Rhodium-Catalyzed Hydroformylation in Supercritical Carbon Dioxide

Abstract

Supercritical carbon dioxide (scCO2) is an environmentally benign reaction medium for highly efficient rhodium-catalyzed hydroformylation reactions. Olefinic substrates can be hydroformylated in scCO2 at 40−65 °C to give the corresponding aldehydes in practically quantitative yields. The reaction course of the hydroformylation of 1-octene in scCO2 was analyzed in detail by online-GC monitoring. The influence of reaction parameters such as temperature, synthesis gas pressure, and [P]/Rh ratio on reaction rates and selectivities is grossly similar to the effects observed in conventional solvents. Maximum turnover frequencies of 1375, 500, and 115 h-1 were determined as lower limits for the catalytic activities under the present conditions for the unmodfied, phosphine-modified, and phosphite-modified systems, respectively. With unmodified catalysts, the hydroformylation rates are considerably higher in scCO2 than in organic solvents or liquid CO2 under otherwise identical conditions. Modified catalytic systems formed with perfluoralkyl-substituted triarylphosphine and triaralyphosphite ligands lead to higher regioselectivities than those found in conventional solvents. A constant overall n/iso ratio of 5−6 was achieved with "CO2-philic" triarylphosphines, whereas it increased in an apparent linear fashion during the reaction from approximately 6 to over 9 with the phosphite ligand. Olefin isomerization, which is a typical side reaction for phosphite-modified systems in conventional solvents, was effectively suppressed in scCO2.