Green Solvent for the Synthesis of Linear α-Olefins from Fatty Acids

Whereas transition-metal-catalyzed decarbonylative dehydration of fatty acids shows promise as a more sustainable route to alpha-olefins, the solvents used for this process have so far been toxic compounds such as N-methyl-2-pyrrolidone and 1,3-dimethyl-3,4,5,6-tetrahydro-2(1H)-pyrimidinone. Here, potential greener solvents are surveyed, using the well-defined precatalyst Pd(cinnamyl)Cl(DPEPhos) (1). In general, the superiority of aprotic and polar solvents for this process is striking. An analysis of the experimental observations and of mechanistic density functional theory calculations suggests that this superiority originates from the ability of polar solvents to stabilize the rate-determining transition state, located in the olefin-forming beta-hydrogen transfer step. To create electronic and steric room for the transfer, a ligand must be dissociated. In polar solvents, the corresponding hydrogen acceptor (the anionic Bronsted base), dissociates, which facilitates the transfer. Conversely, in apolar solvents the bidentate phosphine ligand dissociates, which leads to a higher barrier. Importantly, the study identified gamma-valerolactone, which can be obtained from biomass, as a solvent offering almost the same efficiency for the decarbonylative dehydration reaction as the traditional, toxic solvents. Other green solvents tend to either have too low boiling points (below the reaction temperature, 110 degrees C) or to react with the substrate, the catalyst, or side products of the reaction.