Aurora Ruiz

Chiral diphosphites derived from D-glucose: new ligands for the asymmetric catalytic hydroformylation of vinyl arenes.

A series of novel diphosphite ligands derived from readily available D-(+)-glucose has been synthesized. These ligands have been applied to the Rh-catalyzed hydroformylation of vinyl arenes. Both excellent enantioselectivities (up to 91%) and regioselectivities (up to 98.8%) were achieved under mild conditions. The advantage of these ligands is that their modular natures allow facile, systematic variation in the configurations at the stereocenters [C(3), C(5)] at the ligand bridge and in the biphenyl substituents, enabling their effects on the stereoselectivity to be studied. Results show that the absolute configuration of the product is governed by the configuration at the stereogenic center C(3), while the level of the enantioselectivity is influenced by a cooperative effect between stereocenters C(3) and C(5). Replacement of the tert-butyl substituent by methoxy substituents at the para positions of the biphenyl moieties improved the enantioselectivities. We have characterized the rhodium complexes formed under CO/H2 by NMR techniques and in situ IR spectroscopy and have observed that there is a relationship between the structure of the [HRh(CO)2(PP)] species and their enantiodiscriminating performance in hydroformylation. Enantioselectivities were highest with ligands with a strong bis-equatorial coordination preference, while an equilibrium of species with bis-equatorial and equatorial-axial coordination modes considerably reduced the ees.

Tunable furanoside diphosphite ligands. A powerful approach in asymmetric catalysis

A series of highly tunable furanoside diphosphite ligands, derived from readily available D-(+)-xylose and D-(+)-glucose, are discussed. Their modular nature allows a facile systematic variation in the configuration of the stereocentres at the ligand bridge and in the biaryl substituents. This enabled to select a ligand for each particular reaction that provided enantioselectivities that are comparable to those of the best catalysts previously reported in different asymmetric reactions.

Asymmetric hydroformylation of styrene catalyzed by carbohydrate diphosphite-Rh(I) complexes

A series of new chelating diphosphite ligands with a furanoside backbone and axially chiral biphenyl or binaphthyl moieties have been synthesized. Their Rh(I) complexes have been tested as catalyst precursors for the asymmetric hydroformylation of styrene. Systematic variation in chirality at both chiral sugar backbone stereocenters (C-3 and C-5) and either the axial chiral biphenyl or binaphthyl substituents revealed a remarkable effect on the selectivity of the hydroformylation catalysts. In this way, by judicious choice of these elements, both regio- and enantioselectivities can be optimized. Thus, both high enantioselectivity (up to 93% S) and regioselectivity in 2-phenylpropanal (up to 98.8%) were found under mild reaction conditions (15–40°C, 10 bar of syngas) for the ligand with a glucofuranoside backbone and bis(trimethylsilyl)-2,2′-biphenyl moiety. The solution structures of HRh(L–L)(CO)2 complexes have been studied by high pressure NMR and IR. Varying the configuration of the binaphthyl moieties revealed a remarkable effect on the diphosphite coordination modes on the intermediate HRh(L–L)(CO)2 species and, therefore, on enantioselectivity. Enantioselectivity was highest for ligands with a strong bis-equatorial coordination preference.

ENANTIOSELECTIVE COPPER-CATALYSED 1,4-ADDITION OF DIETHYLZINC TO CYCLOHEXENONE USING CHIRAL DIPHOSPHITE LIGANDS

Abstract Bulky diphosphite ligands derived from ribo- and xylofuranose were tested in the copper asymmetric catalytic addition of diethylzinc to 2-cyclohexenone. The xylose derivatives provide enantiomeric excesses in the range of 22–53%. We also describe the preparation of the ribofuranose diphosphite ligands.

Diphosphite ligands based on ribose backbone as suitable ligands in the hydrogenation and hydroformylation of prochiral olefins

Abstract Rh(I) and Ir(I) cationic complexes [M(cod)(PP)]BF 4 have been synthesised from diphosphite ligands 4 – 6 derived from ribofuranose. They have been used in the rhodium and iridium catalysed asymmetric hydrogenation of acrylic acid derivatives. Ribose derivative ligands 4 – 6 have also been used as auxiliaries in the Rh-catalysed hydroformylation of styrene. Hydroformylation results have been explained on the basis of the species formed under hydroformylation conditions. Comparative experiments with the related epimer d -(+)-xylose derivatives showed that the configuration of the product is controlled by the absolute configuration of the stereogenic carbon atom C-3.

Chiral diphosphites derived from d-glucose in the copper-catalyzed conjugate addition of diethylzinc to cyclohexenone

Abstract A series of diphosphite ligands 1 – 3 derived from readily available d -(+)-glucose and bisphenol or binaphthol derivatives have been applied as ligands in the Cu-catalyzed 1,4-addition of diethylzinc to cyclohexenone. Excellent reaction rates (TOF>1200 h −1 ) and good enantioselectivities (e.e. of up to 84%) were achieved. The modular nature of these ligands allows easy systematic variation in the configuration of the stereocenters (C(3), C(5)) at the ligand backbone and in the biaryl substituents, so the optimum configuration for maximum enantioselectivity in the asymmetric 1,4-addition can be determined. The results obtained show that the enantioselectivity induced by the ligand depends strongly on the absolute configuration of the C(3) stereogenic centre, while the sense of the enantiodiscrimination is predominantly controlled by the configuration of the biaryl groups of the phosphite moieties.

Copper-catalysed asymmetric conjugate addition of organometallic reagents to enones using S,O-ligands with a xylofuranose backbone

Abstract The copper-catalysed enantioselective 1,4-addition reactions of diethylzinc to cyclohexenone and trimethylaluminium to E -non-3-en-2-one in the presence of thioether-alcohol ligands, which are easily prepared from d -(+)-xylose, resulted in e.e.s of up to 62% for cyclohexenone and 34% for E -non-3-en-2-one.

Highly active and enantioselective copper-catalyzed conjugate addition of diethylzinc to cyclohexenone using sugar derivative diphosphites

Abstract High enantioselectivity combined with unprecedented high catalytic activity (81% ee and TOF≈1200 h −1 ) are achieved in the copper-catalyzed 1,4-addition of diethylzinc to 2-cyclohexenone with new C 1 -chiral diphosphites derived from the readily available d -(+)-xylose.

Chiral furanoside phosphite–phosphoroamidites: new ligands for asymmetric catalytic hydroformylation

Abstract We have designed a new series of phosphite–phosphoroamidites ligands 1–4 based on a furanoside backbone. These ligands were screened in the Rh-catalyzed asymmetric hydroformylation of styrene, inducing high regioselectivities with 2-phenylpropanal and moderate enantioselectivities (up to 65% e.e.). The results showed that the configuration of the stereogenic carbon atom C(3) at the ligand backbone had remarkable effects on the activity and enantioselectivity. Replacing the tert-butyl substituents with methoxy substituents at the para positions of the biphenyl moieties improved the enantioselectivities. We have also studied the solution structures of HRh(PP)(CO)2 complexes.

New catalysts for the alternating copolymerization of 4-tert-butylstyrene/CO

New cationic palladium(II) complexes, containing CS-nitrogen chelate ligands with pyrazol moieties, catalyze the copolymerization of styrene with CO under mild conditions, to produce a syndiotactic copolymer. Molecular weight and polydispersity for the obtained polymers are among the best reported for bisnitrogen planar ligands.