Hydroformylation of alkenes in supercritical carbon dioxide catalysed by rhodium trialkylphosphine complexes

Rhodium complexes modified by simple trialkylphosphines can be used to carry out homogeneous hydroformylation in supercritical carbon dioxide (scCO₂). The catalyst derived from PEt₃ is more active and slightly more selective for the linear products in scCO₂ than in toluene, and under the same reaction conditions [100°C, 40 bar of CO/H₂ (1:1)] P(OPrⁱ)₃ is also an effective ligand giving good catalyst solubility and activity. Other ligands such as PPh₃, POct₃, PCy₃, and P(4-C₆H₄Bu^t)₃ are less effective because of the low solubility of their rhodium complexes in scCO₂. P(4-C₆H₄SiM₃),Ph_3-n, (n = 3 or 1) and P(OPh)₃ impart activity despite their complexes only being poorly soluble in scCO₂. Under subcritical conditions, using PEt₃ as the ligand, C7-alcohols from hydrogenation of the first formed aldehydes are the main products whilst above a total pressure of 200 bar, where the solution remains supercritical (monophasic) throughout the reaction, aldehydes are obtained with 97% selectivity. High pressure IR studies in scCO₂ using PEt₃ as the ligand are reported.