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

The homogeneous catalytic hydroformylation of cyclohexene to cyclohexanecar boxaldehyde was studied, starting with Rh-4(CO)(12) as catalyst precursor i n n-hexane as solvent, The reaction conditions were T = 298 K, P-H2 = 2.0 M Pa (0.018 mol fraction), P-CO = 6.0 MPa (0.095 mol fraction), and [C6H10](0 ) = 0.144 mol fraction. The nominal rhodium concentration was varied in fou r experiments as [Rh-4(CO)(12)](0) = 3.5 x 10(-5), 6.6 x 10(-5), 1.7 x 10(- 5), and 3.3 x 10(-4) mol fraction (where the last value represents the appr oximate solubility limit). In each of the four experiments the time-depende nt concentrations of the precursor Rh-4(CO)(12), the organometallic interme diate C6H11CORh(CO)(4), and the organic product C6H11CHO were measured unde r isobaric and isothermal conditions using in situ high-pressure infrared s pectroscopy. Complete conversion of the precursor Rh-4(CO)(12) to the inter mediate C6H11CORh(CO)(4) was never observed during the 8 h experiments. Ins tead, after approximately 30 min, a pseudo steady state was achieved betwee n the species Rh-4(CO)(12) and C6H11CORh(CO)(4). The hydrogenolysis of C6H1 1CORh(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 calc ulated at 15 min intervals for all four experiments, and the data were regr essed according to the polynomial TOF = (k(1) + k(2)K(eq)[Rh-4(CO)(12)](0.2 5)). The evaluated coefficients were k(1) = (2.11 +/- 0.06) x 10(-3) s(-1) and k(2)K(eq) = (0.50 +/- 0.60) x 10(-3) s(-1), where the term k(2)K(eq) [R h-4(CO)(12)](0.25) is ca. 5 x 10(-5) s(-1). These results strongly suggest that there is no statistically significant contribution to aldehyde formati on from a monometallic catalytic binuclear elimination in the rhodium-catal yzed hydroformylation reaction under these conditions.