Kinetics and mechanisms of homogeneous catalytic reactions: Part 6. Hydroformylation of 1-hexene by use of Rh(acac)(CO)2/dppe [dppe = 1,2-bis(diphenylphosphino)ethane] as the precatalyst

A kinetic study of the homogeneous hydroformylation of 1-hexene to the corresponding aldehydes (heptanal and 2-methyl-hexanal) was carried out using a rhodium catalyst formed by addition of 1 equiv. of 1,2-bis(diphenylphosphino)ethane (dppe) to Rh(acac)(CO)₂ under mild reaction conditions (80 °C, 1–7 atm H₂ and 1–7 atm CO) in toluene; in all cases linear to branched ratios were close to 2. The reaction rate is first-order in dissolved hydrogen concentration at pressures below 3 atm, but independent of this parameter at higher pressures. In both regimes (low and high H₂ pressure), the initial rate was first-order with respect to the concentration of Rh and fractional order with respect to 1-hexene concentration. Increasing CO pressure had a positive effect on the rate up to a threshold value above which inhibition of the reaction was observed; the range of positive order on CO concentration is smaller when the total pressure is increased. The kinetic data and related coordination chemistry are consistent with a mechanism involving RhH(CO)(dppe) as the active species initiating the cycle, hydrogenolysis of the acyl intermediate as the rate-determining step of the catalytic cycle at low hydrogen pressure, and migratory insertion of the olefin into the metal-hydride bond as rate limiting at high hydrogen pressure. This catalytic cycle is similar to the one commonly accepted for RhH(CO)(PPh₃)₃ but different from previous proposals for Rh-diphosphine catalysts.