Asymmetric dendronization

A further interest in dendrimers for EL applications is the possibility of assigning different functional properties to the individual components of the macromo-lecular structure. For instance, it is conceivable to attach moieties with hole-transporting properties to the core region and electron-transporting properties to the dendrons, or even to integrate bipolar transport functionality in one molecule by virtue of asymmetric dendronization [41, 70]. Many of the fundamental aspects relating to the physical structure and architecture of dendrimers can be mapped... 

Compared to dendrimers, the size (from nanoscale up to microscale), shape (from spherical to rod-like), and structure of dendrons can be engineered more extensively. This structural/morphological flexibility of the materials has allowed for a library of hundreds of dendrons to be synthesized.[14] Some of the unique examples include Janus dendron hybrids and asymmetrically dendronized complexes that have been created via a mix-and-match approach of several dendron scaffolds with different architecture and topology.[14,ll,16]... 

Conjugated polymers, including optically active polymers and dendronized polymers that are very useful in electrical and optical fields and asymmetric catalysis, will continue to attract interest from chemists and materials scientists. It is well anticipated that more and more polymers with interesting structures and properties will be synthesized from the transition metal coupling strategy. 

Fujiki and coworkers [290] synthesized asymmetrically substituted PFs, bearing a bulky Frechet-type dendron and a less bulky 3,6-dioxaoctyl group in position 9. The polymers 215-217 showed a pure blue PL emission with rather low green emission band (at 520 nm) for the films annealed at 200°C for 3 h (in vacuum) (Chart 2.51). 

A. Kimoto, J. Cho, M. Higuchi, and K. Yamamoto, Synthesis of asymmetrically arranged dendrimers with a carbazole dendron and a phenylazomethine dendron, Macromolecules, 37 5531-5537 (2004). 

A number of groups have reported the preparation and in situ application of several types of dendrimers with chiral auxiliaries at their periphery in asymmetric catalysis. These chiral dendrimer ligands can be subdivided into three different classes based on the specific position of the chiral auxiliary in the dendrimer structure. The chiral positions may be located at, (1) the periphery, (2) the dendritic core (in the case of a dendron), or (3) throughout the structure. An example of the first class was reported by Meijer et al. [22] who prepared different generations of polypropylene imine) dendrimers which were substituted at the periphery of the dendrimer with chiral aminoalcohols. These surface functionalities act as chiral ligand sites from which chiral alkylzinc aminoalcoholate catalysts can be generated in situ at the dendrimer periphery. These dendrimer systems were tested as catalyst precursors in the catalytic 1,2-addition of diethylzinc to benzaldehyde (see e.g. 13, Scheme 14). 

Klarner et al. undertook detailed studies of the host/guest relationships between molecular tweezers formed from condensed hydrocarbon arenes and symmetrically and asymmetrically dendryl-substituted viologens - with a 4,4 -bi-pyridinium core acting as -electron acceptor and a Frechet-type dendron as 71-electron donor (Fig. 6.23). The strong fluorescence of the 1,3-dimethylenoxyben-zene units of the Frechet dendron is quenched as a result of the donor/acceptor interaction. Accordingly, in dichloromethane solution the 4,4 -bipyridinium core... 

On use as homogeneous catalysts in the asymmetric reductive alkylation of benzaldehyde with diethylzinc to form secondary alcohols, the corresponding dendritic titanium-TADDOL complexes having either chiral or achiral dendrons gave enantiomeric excesses (ee) of up to 98.5 1.5 at a conversion of 98.7% (for the catalyst with GO dendrons). With larger dendrons the reduction of the ee to 94.5 5.5 (G4) remained within reasonable limits, while the drop in conversion to 46.8% (G4) proved to be drastic. In comparison, the unsubstituted Ti-TAD-DOL complex gave an ee of 99 1 with complete conversion. This negative den-... 

The same group also developed optically active dendronized polymeric BINAP ligands (see also Sect. 5) as a new type of macromolecular chiral catalyst for asymmetric hydrogenation. They could be synthesized by condensation of 5,5 -diamino-BINAP with dendritic dicarboxylic acid monomers (Scheme 5) [44],... 

Fig. 25 Dendronized catalysts for asymmetric catalytic alkylation in a biphasic system...

Scheme 8 Asymmetric alkylation of N-(diphenylmethylene)glycine isopropyl ester with benzyl bromide catalyzed by the dendronized catalysts shown in Fig. 25...

Scheme 7.10. Preparation of chiral dendrimers via the attachment of benzyl ether dendrons to asymmetric, trigonal cores.

Recently, four generations of chiral diphosphine BI NAP-centered dendrimers were synthesized by Fan and coworkers via the condensation of Frechet-type dendrons with fR)-5,5 -diamino-BINAP (Figure 4.3) [32, 33]. The first- to third-generation BINAP dendrimers were tested in the Ru-catalyzed asymmetric hydrogenation of 2-[p-(2-methylpropyl)phenyl]-acrylic acid in methanol toluene (1 1, v/v) at room temperature (Scheme 4.2) [32]. The size of the dendrihc wedges was... 

In order to facilitate recycling of the multiple TsDPEN-functionalized dendrimer catalysts, the same group recently reported the synthesis of a novel form of hybrid dendrimer ligands by coupling polyether dendrons with peripherally TsDPEN-functionahzed Newkome-type poly(ether-amide) dendrimer (Figure 4.28) [90]. The solubility of these hybrid dendrimers was found to be affected by the generation of the polyether dendron. The ruthenium complexes produced were applied in the asymmetric transfer hydrogenation of ketones, enones, imines and activated... 

Fluorescence lifetimes and quantum yields of the symmetrical dendrons (27 and 28) were higher then those of their asymmetric counterparts (29 and 30)... 

The field of catalytic metallodendrimers has seen many advances, particularly in enantioselective catalysis. " Thanks to the early work of Seebach and coworkers, the synthesis of core-functionalised TADDOL dendrimers 40 has been achieved by using four Frechet dendrons (Figure 7.2). These dendrimers were used as ligands in the synthesis of titanium TADDOLates, which were evaluated as catalysts in the asymmetric addition of diethylzinc to benzaldehyde. In a general manner, the good stereoselectivities (89-97% enantiomeric excesses) detected were found to be comparable to those observed with the monomeric TADDOL ligand (98% enantiomeric excess). No attempt to recycle the catalyst or the ligand was mentioned by the authors. 

In this example, the thermal stability decreased with increasing dendritic generation, for both the dendrons and the symmetric dendrimers. TEM and XRD studies indicated a lamellar structure. Interestingly, some of these asymmetric phase-separated dendrons also possess amphiphilic properties and... 

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