Dendrimers aromatic hydrocarbon

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)...

Scheme 5.10. Employment of Suzuki coupling leads to the construction of all aromatic hydrocarbon dendrimers.

Cyclohydrogenation of poly(phenylene) dendrimers has to be mentioned as a special case of topochemical polymerization proceeding in solution. The reaction yielded extended polycyclic aromatic hydrocarbons containing up to 132 carbon atoms (Figure 17b) [188]. 

Poly(amidoamine) dendrimers such as 91, which differ only in the presence of an ethylenediamine (instead of ammonia) core, were used to host the electron transfer reaetion between photoexcited polycyelic aromatic hydrocarbons (PAHs) and ni-tromethane [169]. Quenching studies in aqueous solution indicate that the PAHs associate with the dendrimer, residing in the interior region, far from the charged surface. It was also found that within the dendrimer structure, nitromethane quenches alternant PAHs, while nonalternant PAHs are not quenched. This selective behavior of nitromethane parallels that commonly observed in solution, while in the presence of traditional micelles both classes of PAHs are quenched by nitromethane. 

Polypropylene amines, poly(amidoamines) (PAMAM) and silicon dendrimers set records for the number of generations, whereas the polyacetylene dendrimer prepared by Moore et al. [6] with the formula C1398H1278 and a mass of 18079.53 holds the heavyweight championship title of pure hydrocarbons. Mullen et al. have recently published a series of aromatic hydrocarbon dendrimers consisting of phenylene-bridged benzene rings. [7] Interestingly, these species form two-dimensional planes rather than three-dimensionally extended structures. 

Synthetic peptide dendrimers, catalytic antibodies, RNA catalysts, peptide foldamers as well as other native or modified enzymes with completely different fxmctions were discovered to catalyze carbon-carbon bond formation [15]. 4-Oxalocrotonate tau-tomerase (4-OT) catalyzes in vivo the conversion of 2-hydroxy-2,4-hexadienedioate (136) to 2-oxo-3-hexenedioate (137) (Scheme 10.33a), and it belongs to the catabolic pathway for aromatic hydrocarbons in P. putida mt-2 [200]. This enzyme carries a catalytic amino-terminal proline, which could act as catalyst in the same fashion as the proline mediated by organocatalytic reactions. Initial studies demonstrate that this enzyme was able to catalyze aldol condensations of acetaldehyde to a variety of electrophiles 138 (Scheme 10.33b) [200]. This enzyme was also examined as a potential catalyst for carbon-carbon bond forming Michael-type reactions of acetaldehyde to nitroolefins 139 (Scheme 10.33c) [201,202]. 

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. 

Besides these classical aromatics and polyaromatic hydrocarbons, other very important classes or arene molecules are porphyrins [60, 61], phthalocyanins [61, 62], porphycens [63], calixarenes [64], resorcarenes [64], cydophanes [47], dendrimers [65], elementa-arenes [66], organometallic arene (hexahapto) [67], benzyne (dihapto), and aryl- and benzyl (monohapto) complexes [68], inorganic pyridine and polypyridine complexes [69], fullerenes [70, 71], and... 

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