The mechanism of phosphine-modified rhodium-catalyzed hydroformylation studied by CIR-FTIR

The mechanism of phosphine-modified rhodium-catalyzed hydrofonnylation 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(PR3)2 complexes 1 were synthesized using p-substituted triarylphosphines where the electron density on rhodium was varied by using p-N(CH3)2, p-OCH3, p-H, p-F,p-Cl or p-CF3. 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 IR spectra, measured under autogenous conditions of 70 †C and 200 psi (1.38 MPa), showed the presence of various intermediates in the catalytic cycle depending upon the phosphine modification, P/Rh ratio, total syngas pressure, and degree of olefin conversion. The rate and spectroscopic data permitted the assignment of a reaction mechanism involving CO dissociation from RhH(CO)2L2 1 as the primary selective hydroformylation pathway.