Mechanistic Aspects of a Highly Regioselective Catalytic Alkene Hydroformylation using a Rhodium Chelating Bis(phosphite) Complex

The rhodium-catalyzed hydroformylation of 1-octene using the bis(phosphite) ligand bbmb was studied using in situ, high-pressure ¹H and ³¹P NMR and FT-IR spectroscopy. Four species, Rh(bbmb)(acac), Rh(bbmb)(CO)₂H, and two dimeric complexes, appeared sequentially during different stages of the catalysis when Rh(acac)(CO)₂ was used as the catalyst precursor. These were independently synthesized and their reactivity studied. The major species present during catalysis was determined to be Rh(bbmb)(CO)₂H (1), which was fully characterized. This hydride complex was shown to be an effective alkene isomerization catalyst. Using magnetization transfer, the rate of exchange between 1 and the terminal and internal vinyl hydrogens of 3,3-dimethylbutene were 0.62 and 0.51 s^-1 respectively. The rapid, reversible nature of the alkene insertion establishes that the regiochemistry of the final aldehyde is not determined at the alkene insertion step or any event prior to it. The dimeric species were shown to convert to 1 via reversible addition of dihydrogen.