Very Low Pressure Rh-Catalyzed Hydroformylation of Styrene with (S,S,S-Bisdiazaphos): Regioselectivity Inversion and Mechanistic Insights

Rates and selectivities of styrene hydroformylation as catalyzed by the rhodium coordination complex of (S,S,S)-bis-diazaphos (BDP) under extremely low pressures of syngas (CO/H2) are reported. At pressures <10 psia, the catalyst system is selective for the linear regioisomer, 3-phenyl-1-propanal, whereas at high pressure, it is highly selective for branched (R)-2-phenylpropanal. Lowered pressures severely degrade the enantioselectivity of the branched product. Qualitative kinetic data reveal large changes in the form of the apparent rate law, suggesting significant changes in catalyst speciation prior to selectivity-determining transformations. Most strikingly, under low-pressure conditions, the qualitative kinetic data imply that the catalyst accumulates as either 4-coordinate (bisphosphine)Rh(CO)(alkyl) (4) or 5-coordinate (bisphosphine)Rh(CO)(alkyl)(styrene) (5) complexes, neither of which have been previously observed as hydroformylation resting states under catalytic conditions. Although styrene is electronically biased to yield branched product, simple changes in the pressure of CO change the rate law and regio- and enantioselectivity while maintaining useful catalytic rates. These observations suggest that a broad range of selectivity regimes can be accessed with a single catalyst, instead of a catalyst library, simply by varying across different magnitudes of CO pressure.