In the Heck coupling of bromobenzene and styrene over conventionally prepared Pd particles supported on SiO2 at 135 °C under Ar with Na2CO3 as the base, the catalysis of 2−4 ppm of leached Pd in solution is considerably affected by the supported Pd particles. Although such low concentration leached Pd alone is very active for the Heck reaction, its catalytic activity is suppressed in the presence of the supported Pd particles. Despite this, the variation of TON with leached Pd concentration follows the same trend of a homogeneous Pd catalyst. Homogeneous catalysis in nature is suggested based on the fact that the solid−liquid reaction rate is dependent on the leached Pd concentration rather than the supported Pd loading. The change of homogeneous catalysis relating to the variation of active Pd fraction in the presence and absence of the supported Pd particles is discussed.

We report the formation and characterization of stable dispersions of hybrid nanoparticles in solution formed via stereocomplexation of enantiomeric poly(lactide) hybrid star polymers. The hybrid starlike polymers, having polyhedral oligomeric silsesquioxane (POSS) nanocages as the core and either poly(L-lactide) (PLLA) or poly(D-lactide) (PDLA) as the arms, are synthesized via ring-opening polymerization of lactide using octafunctional POSS as the macroinitiator. In the solid state, differential scanning calorimetry and wide-angle X-ray scattering measurements confirmed the formation of the stereocomplex in the mixture of POSS-star-PLLA and POSS-star-PDLA (50:50, wt %). In a solution of the same mixture in tetrahydrofuran (THF), sterocomplexation leads to formation of hybrid nanaoparticles. Detailed accounts of the nanoparticle formation and influence of aging and concentration have been presented. It was observed that at low concentration the stereocomplexed nanaoparticles remain stable over 45 days and are not sensitive to dilution, suggesting the formation of a stable hybrid nanoparticle dispersion in solution. In contrast, the aggregates of the individual POSS-star-PLLA or POSS-star-PDLA in THF, formed via weak solvophobic interactions, tended to disintegrate into smaller aggregates on dilution. Exploiting the PLLA-PDLA stereocomplexation with an appropriate molecular design can be a versatile route to develop stable organic/inorganic hybrid nanoparticle dispersions.

A ligand-free heterogeneous metal catalyst system (represented as Pd/SiO 2 (O)) derived by calcination of Pd(acac) 2 /SiO 2 in air and its catalytic properties toward the Heck coupling of bromobenzene (PhBr) and styrene have been studied. X-ray photoelectron spectroscopy (XPS) and catalytic results demonstrate that most of Pd 2+ is reduced to Pd 0 on SiO 2 by N,N-dimethylacetamide (DMA) during the Heck reaction and that the resulting Pd 0 /SiO 2 is highly active for the Heck reaction, the remaining Pd 2+ /SiO 2 is not responsible for the high activity. Pd/SiO 2 (O) possesses incomparable advantages over a heterogeneous homolog (represented as Pd/SiO 2 (H)) prepared by reduction of Pd(acac) 2 /SiO 2 in H 2 as a pre-catalyst in both activity and catalyst recycling. The activity over Pd/SiO 2 (O) is comparable to that over a homogeneous Pd system. Transmission electron microscopy (TEM) analysis illustrates that the high activity over Pd/SiO 2 (O) consists in the small size of supported Pd particles generated in-situ with gentle reducing agents at a mild temperature.

65 7.0 C6HgCH2N(CH3)2 (100, 85s) 25 2.0 (< 5) ch3(ch2)5n(ch3)3i 15066-77-0 65 4.0 CH3(CH2)5N(CH3)2 (100, 884) 25 2.0 (< 5) ch3c(ch3)2ch2c(ch3)2n(ch3)3i 53624-41-8 65 1.0 ch3c(ch3)2ch2c(ch3)2n(ch3)2 (100) " Reductions were performed using 1.0 mmol of salt and 1.5 mmol of LiEt3BH in 5 ml of dry THF under nitrogen.6 All products identified by comparison with authentic samples or through preparation of known derivatives of isolated products. c Yields were determined by glpc analysis with the aid of an internal standard unless otherwise noted.d Yield of isolated picrate salt.