Hydroformylation of 1-octene in supercritical carbon dioxide: Can alkylation of arylphosphines with tertbutyl groups lead to soluble and active catalytic systems?

The possibility of increasing the solubility and activity of rhodium catalysts in the hydroformylation of 1-octene in supercritical carbon dioxide (scCO2) by attachment of tertbutyl (tBu) groups to the triarylphosphine ligand was investigated. The solubility of the parent and monoalkylated ligands in a mixture of gaseous components (CO2/CO/H2) was evaluated by visual observation in a view cell. In the case of triphenylphosphine, introduction of one tertbutyl group does not seem to affect the solubility of the phosphine. When the behaviour of (diphenyl)biphenylphosphine and (para-tertbutyldiphenyl)biphenylphosphine was compared a clear solubilising effect of the tBu group was observed. Ligands where tested in experiments using 0.22 ± 0.01 mM Rh concentration with 1-octene/Rh molar ratio close to 800, syngas pressure at 25 °C of 2 MPa and total pressure of 31–38 MPa at 60 °C. From the catalytic point of view, attachment of tBu groups on the triphenylphosphine had a detrimental effect on the catalyst activity. In the case of (diphenyl)biphenylphosphine, presence of a branched alkyl group on the diphenyl part of the phosphine allowed the catalyst activity to increase. The results presented in this work suggest that addition of tertbutyl groups to arylphosphines is not always effective to increase the scCO2 solubility and activity of Rh-based catalyst for hydroformylation in scCO2 and that the strategy for molecular design of ligands for hydroformylation in scCO2 must simultaneously take into account solubility, steric and electronic considerations.