechanistic Study

In this work, a green‐catalytic procedure is used to demonstrate the synthesis of long‐chain anhydrides. This is accomplished by the carbonylation of alkenes with carboxylic acids. Anhydrides are important intermediates, finding specific applications as acetylation agents. The process is catalyzed by a Pd phosphine catalyst system in the presence of an acid promotor. Through optimization experiments, the importance of the phosphine ligand and acid promotor is demonstrated, without which the catalyst shows no activity. Although the yield was limited because of catalyst deactivation (42 %), to the best of our knowledge, the synthesis of long‐chain anhydrides through this route have not been reported previously. It therefore represents a significant result. Isolation of the anhydrides were also successfully demonstrated, however, as a result of the sensitivity of the anhydrides to water, the isolated yields were much lower than the HPLC yield. Furthermore, a mechanistic study by means of HP‐NMR (High Pressure Nuclear Magnetic Resonance) spectroscopy provided significant insights into the mechanism of the reaction. Symmetric anhydrides (nonanoic anhydride) were synthesized through the Pd‐catalyzed Reppe‐carbonylation of 1‐octene with nonanoic acid. The role of the phosphine ligand and the acid additive were investigated, which showed that these are necessary to produce an active catalyst. Furthermore, various phosphine ligands, including mono‐ and bidendate ligands, as well as Pd‐precursors were also investigated. Utilizing 1‐hexene and 1‐decene in combination with nonanoic acid produced unsymmetric anhydrides which underwent disproprionation to symmetric counterparts. Valuable insights regarding the mechanism of the reaction were obtained from 31P and 1H NMR spectroscopy.