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

摘要

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.