Rhodium-catalyzed hydroformylation in supercritical carbon dioxide

Supercritical carbon dioxide (scCO(2)) is an environmentally benign reactio n medium for highly efficient rhodium-catalyzed hydroformylation reactions. Olefinic substrates can be hydroformylated in:scCO(2) at 40-65 degrees C t o give the corresponding aldehydes in practically quantitative yields. The reaction course of the hydroformylation of 1-octene in scCO(2) was analyzed in detail by online-GC monitoring. The influence of reaction parameters su ch as temperature, synthesis gas pressure, and [P]/Rh ratio on reaction rat es and selectivities is grossly similar to the effects observed in conventi onal solvents. Maximum turnover frequencies of 1375, 500, and 115 h(-1) wer e determined as lower limits for the catalytic activities under the present conditions for the unmodfied, phosphine-modified, and phosphite-modified s ystems, respectively. With unmodified catalysts, the hydroformylation rates are considerably higher in scCO(2) than in organic solvents or liquid CO2 under otherwise identical conditions. Modified catalytic systems formed wit h 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" tria rylphosphines, whereas it increased in an apparent linear fashion during th e reaction from approximately 6 to over 9 with the phosphite ligand. Olefin isomerization, which is a typical side reaction for phosphite-modified sys tems in conventional solvents, was effectively suppressed in scCO(2).