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Abstract
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, (PPh3)2Rh(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.
Highlights
Linear alpha olefins (LAOs) are key commodity chemicals and petrochemical intermediates that are used in applications, such as polyethylene production, synthesis of oxo alcohols to give detergents and plasticizers, and the manufacturing of poly-α-olefins that are used in drilling fluids and synthetic lubricants, to mention but a few [1,2,3]
Because of the limited supply of fossil resources and the fact that their use releases carbon dioxide to the atmosphere, research on the utilization of renewable feedstocks has gained extraordinary importance [6,7,8]. α-Olefins are produced on a scale of tens of millions of tons per year [3]
Fatty acids are attractive as renewable starting materials, and deoxygenation of fatty acids and their derivatives is currently being explored as an alternative and more sustainable route to uncommon, odd-numbered LAOs [2,9,10,11]
Summary
Linear alpha olefins (LAOs) are key commodity chemicals and petrochemical intermediates that are used in applications, such as polyethylene production (where the LAOs are used as co-monomers), synthesis of oxo alcohols to give detergents and plasticizers, and the manufacturing of poly-α-olefins that are used in drilling fluids and synthetic lubricants, to mention but a few [1,2,3]. LAOs are currently produced via oligomerization of ethylene derived from fossil resources [4,5]. The development of a viable route from renewable feedstocks to α-olefins would be a major breakthrough, with a potentially transformative impact on sustainable chemical production. Fatty acids are attractive as renewable starting materials, and deoxygenation (oxidative decarboxylation or decarbonylative dehydration) of fatty acids and their derivatives is currently being explored as an alternative and more sustainable route to uncommon, odd-numbered LAOs [2,9,10,11]. LAOs may be obtained from fatty acids using homogenous- [12,13,14,15,16,17], heterogeneous- [18], and bio-catalysts [19,20,21].
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