Dendritic hydrocarbon

Hardly any purely aliphatic hydrocarbon dendrimers have hitherto been prepared (see also Section 6.2.3.3), probably owing to fundamental synthetic problems in the formation of unsymmetrical C-C bonds. The dendritic hydrocarbons described so far are nearly all made up of condensed or coupled arene or/ and multiply bonded components, which is easier to achieve synthetically. As a rule, such molecules are generally much more conformationally rigid than their aliphatic analogues made up of [CH2]n units. 

Further elements of the periodic system will be incorporated into the core, branches, and periphery of dendrimers. High-purity dendritic silicones still appear to be absent. Whereas purely aromatic dendritic hydrocarbons have been described, aliphatic CxHy dendrimer are hardly known. This will require efficient non-symmetrical C-C coupling reactions posing a challenge to develop fundamentally new synthetic approaches yielding minimal amounts of byproducts. 

Dendritic Poly(phenylene)s and Giant Polyaromatic Hydrocarbons (PAHs)... 

This volume of Topics in Current Chemistry aims to highlight major developments in the field and it shows the efforts of chemists from various subdisciplines of chemistry to design new dendritic molecules focusing on novel properties, functions, and potential applications. Hyperbranched/dendritic molecules containing silicon, phosphorus, and other elements are reported apart from hydrocarbon, carbohydrate and nucleic acid cascade molecules. 

Based on the various hybridization states of carbon, (Figure 1.2) at least four major carboskeletal architectures are known [6, 15]. They are recognized as (I) linear, (II) bridged (2D/3D), (III) branched and (IV) dendritic. In adherence with skeletal isomerism principles demonstrated by Berzelius (1832) these major architectural classes determine very important differentiated physicochemical properties that define major areas within traditional organic chemistry (e.g. linear versus branched hydrocarbons). It is interesting to note that analogous... 

The versatility in the construction of dendritic macromolecules, and the unique advantages offered by a controlled step-wise approach, are perhaps best illustrated by the all hydrocarbon family of arylacetylenic dendrimers devel-... 

Recently, dendrimers, which are hyperbranched macromolecules, were found to be an appropriate support for polymer catalysts, because chiral sites can be designed at the peripheral region of the dendrimers (Scheme 5). Seebach synthesized chiral dendrimer 14, which has TADDOLs on its periphery and used an efficient chiral ligand in the Ti(IV)-promoted enantioselective alkylation [21]. We developed chiral hyperbranched hydrocarbon chain 15 which has six p-ami-no alcohols [22], It catalyzes the enantioselective addition of diethylzinc to aldehydes. We also reported dendritic chiral catalysts with flexible carbosilane backbones [23]. 

The stilbene carbon unit has also been peripherally bound to POPAM cores. Although it does not strictly belong to the hydrocarbon dendrimers, the formula of a G2 dendrimer of this type (Fig. 4.21) is depicted here as an example. It was obtained by alkylation of the corresponding eightfold mono-sulphonamide with 4-(bromomethyl)stilbene. Its fluorescence, E/Z isomerisation, photoisomerisation (see Section 5.2.2), and excimer formation were compared with those of non-dendritic stilbenes. The quantum yields of photoisomerisation (0.30) and fluorescence of the E isomer (0.014) of the dendrimer proved to be substantially lower [38]. 

The formation of carbon-mineral adsorbents containing carbon deposits in the form of dendrites, whiskers or carbon black is not advantageous because of the poor mechanical properties of these deposits. The morphology of the coke depends on the mechanism and conditions of its formation on the mineral surface. Two main mechanisms of formation of carbon deposits can be distinguished consecutive reactions and carbide-forming. The latter mode consists in the thermal decomposition of hydrocarbons. 

Recently, Soai et al. reported the synthesis of series of chiral dendrimer amino alcohol ligands based on PAMAM, hydrocarbon and carbosilane dendritic backbones (Figure 4.31) [99-102]. These chiral dendrimers were used as catalysts for the enantioselective addition of dialkylzincs to aldehydes and N-diphenylphosphi-nylimines (Scheme 4.25). The molecular structures of the dendrimer supports were shown to have a significant influence on the catalytic properties. The negative dendrimer effect for the PAMAM-bound catalysts was considered due to the fact that the nitrogen and oxygen atoms on the dendrimer skeleton could coordinate to zinc. 

Khan S, Liu S, Stoner M, Safe S (2007) Cobaltous chloride and hypoxia inhibit aryl hydrocarbon receptor-mediated responses in breast cancer cells. Toxicol Appl Pharmacol 223 28-38 Kim MD, Jan LY, Jan YN (2006) The bHLH-PAS protein Spineless is necessary for the diversification of dendrite morphology of Drosophila dendritic arborization neurons. Genes Dev 20 2806-2819... 

A. Simonyan, I. Gitsov, Linear-dendritic supramolecular complexes as nanoscale reaction vessels for green chemistry. Diels-Alder reactions between fullerene Cgo and polycydhc aromatic hydrocarbons in aqueous medium, Langmuir, 2008, 24, 11431. 

It is worth noting that Newkome s structures have multiple surface hydroxyl groups. As outlined in the Introduction, it is these surface groups that determine the compatibiUty of dendritic architectures with bulk environments such as the surrounding solvent. These hydroxyl groups therefore ensure the compatibility of these systems with water. Furthermore, the relatively apolar hydrocarbon chain is shielded within the dendritic superstructure, and it may be expected that the structure will attempt to phase separate in some way in order to yield stabilising hydrophobic-hydrophobic and hydrophiHc-hydrophiHc interactions. 

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