Kinetics and mechanism of propene hydroformylation catalyzed by rhodium complexes with a diphosphite ligand

The kinetics of propene hydroformylation in the presence of the catalytic system Rh(acac)(CO)2/nL (L = 2,2′-bis[(1,1′-diphenyl-2,2′-diyl)phosphito]-3,3′,5,5′-tetra-tert-butyl-1,1′-diphenyl, 0.5 < n < 20) in para-xylene at 90°C is reported. At n ≥ 2, the rate and regioselectivity of the process are independent of the L concentration. The reaction is of positive fractional order with respect to propene and hydrogen and of negative order with respect to CO. The molar ratio between the linear product and the branched product decreases with an increasing CO pressure and increases with an increasing H2 pressure. The kinetic data are consistent with a process mechanism involving irreversible propene addition to the unsaturated hydride complex HRh(CO)L with the formation of the π-complex HRh(CO)L(C3H6). The insertion of coordinated propene into the H-Rh bond of this complex is reversible in the linear aldehyde formation route and is quasi-equilibrium in the branched isomer formation route. The conclusions as to the character of these reaction steps are corroborated by the compositions of the but-1-ene and but-2-ene hydro-formylation products.