Carboxylated core

Self-assembly of functionalized carboxylate-core dendrons around Er +, Tb +, or Eu + ions leads to the formation of dendrimers [19]. Experiments carried out in toluene solution showed that UV excitation of the chromophoric groups contained in the branches caused the sensitized emission of the lanthanide ion, presumably by an energy transfer Forster mechanism. The much lower sensitization effect found for Eu + compared with Tb + was ascribed to a weaker spectral overlap, but it could be related to the fact that Eu + can quench the donor excited state by electron transfer [20]. 

Rhodium and palladium catalysts that contain 4 display high enantioselectivities for the asymmetric hydrogenation of enamides, itaconates, P-keto esters, asymmetric hydroboration, and asymmetric allylic alkylation,80 82 but this ligand system distinguishes itself from other chiral bisphos-phines in the asymmetric reduction of tetrahydropyrazines and tetrasubstituted olefins (see also Chapter 15). The reduction of tetrahydropyrazines produces the piperazine-2-carboxylate core,... 

Dioxygen binding rates to diiron-carboxylate cores can be retarded and oxygenated intermediate can be trapped by embedding the metal complex into dendrimers.96 A third-generation dendrimer assembled around a paddlewheel diiron complex with 02CArTo1 reacts with 02 300 times slower than its nondendritic counterpart. 

The formation of nanocapsules was achieved by a variety of approaches. One of the earliest processes for making hollow latex particles was developed in the research laboratories of the Rohm and Haas Company [51-54]. Their concept involved making a structured particle with a carboxylated core polymer and one or more outer shells. Ionization of the carboxylated core with base under the appropriate temperature conditions expands the core by osmotic swelhng to produce hollow particles with water and polyelectrolyte in the interior. In addition to this approach, a number of alternative processes have also been patented that are complex in terms of process stages and chemistry [55-57]. 

Wang, X.M., Sun, D.H., Peng, C.Y. and Zheng, G.L (2005) Carboxyl-cored dendrimer and toluene-assisted fabrication of uniform platinum nanodendrites at a water/oil interface and their potential application as a catalyst. Nanotechnology, 17,1599. 

Efforts have been made to reproduce these characteristics in model systems, and molecules with the core structures already described have been prepared. Though none as yet has shown any photoredox activity the 270 pm distance has been shown to be consistent with (/u-oxo)2 bridges and 330 pm with /u-oxo or /u,-oxo-/u-carboxylate bridges. Several mechanistic proposals have been made incorporating these features. 

The imidazole ring is a privileged structure in medicinal chemistry since it is found in the core structure of a wide range of pharmaceutically active compounds efficient methods for the preparation of substituted imidazole libraries are therefore of great interest. Recently, a rapid synthetic route to imidazole-4-carboxylic acids using Wang resin was reported by Henkel (Fig. 17) [64]. An excess aliphatic or aromatic amine was added to the commercially available Wang-resin-bound 3-Ar,M-(dimethylamino)isocyano-acrylate, and the mixture was heated in a sealed vial with microwave irradi-... 

Ionic interactions have been used to prepare lanthanide-core dendrimers. This has been achieved using a convergent synthesis, in which polyether den-drons with a carboxylic acid group at the focal point were assembled around a lanathanide cation. This involved a metathetical reaction with compounds such as Er(OAc)3, Tb(OAc)3 or Eu(OAc)3 to introduce the appropriate lanthanide ion. 

Binger and Hanna [76] have considered the alignment of 4-n-octyl-2-flurophenyl-4-n-octyloxybiphenyl-4 -carboxylate (MBF) and 8CB at room temperature on the (110) surface of polyethylene and the (100) surface of nylon. They find that alignment of both molecules is dominated by the behaviour of the flexible tails that prefer to align parallel between two polymer chains. This forces the mesogenic core to straddle one or more chains. This is illustrated in Fig. 4 for the specific case of MBF on the (110) surface of... 

Zareba et al. [165] described the crystal structure of the chiral 4-(l-methyl-heptyloxycarbonyl)-phenyl 4-heptyloxytolane-4 -carboxylate (C7-tolane) which shows monotropic antiferroelectric and ferroelectric phases. The single-crystal X-ray analysis of this compound shows that the crystal has a smectic-like layer structure composed of largely bent molecules where the chain of the chiral group is almost perpendicular (86°) to the core moiety. Within the layers, the molecules are tilted. The central tolane group of the molecule is roughly planar. 

Electrochemical studies performed in the 7 x Cys-Aspl4 D. afri-canus Fdlll indicate that the reduced [3Fe-4S] center can react rapidly with Fe to form a [4Fe-4S] core that must include noncysteinyl coordination (101). The carboxylate side chain of Asp 14 was proposed as the most likely candidate, since this amino acid occupies the cysteine position in the typical sequence of a 8Fe protein as indicated before. The novel [4Fe-4S] cluster with mixed S and O coordination has a midpoint redox potential of 400 mV (88). This novel coordinated state with an oxygen coordination to the iron-sulfur core is a plausible model for a [4Fe-4S] core showing unusual spin states present in complex proteins (113, 114). 

Inspired by Gif or GoAgg type chemistry [77], iron carboxylates were investigated for the oxidation of cyclohexane, recently. For example, Schmid and coworkers showed that a hexanuclear iron /t-nitrobenzoate [Fe603(0H) (p-N02C6H4C00)n(dmf)4] with an unprecedented [Fe6 03(p3-0)(p2-0H)] " core is the most active catalyst [86]. In the oxidation of cyclohexane with only 0.3 mol% of the hexanuclear iron complex, total yields up to 30% of the corresponding alcohol and ketone were achieved with 50% H2O2 (5.5-8 equiv.) as terminal oxidant. The ratio of the obtained products was between 1 1 and 1 1.5 and suggests a Haber-Weiss radical chain mechanism [87, 88] or a cyclohexyl hydroperoxide as primary oxidation product. 

The effect of core shielding of a porphyrin moiety by peripheral dendrons has been carefully investigated on two series of Zn-phthalocyanine-cored dendrimers with aryl-ether branches [60]. Generation 0,1, and 2 (dendrimer 27) species, terminated with ester groups, are soluble in organic solvents, while the species terminated with carboxylate units (e.g., 28) are soluble in water. 

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