A new approach for the preparation of well-defined Rh and Pt nanoparticles stabilized by phosphine-functionalized silica for selective hydrogenation reactions.

In this work, a new methodology for the synthesis of well-defined metallic nanoparticles supported on silica is described. This methodology is based on the surface control provided by SOMC. The nanoparticles are formed via the organometallic approach and are catalytically active in the hydrogenation of p-xylene, 3-hexyne, 4-phenyl-2 butanone, benzaldehyde, and furfural.

Modular furanoside phosphite ligands for asymmetric Pd-catalyzed allylic substitution.

A series of diphosphite, phosphine-phosphite, and thioether-phosphite ligands 1-5 with a furanoside backbone have been used in the enantioselective palladium-catalyzed allylic substitution of rac-1,3-diphenyl-2-propenyl acetate giving low to high enantioselectivies (from close to 0% to 97% ee). The modular nature of these ligands enables systematic investigations of the effect of the ligand structure on the enantioselectivity. The enantioselectivity is mainly determined by the configuration of the stereogenic center C-3 of the furanose backbone. From this we conclude that the attack of the nucleophile takes place trans toward the donating group at the stereogenic C-5 atom. Systematic variation of the donor group attached to the carbon atom C-5 indicated that the presence of a bulky phosphite functionality has a positive effect on enantioselectivity. Thus, the highest ee's are obtained using the bulky diphosphite ligand 1b containing a xylofuranoside backbone.

Highly enantioselective Rh-catalyzed hydrogenation based on phosphine-phosphite ligands derived from carbohydrates.

A new class of efficient catalysts was developed for the asymmetric hydrogenation of alpha,beta-unsaturated carboxylic acid derivatives by synthesizing a series of novel phosphine-phosphite ligands (4a-d) derived from readily available D-(+)-xylose. Excellent enantioselectivities (> 99%) were achieved under very mild reaction conditions (1 bar H(2) and 20 degrees C). Varying the biphenyl substituents in the phosphite moiety greatly affected the enantioselectivity in the hydrogenation reactions. The results also indicate that the sense of enantioselectivity is mainly controlled by the configuration of the phosphite moiety. (31)P[(1)H] NMR and kinetic studies on intermediates of the catalytic cycle show that the [Rh(P(1)-P(2))(enamide)]BF(4) species is the resting state and that the rate dependence is first order in rhodium and hydrogen pressure and zero order in enamide concentration.

Chiral phosphine-phosphite ligands in the highly enantioselective rhodium-catalyzed asymmetric hydrogenation.

We have investigated a series of enantiopure phosphine-phosphite ligands (P(1)-P(2) = ligands 1-4) in the rhodium-catalyzed asymmetric hydrogenation reaction. Intermediate [Rh(P(1)-P(2))(cod)]BF(4) and [Rh(P(1)-P(2))(5)]BF(4) complexes (cod = 1,5-cyclooctadiene; 5 = methyl acetamidoacrylate ester) were observed by (31)P[(1)H] NMR. The [Rh(P(1)-P(2))(cod)]BF(4) complexes were precursors to active catalysts of the asymmetric hydrogenation reaction of several prochiral dehydroamino acid derivatives under mild reaction conditions (1 bar of hydrogen and 20 degrees C). The enantiomeric excess reached up to 99%.

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 ee's.

An unprecedented recyclable catalyst system for asymmetric hydroboration.

Immobilised preformed chiral homogeneous catalysts were subjected to catalytic hydroboration of styrene with catecholborane and the activity, regio- and enantioselectivity observed were similar to those found when the corresponding homogeneous catalyst was used, remaining constant for several consecutive runs.

Decay of the GRB 990123 optical afterglow: implications for the fireball model

Broad-band (ultraviolet to near-infrared) observations of the intense gamma ray burst GRB 990123 started approximately 8.5 hours after the event and continued until 18 February 1999. When combined with other data, in particular from the Robotic Telescope and Transient Source Experiment (ROTSE) and the Hubble Space Telescope (HST), evidence emerges for a smoothly declining light curve, suggesting some color dependence that could be related to a cooling break passing the ultraviolet-optical band at about 1 day after the high-energy event. The steeper decline rate seen after 1.5 to 2 days may be evidence for a collimated jet pointing toward the observer.