Rhodium-sulfonated diphosphine catalysts in aqueous hydroformylation of vinyl arenes: high-pressure NMR and IR studies
Hydroformylation of vinyl arenes (p-metboxystyrene and p-fluorostyrene) was performed in aqueous solutions using as catalyst precursor [Rh(mu-OMe)(cod)](2) (cod = 1,5-cyclooctadiene) associated with the sulfonated 1,3-diarylphosphines (tetra-sulfonated 1,3-bis(diphenylphosphino)propane (dpppts)) and the chiral (S,S)-bdppts (2,4-bis(diphenylphosphino)pentane). The influence of pH on the reaction rate was studied. After 24 h conversion was practically total with the achiral system in basic medium for the substituted styrene substrates. Selectivities in aldehydes were >85%. At neutral pH, the asymmetric hydroforrnylation of p-substituted styrenes using the rhodium-bdppts systems provides low conversion but the enantioselectivities were as high as 66%, the highest reported so far for this kind of substrates in aqueous systems. Comparison experiments using rhodium precursors with the non-sulfonated bdpp in organic solvents indicated that the enantioselectivity was higher in aqueous solutions for the p-methoxystyrene derivative and slightly lower for p-fluorostyrene. However, in both the cases the conversions in aqueous systems were low. High-pressure NMR and IR experiments in water/methanol indicate that [RhH(CO)(2)(sulfonated diphosphine)] species form under catalytic conditions in basic medium. At neutral pH, the main species observed in the case of the bdppts ligand is [Rh(bdppts)(2)](+) which may account for the low conversion in this medium. (C) 2002 Elsevier Science B.V. All rights reserved.