Synthesis of Polyphenylene-Dendrimers

Scheme 6. Palladium catalyzed synthesis of polyphenylene dendrimers...

Wiesler, U.-M., Berresheim, A.J., Morgenroth, F., Lieser, G. and Miillen, K. (2001) Divergent synthesis of polyphenylene dendrimers the role of core and branching reagents upon size and shape. Macromolecules, 34,187-99. Weil, T., Wiesler, U.M., Herrmann, A., Bauer, R., Hofkens,)., De Schryver, F.C. and Mullen, K. (2001) Polyphenylene dendrimers with different fluorescent chromophores asymmetrically distributed at the periphery. Journal of the American Chemical Society, 123, 8101-8. 

Figure 8.6 Divergent synthesis of polyphenylene dendrimers through Diels-Alder reaction (Morgenroth et al, 1998). (Reproduced from F. Morgenroth, A.J. Berresheim, M. Wagner and K. Mullen, Spherical polyphenylene dendrimers via Diels-Alder reactions The first example of an A(4)B building block in dendrimer chemistry, Chemical Communications, 1139-1140,1998, by permission of the Royal Society of Chemistry.

Scheme 10 Synthesis of polyphenylene dendrimers with tetraphenylmethane core by a divergent approach. TiPS triisopropylsilyl...

Using related synthetic tools as above, Bunz et al. also performed the synthesis of polyphenylene dendrimers containing a cyclobutadiene CoCP at the core of the dendrimers. The necessary ethynyl phenyl starting materials 292 were prepared using a Pd-catalyzed Sonogashira-type coupling reaction in good yields. The Co-initiated [2-1-21-cycloaddition of the ethynyl units with CpCo(CO)2 proceeded exclusively between the desired internal alkynes to afford 293 (Scheme 50). ... 

Wessjohann LA, Ruijter E (2005) Strategies for Total and Diversity-Oriented Synthesis of Natural Product(-Like) Macrocycles. 243 137-184 Wiesler U-M,Weil T, Mullen K (2001) Nanosized Polyphenylene Dendrimers.212 1-40 Williams P, see Chhabra SR (2005) 240 279-315... 

Scheme 3. Divergent synthesis of the polyphenylene dendrimers based on the Diels-Alder cycloaddition... 

Kimura et al. also presented the synthesis and characterization of a second-generation ruthenium(II) bis(terphenyl)polyphenylene dendrimer 52, an attractive molecule with regard to its electrochemical and photochemical pro-... 

The convergent synthesis of a range of aryl ester dendrimers with peripheral tetrathiafulvalene units was also reported (Devonport et al. 1998). The dendrimers acquire some amount of the cation-radical tetrathiafulvalene tips on reacting with iodine in solutions. Another promising material is polyphenylene dendrimers functionalized with benzophenone units. On being transformed into the potassium ketyl derivatives, the dendrimers bring forth intermolecular biradicals. These technically attractive species contain four-membered cycles that coordinatively combine two potassium cations... 

Hyperbranched polymers are formed by polymerization of AB,-monomers as first theoretically discussed by Flory. A wide variety of hyperbranched polymer structures such as aromatic polyethers and polyesters, aliphatic polyesters. polyphenylenes, and aromatic polyamides have been described in the literature. The structure of hyperbranched polymers allows some defects, i.e. the degree of branching (DB) is less than one. The synthesis of hyperbranched polymers can often be simplified compared to the one of dendrimers since it is not necessary to use protection/deprotection steps. The most common synthetic route follows a one-pot procedure " where AB,-monomers are condensated in the presence of a catalyst. Another method using a core molecule and an AB,-monomer has been described. ... 

Since the first synthesis of a polypropylene inline) (PPI) dendrimer [1], several families of dendrimers have been prepared including the poly(amidoamines) (PAMAM), polyethers, polyphenylenes), polyphenylacetylenes), polysilanes, silicon, phosphorus and nitrogen-based dendrimers... 

Synthesis of a series of azobenzene-cored polyphenylene dendrimers was reported. The photoresponsive properties of these dendrimers were investigated by optieal speetroseopy and gel permeation chromatography The polyphenylene dendrimers exhibit a photoresponsive behavior upon UV and visible light irradiation, resulting in a reversible and appreciable changes of the dendrimer struetures. 

Wang F, Kon AB, RauhRD (2000) Synthesis of a terminally functionalized bromothiophene polyphenylene dendrimer by a diveigent method. Macromolecules 33 5300-5302... 

The synthesis of the polyphenylene then followed a procedure exploited by Mullen et al. for the obtention of its dendrimers. This procedure provided the access of extremely large ligands around the metal center that may alter the reactivity of the organometallic unit. 

Special emphasis is paid to a modular approach in which functionalized benzene moieties serve as building blocks for chain-type (ID), disc-type (2D), and dendrimer-type (3D) graphene derivatives (Fig. 1) [104,105]. The critical question, when taking on graphene as a challenge for polymer synthesis, is whether any of these polyphenylenes can serve as precursors for graphenes and how the necessary chemical transformation to flat graphene sheets can be accomplished. 

The following case created a breakthrough. Instead of the above AA+BB combination or the AB2-type tetrapenylcyclopentadienone 33 for dendrimer synthesis, we made the AB-type species 33b, which was additionally equipped with long linear or branched aUcyl tails. One readily envisages that a polyphenylene 33c with a rod-like shape is formed in a repetitive Diels-Alder cycloaddition (Scheme 12). 

Concerted efforts to examine the synthesis and polymerization of such ABx monomers was however not attempted until 1989 when Kim and Webster (17) reported their preparation of hyperbranched polyphenylenes fi-om the AB2 monomers, 3,5-dibromophenyl boronic acid, 7, or 3,5-dibromophenyl magnesium bromide, 8 (Figure 1). In a similar way to dendrimers, the field of hyperbranched macromolecules has been extended considerably since this initial report. Hyperbranched polyesters based on fully aromatic, fiilly aliphatic, or partially aliphatic/aromatic systems have been extensively studied by a number of groups (18-20). Similarly, hyperbranched poly(etherketones) (21), poly(ethers) (22), poly(urethanes) (23), poly(siloxanes) (24), etc. have been prepared using classical step growth chemistry. 

---