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

Abstract

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.

Selective production of linear α-olefins via catalytic deoxygenation of fatty acids and derivatives

“…Indeed, when investigating the SMD solvent effect on the Pd(PPh3)2 catalyzed mechanism 81 by changing from the apolar protic solvent (butanoic acid, εr=2,9) to a aprotic polar solvent (DMA, εr=37,8), it was found that the cationic β-hydride elimination via [M6-7 + ] ‡ experiences a massive decrease in energy (from 66.7 kcal/mol to 34.6 kcal/mol) (Scheme 11). 83 The preferred pathway however was still found to be the neutral low-coordinated Pd complex via [M5-7 + ] ‡ , which barrier was reduced from 38. (dashed lines), starting from geometries reported for Pd(PPh3)2-catalyzed decarbonylative dehydration.…”

“…pivalic anhydride) may further disfavor this side reaction. 61,64,81,83 Scheme 6. Investigated pathways of substrate activation and oxidative addition.…”

“…An exception being FeCl2 catalyzed decarbonylative dehydration which interestingly requires a CO overpressure of 5 bar for activity, presumably because an iron-carbonyl is the active specie. 83 Indeed, all the computational studies have found that if CO is retained at the metal very stable M(CO) complexes, that act as thermodynamic sinks for the reaction, are formed. 74,81,84 To account for this the experiments facilitate dissociation of CO from the reaction mixture by combining high temperature, stirring and open systems or even vacuum purges.…”

“…This is surprising seeing, as observed, that polar solvents give better activity and the rate-determining step is assumed to be alkene formation. 66,74,81,[83][84] An alternate pathway, where the hydride hydrogen is transferred to the carboxylate prior to olefin dissociation, was also explored computationally, but whenever the pivalate approached the hydride in AF5like complexes, the pivalate associated with Pd to complexes similar to AF1 and subsequent hydrogen transfer following the neutral pathway. Finally, the preferred TS, TS_AF1-2 (Figure 3), is overall neutral, but also zwitterionic, and the pivalate-Pd While the polar solvent does not seem to facilitate the alkene formation, it may instead promote isomerization.…”

“…Only during the work with this thesis, have the first computational studies of the reaction arrived. 66,74,[81][82][83][84] This lack of mechanistic information has stifled the understanding of different factors of the reaction and rational design of better catalysts, and was partly the motivation for synthesizing the molecular well-defined precatalyst A1, Pd(cinnamyl)Cl(DPEphos). 85 While previous catalysts for decarbonylative dehydration have been formed in situ, this catalytic system was able to transform fatty acids to α-olefin at relatively mild conditions (110°C) and, importantly, without excess of phosphine.…”