Rhodium-catalyzed hydroformylation of alkenes using in situ high-pressure IR and polymer matrix techniques

This paper exploits an unusual high-pressure cell together with in situ FTIR and polymer matrix techniques to study Rh-catalyzed hydroformylation of 1-octene, 1-butene, propene, and ethene, using either Rh(acac)(CO)₂ or Rh(acac)(CO)(PPh₃) in a polyethylene matrix. The technique requires only micrograms of catalyst. Using Rh(acac)(CO)₂ as the catalyst precursor, we were able to characterize the previously observed acyl rhodium tetracarbonyl intermediates, RC(O)Rh(CO)₄, using IR spectroscopy. For the PPh₃-modified reactions, the analogous acyl rhodium tricarbonyl triphenylphosphine intermediates, RC(O)Rh(CO)₃(PPh₃), have been observed. The IR spectra suggest a trigonal-bipyramidal structure with the acyl ligand and the PPh₃ ligand occupying the axial positions. The acyl rhodium tetracarbonyl intermediates, RC(O)Rh(CO)₄, can easily react with ethene to form acyl rhodium tricarbonyl ethene species RC(O)Rh(CO)₃(C₂H₄), again identified via IR. The resulting hydroformylation products were extracted from the polyethylene film after the reaction and analyzed by GC. The results indicate that, in the PE matrix, the PPh₃-modified hydroformylation gives a higher linear-to-branched ratio for the unsymmetrical alkenes.