Experimental evidence for a significant homometallic catalytic binuclear elimination reaction: Linear-quadratic kinetics in the rhodium catalyzed hydroformylation of cyclooctene

The hydroformylation of cyclooctene was studied using Rh4(CO)12 as precursor in n-hexane solvent in the temperature range 293–308 K and PT of 4.0–8.0 MPa, using quantitative in situ infrared spectroscopy. During the course of reaction, the degradation of Rh4(CO)12 to the intermediate RCORh(CO)4 was observed, with accompanying formation of cyclooctane carboxaldehyde. The limited conversion of Rh4(CO)12 to RCORh(CO)4 was shown to be equilibrium-controlled. Some ketone was also formed. Spectral deconvolution was performed with band-target entropy minimization (BTEM). The reaction kinetics for product formation, in terms of the observable organometallics, were rate = image[RCORh(CO)4][CO]−1[H2] + image[RCORh(CO)4][Rh4(CO)12]0.25[H2]0.5[CO]Y. This can be rewritten as rate = image[RCORh(CO)4][CO]−1[H2] + image[RCORh(CO)4][HRh(CO)4][CO]X. The hydride HRh(CO)4 could be identified by BTEM but not accurately quantified. This unusual linear-quadratic expression in rhodium species represents the kinetic form for a simultaneous interconnected unicycle catalytic mechanism and a homometallic catalytic binuclear elimination reaction (CBER). The second term accounted for ca. 40% of the observed product formation at the mean reaction conditions used in this study. The implications and opportunities presented by homometallic CBER are discussed. In particular, a form of modified homometallic CBER is proposed that will permit greater utilization of the nonlinear kinetics.