The unmodified homogeneous rhodium-catalyzed hydroformylation of cyclohexene and the search for monometallic catalytic binuclear elimination

Abstract

The homogeneous catalytic hydroformylation of cyclohexene to cyclohexanecarboxaldehyde was studied, starting with Rh4(CO)12 as catalyst precursor in n-hexane as solvent. The reaction conditions were T = 298 K, PH2 = 2.0 MPa (0.018 mol fraction), PCO = 6.0 MPa (0.095 mol fraction), and [C6H10]0 = 0.144 mol fraction. The nominal rhodium concentration was varied in four experiments as [Rh4(CO)12]0 = 3.5 × 10-5, 6.6 × 10-5, 1.7 × 10-4, and 3.3 × 10-4 mol fraction (where the last value represents the approximate solubility limit). In each of the four experiments the time-dependent concentrations of the precursor Rh4(CO)12, the organometallic intermediate C6H11CORh(CO)4, and the organic product C6H11CHO were measured under isobaric and isothermal conditions using in situ high-pressure infrared spectroscopy. Complete conversion of the precursor Rh4(CO)12 to the intermediate C6H11-CORh(CO)4 was never observed during the 8 h experiments. Instead, after approximately 30 min, a pseudo steady state was achieved between the species Rh4(CO)12 and C6H11CORh-(CO)4. The hydrogenolysis of C6H11CORh(CO)4 resulted in aldehyde formation, and only 1.04-1.94% conversion of C6H10 was observed in any 8 h experiment. The turnover frequencies (TOF), based on the instantaneous concentrations of C6H11CORh(CO)4, were calculated at 15 min intervals for all four experiments, and the data were regressed according to the polynomial TOF = (k1 + k2Keq[Rh4(CO)12]0.25). The evaluated coefficients were k1 = (2.11 ± 0.06) × 10-3 s-1 and k2Keq = (0.50 ± 0.60) × 10-3 s-1, where the term k2Keq [Rh4(CO)12]0.25 is ca. 5 × 10-5 s-1. These results strongly suggest that there is no statistically significant contribution to aldehyde formation from a monometallic catalytic binuclear elimination in the rhodium-catalyzed hydroformylation reaction under these conditions.