Benefit of a hemilabile ligand in deoxygenation of fatty acids to 1-alkenes

Abstract

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

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

“…Alternatively, an acyl group will not be formed if the wrong (O=)C-O bond is cleaved during the addition of the substrate, TS_OA4-5, and will not lead to the desired olefin product. This pathway has however been found to be higher in energy [81][82] and, while the type of anhydride seems to be of less importance, the use of a bulky anhydride (e.g. pivalic anhydride) may further disfavor this side reaction.…”

“…Below, the different steps will be summarized as identified by the different computational studies done for homogenous decarbonylative dehydration of fatty acid substrates. 66,74,[81][82]142…”

“…In computational studies the effect of this removal of CO from the mixture has been modelled by using a reduced partial CO pressure of 10 -5 atm (see the computational details and standard state corrections). 66,82,[84][85] The immediate effect of this is that all steps following CO dissociation is lowered in free energy (e.g. by -8.5 kcal/mol at 100°C as used in Paper III).…”

“…show high selectivity due to the steric bulk and strong coordination. 63,66,[82][83] In fact a partial DPEphos dissociation, as in the formation of 1-alkene, was investigated, but all attempts at stabilizing isomerization-potent Pd-hydride complexes with partially dissociated DPEphos ligands resulted in the rebinding of phosphine and regular, bidentate coordination of DPEphos. Finally, also the carboxylate anion influences the selectivity, as it must dissociate for the olefin rotation to take place instead of the hydrogen transfer to release the α-olefin.…”

“…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.…”