Hydrocarbon dendrimers

A large family of new dendrimers has been synthesized following this divergent method. Hawker and Frechet developed polyaryl(-benzyl)ether dendrimers [27], Miller andNeenan [28], and also Moore and Xu [29] prepared hydrocarbon dendrimers. The latter have reported the largest monodisperse organic hydrocarbon dendrimer with a molecular mass of 18 kDa and a diameter of 12.5 nm [30]. 

We reported the first synthesis of a symmetrical, four-directional, saturated hydrocarbon dendrimer containing 36 carboxylic acid moieties equidistant... 

Especially, the divergent and convergent strategies have been applied to many dendrimer syntheses. Nowadays, there are dendrimers with branching units which are based on silicon, phosphorus and other heteroatoms,1101 dendrimers which are built up supramolecularly (rather than covalently)1111 and even pure hydrocarbon dendrimers.112,131 A dendrimer of the tenth generation with 3072 gold ions at its periphery, which is 15 nanometers in diameter, has been prepared successfully by Majoral et al.1101... 

A similar approach was taken by Moore [27], utilizing poly(phenyl acetylene) dendrimers with a dimethylbenzene moiety attached at the core of the dendrimer. An anomalous shift (41 nm) in the fluorescence spectra of the probe in various nonpolar hydrocarbon solvents was observed for G5 and G6, but not for G1 to G4. This observation confirmed significant in the size and shape changes for these dendrimers between G4 and G5. 

A working model for dendrimer thermolysis during calcination involves the PA-MAM dendrimer backbone initially reacting with oxygen (which may or may not be activated by a nanoparticle) in a relatively facile process to generate carboxylates and other surface species. Removal of carbonaceous species closely associated with the nanoparticle is required for complete activation of the catalyst. For Pt DENs, the surface carboxylates may be strongly adsorbed to the nanoparticle surface and extended O2 treatments are required for deep oxidation of the hydrocarbon to reach reasonably volatile species. Once formed, however, it appears that they can be removed more readily with a hydrogen treatment than with further oxidation. 

Using the dendrimer route, it is possible to prepare supported catalysts not available via traditional routes. Dendrimer derived Pt-Au catalysts having compositions within the bulk miscibility gap can be prepared on several oxide supports. For all the supports studied, the bimetallic catalysts exhibited synergism with respect to mono- and cometallic catalysts for the CO oxidation and hydrocarbon NOx SCR reactions. The bimetallic Pt-Au catalysts also showed evidence of exchanging surface and subsurface atoms in response to strongly binding ligands such as CO. 

A key feature of our polyphenylene dendrimers is that they can be planarized and thus reduced in dimensionality by intramolecular dehydrogenation [29,35]. This results in large, fused polycyclic aromatic hydrocarbons (PAHs). PAHs serve as structurally distinct, two-dimensional subunits of graphite and show attractive properties such as high charge carrier mobility, liquid crystallinity, and a high thermal stability, which qualifies these materials as vectorial charge transport layers [81]. 

Koch [82] and optimized further by us. The polyphenylene 28, which represents a first-generation dendrimer, forms 28 new bonds yielding the planar polycyclic aromatic hydrocarbon 56 (Scheme 23). Owing to its extreme insolubility in all common solvents, PAH 56 is characterized by laser desorption mass spectrometry based on its M+ peak at miz =1621. 

Scheme 23. Planarization of polyphenylene dendrimers yielding two-dimensional polycyclic aromatic hydrocarbons (PAHs)...

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-... 

Figure 11.3 Chemical structures of the (a) arborol dendrimer and (b) water-soluble, all-hydrocarbon dendrimer.

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]. 

Miller and Neenan succeeded in the very same year in producing the first hydrocarbon dendrimers based exclusively on arene units, also using a convergent synthetic strategy [16] (Fig. 1.7). 

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. 

Meier et al. described hydrocarbon dendrimers with trans-stilbene chromo-phores in the core and in the periphery. Owing to the flexible nature of the arms intra- and intermolecular C-C bonds could be formed by irradiation [28] (see Section 4.1.5.3 for further information). 

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]. 

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. 

In order to avoid this unfavourable effect of the functional groups in the dendrimer structure, a rigid hydrocarbon backbone without heteroatoms was synthesized. Dendrimers with poly(phenylethyne) backbones, bearing three and six ephedrine derivatives at the periphery, were studied in the alkylation of aldehydes and N-diphenylphosphinylimines and proved to be highly en-antioselective catalysts. For example, the system containing six catalytic sites catalyzed the addition of diisopropylzinc to aldehydes with enantioselectivi-ties of up to 86% ee. As a third backbone a polycarbosilane dendrimer was used, which is chemically inert and more flexible than the poly(phenylethyne)... 

Figure 2.11. Computer generated extended quadrant of a hydrocarbon-based dendrimer illustrating segmented self-similarity.

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