Rhodium catalyzed homogeneous hydroformylation in supercritical carbon dioxide using perfluoroalkyl-substituted triarylphosphine ligands

Supercritical carbon dioxide (scCO2) as a reaction solvent offers many potential advantages over conventional organic solvents, including increased reaction rates, higher selectivities, and facile separation of reactants, catalysts, and products. In addition, carbon dioxide is non-toxic, non-flammable, inexpensive, readily available in large quantities, and has a low critical temperature and a moderate critical pressure. With such properties, scCO 2 has the potential to replace harmful or regulated organic solvents in a number of applications, including homogeneous catalysis. ^ In recently published studies, conventional homogeneous catalysts were shown to be active in scCO2, but their solubilities are too low for industrial application. Therefore, we have synthesized and characterized several analogs of the well known hydroformylation catalyst HRh(CO)(PPh 3)3 (1), by replacing the triphenylphosphine ligands with fluoroalkyl- or fluoroether-substituted triarylphosphines. The new fluorinated complexes have greatly enhanced solubilities, up to 1000 times higher than those of conventional homogeneous catalysts in scCO2. ^ One of the novel catalysts, HRh(CO)[P(4-CF3C6H 4)3]3 (2), was investigated extensively for its behavior in scCO2 and was shown to be active for the hydroformylation of several different unsaturated compounds. Its kinetic behavior in 1-octene hydroformylation was also investigated. A kinetic rate expression was developed which differed significantly from the expression obtained for 1 in organic solvents. ^ In an effort to quantify the effects of the modified phosphine ligands on the kinetic behavior of HRh(CO)L3 (L = modified phosphine) in scCO2, several fluorinated tertiary arylphosphines were synthesized and investigated in the homogeneous catalytic hydroformylation of 1-octene. The activities of the rhodium complexes were found to depend strongly on the basicities of the phosphines, as measured by the carbonyl stretching frequency of HRh(CO)L3. Contrary to the observed behavior of conventional systems, the steric bulk of the phosphine (Tolman cone angle) did not affect either the activities or selectivities of the rhodium catalysts investigated. ^ This study demonstrates the ease with which transition-metal catalysts can be modified for scCO2 applications. However, before industrial application of a scCO2-based hydroformylation system can be realized, more fundamental research is needed. Further study may provide an explanation for the altered kinetic behavior of the scCO2 system, and well-designed ligand modification should lead to facile catalyst/product separation by utilizing the unique properties of scCO2.