Dendritic wedge

Langa and co-workers have prepared fullerodendrimers 18 and 19 in which the phenylenevinylene dendritic wedge is connected to a pyrazoHno [60] fullerene core rather than to a fulleropyrrolidine one as for 12-17 (Fig. 9) [44]. Preliminary photophysical investigations suggest that the efficient energy transfer from the excited antenna moiety to the pyrazolino [60] fullerene core is followed by an electron transfer involving the fullerene moiety and the pyrazoHne N atom. 

Kim et al. synthesized similar dendritic pseudorotaxanes as shown in Fig. 16 [58]. Reaction of triply-branched amine 42 with pseudorotaxane 43 incorporating CB[6] threaded on a string with a carboxylic acid terminus in DMF in the presence of EDC produces branched [4]pseudorotaxane 44, which is a G-1 dendritic pseudorotaxane. Coupling between 42 and pseudorotaxane dendritic wedge 45 under similar conditions yields G-2 dendritic pseudorotaxane 46, which contains 9 beads threaded on a dendritic framework ([lOjpseudorotax-ane). These dendritic pseudorotaxanes have been characterized by NMR spectroscopy, but further characterization remains to be done. 

Dendrimers are taking their space in the tool box of the modern synthetic chemist. Dendritization might offer solutions to problems yet unsolved. Dendritic wedges, i. e. dendryl substituents of well-chosen size and generation allow us to tune molecular properties like solubility, steric accessability of reactive sites, redox behaviour, and other features. Easy-to-make dendrimers and dendrons will thus become extremely helpful for any chemist in the covalent as well as in the supramolecular world . 

Probably the best example of the manifestation of self-assembly processes in dendritic systems via H-bonds is seen in the recent work of Zimmerman et al. [156]. Dendritic wedges possessing tetraacid moieties (67) self-assembling into a hexameric, disc-like framework (68) was confirmed by SEC and -NMR studies. The tetraacid unit (69) is known to form cyclic as well as linear aggregates in solution via carboxylic acid dimerization (Fig. 30). However, with incorporation of larger dendritic wedges on 69, the hexamer form is preferred. 

Practically all dendrimers [ 1 ] known today have cores with a few, typically three to six functional groups to which the corresponding number of dendrons (dendritic wedges) are attached. The fact that these dendrons are connected to one another by a small, almost dot-like molecule results in considerable steric congestion in the space around the core. This congestion is a unique structural feature and has one led to view dendrimers, specifically those of high genera-... 

Attachment of dendritic wedges of either the carbosilane or benzylphenyl ether type to the para-hydroxy aryl site in [2,6-(ArN=CMe)2C5H3N (1 R = Me, Ar = 2-Me-4-OHC6H3), has been shown to proceed in good yield [162], Complexation with iron(II) chloride allows access to dendrimer-supported precatalyst 42 (Scheme 13). Using MAO as a co-catalyst, it was shown that 42 are active in the oligomerisation of ethylene the activity of these new catalysts is not, however, related to the type of dendritic wedge employed. 

Scheme 13 Use of para-hydroxy 1 as a means of linking to dendritic wedges and complexation with iron(II) chloride...

In one of several important studies on dendronized polymers [4c, 4d]. Schluter and coworkers explored the stiffening of polystyrene chains through the incorporation of Frechet-type dendrons as side chains [28, 29]. While the G-l and G-2 dendrons were not sufficiently bulky to effectively stiffen the polystyrene chain, the G-3 dendron provides enough steric bulk to force the hybrid polymer into adopting a cylindrical shape in solution [28b], In a complementary study, Neubert and Schluter demonstrated that adding charges to the dendritic wedges leads to an expansion of the chains of the hybrid copolymer in aqueous solution [29],... 

Miller and co-workers have recently prepared nanometer-scaled molecular dumbbells based on poly(benzyl ether) dendrons (G1-G4) and oligoimide spacers [64], Their synthetic approach involved the coupling of amine-terminated oligoimides to dendrons with a carboxylic acid focal point. The resulting hybrid materials were found to be quite soluble thus allowing their analysis by cyclic voltammetry in DMF. Consistent with Roncali s observation, the kinetics of reduction of the oligoimide core was not found to be limited by the presence of the dendritic wedges. 

Stoddart and co-workers have made use of Frechet-type dendrons as dendritic stoppers for self-assembled [n]rotaxanes [65]. The solubility enhancement that results from incorporating dendritic wedges at the termini facilitated the purification of these materials by column chromatography despite the polycationic nature of their bipyridinium backbone. Again, the dendritic wedges did not alter the electrochemical characteristics of the viologen subunits. Flowever, the enhanced solubility resulting from the presence of the dendritic components en-... 

Figure 16.14 To investigate hydrogen bonding in dendrimers Zimmerman and Moore [187] prepared dendritic wedges (A-B = CH20 or C=C) functionalized with anthypyridine units at the focal point that function as hosts for benzamidinium guest molecules...

In THF at 20°C, dendritic monomers 3a-3c show absorption bands at 335 and 278 nm, due to the focal 1,4-diethynylbenzene unit and the dendritic wedges, respectively. On the other hand, dendritic macromolecular rods 2a-2c display a strong absorption band in the visible region (400-460 nm), characteristic of an extended electronic conjugation in the backbone. Upon excitation of the conjugated backbone at 425 nm in THF (abs425 nm = 0.01) at 20°C, 2a-2c show a strong blue fluorescence at 454 nm, where the quantum yield ( FL) has been evaluated to be virtually 100%. Of much interest is the fact that the fl>FL value of 2c stays at nearly 100%, even when the solution is concentrated until the... 

Similarly to the case of non-dendritic 4a, the trans form of dendritic 4b, upon irradiation of a CHC13 solution at 340 nm, isomerizes to the cis form, which undergoes a backward isomerization to the trans form upon irradiation at 450 nm or heating. On the other hand, when the dendritic wedges are irradiated at 280 nm, cis-4b undergoes an isomerization to the trans form, to reach a photo-stationary state. In contrast, nondendritic 4a does not undergo such a ds-to-trans isomerization under similar conditions. Relative to these observations, a large but nonsymmetrically substituted azodendrimer such as 4c in the cis form,... 

When the catalyst is located in the core of a dendrimer, its stability can also be increased by site-isolation effects. Core-functionalized dendritic catalysts supported on a carbosilane backbone were reported by Oosterom et al. 19). A novel route was developed to synthesize dendritic wedges with arylbromide as the focal point. These wedges were divergently coupled to a ferrocenyl diphosphine core to form dppf-like ligands (5). Other core-functionalized phosphine dendritic ligands have also been prepared by the same strategy 20). 

BINAP core-functionalized dendrimers were synthesized by Fan et al. (36), via condensation of Frechet s polybenzyl ether dendritic wedges to 5,5 -diamino-BINAP (26—28). The various generations of BINAP core-functionalized dendrimers were tested in the ruthenium-catalyzed asymmetric hydrogenation of 2-[p-(2-methyl-propyl)phenyl]acrylic acid in the presence of 80 bar H2 pressure and in a 1 1 (v/v) methanol/toluene mixture. As later generations of the in situ prepared cymeneruthe-nium chloride dendritic catalysts were used, higher activities were observed (TOF values were 6.5, 8.3, and 214 h respectively). Relative to those of the BINAP... 

Complexes 88—91 catalyzed the polymerization of ethene upon activation with MAO. Every dendritic catalyst displayed a lower activity than its reference complex Cp2ZrCl2 or Cp2TiCl2. It was found that the replacement of a C5HJ ligand by a dendritic cyclopentadienide (88 or 89) caused a moderate decrease in activity (4,320 2,064 and 1,720 kg/mol/h for CpTiCls, 88, and 89, respectively), which could be attributed to steric hindrance. The bis-dendritic Cp system 90 gave a lower activity (576kgmor h ). Interaction between a pair of dendritic wedges may restrict the conformations in which they are kept far away from the metal center. 

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