Transition-metal-catalyzed decarbonylative dehydration of even-numbered fatty acids to give valuable odd-numbered alpha olefins has so far been performed using approaches in which the catalyst is formed in situ. We show that well-defined Pd­(bis-phosphine) precatalysts eliminate the need for excess phosphine and selectively produce linear alpha olefins under mild conditions and with substantially increased turnover frequency (TOF) and substrate scope. In particular, the new precatalyst Pd­(cinnamyl)­Cl­(DPEPhos) (5) selectively converts substrates containing unprotected aliphatic-alcohol and amine functional groups to their corresponding linear alpha olefins.

Linear alpha olefins (LAOs) are key commodity chemicals and petrochemical intermediates that are currently produced from fossil resources. Fatty acids are the obvious renewable starting material for LAOs, which can be obtained via transition-metal-catalyzed decarbonylative dehydration. However, even the best catalysts that have been obtained to date, which are based on palladium, are not active and stable enough for industrial use. To provide insight for design of better catalysts, we here present the first computationally derived mechanism for another attractive transition-metal for this reaction, rhodium. By comparing the calculated mechanisms and free energy profiles for the two metals, Pd and Rh, we single out important factors for a facile, low-barrier reaction and for a stable catalyst. While the olefin formation is rate limiting for both of the metals, the rate-determining intermediate for Rh is, in contrast to Pd, the starting complex, (PPh 3) 2 Rh(CO)Cl. This complex largely draws its stability from the strength of the Rh(I)-CO bond. CO is a much less suitable ligand for the high-oxidation state Rh(III). However, for steric reasons, rhodium dissociates a bulkier triphenylphosphine and keeps the carbonyl during the oxidative addition, which is less favorable than for Pd. When compared to Pd, which dissociates two phosphine ligands at the start of the reaction, the catalytic activity of Rh also appears to be hampered by its preference for high coordination numbers. The remaining ancillary ligands leave less space for the metal to mediate the reaction.

Various homogeneous, heterogeneous, and enzyme catalysis strategies for the selective synthesis of linear α-olefins from fatty acids and their derivatives are reviewed.

Whereas transition-metal-catalyzed decarbonylative dehydration of fatty acids shows promise as a more sustainable route to α-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 β-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 Brønsted 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 γ-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°C) or to react with the substrate, the catalyst, or side products of the reaction.

DFT calculations show how the hemilability of DPEphos facilitates the Pd-DPEphos-catalysed decarbonylative dehydration of fatty acids to α-olefins.