The effect of polyunsaturation – insights into the hydroformylation of oleochemicals†

Although the hydroformylation of vegetable oil-derived oleochemicals, such as methyl oleate, is a highly demanded reaction and has been intensively studied, little is known about the influence of naturally occurring polyunsaturated (PU) components and their effect on the homogeneous rhodium catalyst. This is now examined in detail in the presented work by conducting systematic perturbation experiments. For the first time, it can be verified that the isomerisation of double bonds in polyunsaturated oleochemicals generates conjugated double bond systems that form stable η³-Rh allyl species and thus temporarily inhibit the catalyst. However, based on these findings, hydroformylation activity can be significantly increased by selective hydrogenation of PU to monounsaturated components. In the case of sunflower methyl ester, the turnover frequency multiplied by a factor of 8 and reached 3201 h⁻¹, the highest rate reported in the context of methyl oleate hydroformylation. These effects were shown for both phosphine and phosphite ligands under both mono- and biphasic conditions and for methyl esters with different PU content, stressing the magnitude of this effect. This work makes it possible to support long-observed phenomena with the underlying mechanism scientifically. This lays the scientific basis for efficiently converting oleochemicals into valuable intermediates through hydroformylation for an increased share of renewable carbon in the chemical value chain.