THE MECHANISM OF PHOSPHINE-MODIFIED RHODIUM-CATALYZED HYDROFORMYLATION STUDIED BY CIR FTIR

The mechanism of phosphine-modified rhodium-catalyzed hydroformylation of 1-hexene was studied by in situ infrared spectroscopy using high pressure autoclaves equipped with embedded cylindrical internal reflectance crystals (CIR-FTIR). A series of RhH(CO)//2(PR//3)//2 complexes were synthesized using p-substituted triarylphosphines where the electron density on rhodium was varied by using p-N(CH//3)//2, p-OCH//3, p-H, p-F, p-Cl or p-CF//3. The metal carbonyl and metal hydride infrared stretching frequencies were correlated by a standard Hammett treatment of the data. Reaction rates and selectivities for linear aldehydes both increased with increasingly electron-withdrawing phosphines. The rate and spectroscopic data permitted the assignment of a reaction mechanism involving CO dissociation from RhH(CO)//2L//2 as the primary selective hydroformylation pathway.