Displacement of P‑Stereogenic Phosphiranes from Rhodium by CO in Hydroformylation Catalysis

The P-stereogenic phosphiranes syn- and anti-Mes*PCH2CHPh (1–2, Mes* = 2,4,6-(t-Bu)3C6H2) formed precatalysts for the hydroformylation of propylene using the precursor Rh(acac)(CO)2. Stoichiometric reactions gave the complexes Rh(acac)(L)(CO) (3–4). High-pressure NMR spectroscopy showed that Rh complex 3 was formed in the catalytic mixture, but on treatment with syngas (CO/H2), the phosphirane was displaced from rhodium. These observations were consistent with a density functional theory study, which found that the equilibrium between Rh(acac)(CO)2, phosphiranes 1–2, CO, and Rh(acac)(L)(CO) (3–4) favored the starting materials. These results emphasized the competition between CO and phosphine ligands for coordination to rhodium during hydroformylation and the importance of strong Rh-L coordination to ensure control of catalyst properties.