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

Chiral S,S-donor ligands in palladium-catalysed allylic alkylation

Chiral dithioether ligands have been tested in the model Pd-catalysed allylic alkylation reaction of (+/-)-3-acetoxy-1,3-diphenyl-1-propene with dimethyl malonate. giving high enantioselectivity (up to 81% e.e.) for the first time in this type of system. Pd(Il)-allylic intermediates, [Pd(eta (3)-1,3-Ph-2-C3H3)(dithioether)]PF6 were prepared and characterised both in solution by NMR spectroscopy and solid state. The X-ray structure for [Pd(eta (3)-1,3-Ph-2-C3H3)(L)]PF6 (L=(R,R)-7,8-O-isopropylidene-1,5-dithiacyclononane) was determined. (C) 2001 Elsevier Science Ltd. All rights reserved

Allylic alkylations catalyzed by palladium systems containing modular chiral dithioethers. A structural study of the allylic intermediates

Palladium allylic systems containing modular chiral dithioether ligands were chosen as catalysts for a systematic study of homodonor ligands in allylic alkylation reactions. For this purpose, new type-DMPS (4-6) and -DEGUS (8, 9) ligands were synthesized. Dithioethers 4-6 afforded high activities and excellent selectivities in all Pd-catalyzed allylic reactions. Particularly the Pd/6 catalytic system provided b-VIII with an ee > 99% and a regioselectivity 1/b (VIII) = 1/1.6. A family of intermediate complexes containing several allyl groups (symmetrical, 13-16 and 20, and nonsymmetrical, 17-19, moieties) and dithioether ligands (4-6, 8, 9, 11, and 12) is described. The complexes were fully characterized both in solution and in the solid state. X-ray structures of four of these complexes (16-18 and 20) were determined. The diastereomers present in solution were studied by NMR spectroscopy, and in some cases it was possible to establish a relationship between the diastereomeric excesses and the selectivities found in the catalytic process