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.