Dendron ligands

Another approach was followed by Bolm et al. [24], who prepared dendron ligands consisting of a chiral pyridyl alcohol connected to the core of Frechet type dendrons [25]. The chiral dendron ligands were used for the in situ generation of ethylzinc dendron ligand complexes which catalyze the addition of diethylzinc to benzaldehyde (see e.g., 15, Scheme 16). The size of the dendron appeared to have practically no influence on the enantioselectivity of this reaction. 

Fig. 21 Nanoparticles decorated with dendronized ligands gold nanoparticles forming a cubic phase (15) [543], and non-mesogenic y-Fe203 Fe304 core-shell nanoparticles (16) [131, 545]...

FIGURE 6.18 Schematic illustration of the formation of the core-shell CdSe/ZnS QDs by using bidentate dendron ligands. (Reprinted from Ref [52] with permission of Elsevier. Copyright 2009.)... 

Bidentate ligands were used by Alper and coworkers. The bis(diphenylphospino-methyl)amine ligands were prepared on primary amine-terminated PAMAM dendrons on silica as well as polyamido dendrons on polystyrene via the double Mannich-like reaction with formaldehyde and diphenylphosphine (Scheme 15.36a). " Subsequently, Alper and coworkers subjected the dendronized ligand-decorated supports to complexation with rhodium and palladium precursors in order to prepare active catalysts for a number of important chemical transformations (Scheme 15.36b). Initially, the dendronized rhodium catalysts were tested in the hydroformylation reaction and carbonylative ring expansion of... 

PBE dendrons coordinate to the surface of II-VI semiconductor nanocrystals (e.g., CdSe [33] and CdSe/ZnS core/shell structure [34, 35]) to modulate the photoluminescence of the nanocrystals [32]. Trioctylphosphine oxide (TOPO)-capped II-VI semiconductor nanocrystals of several-nanometers diameter have been synthesized by a pyrolysis reaction of organometallics in TOPO [33-35]. The capping ligand (TOPO) can be replaced by stronger ligands such as thiol compounds [36], suggesting that dendrons bearing sulfur atom(s) at the focal point replace TOPO as well. 

Fig. 20. Glycocyclodextrins and dendronized cyclodextrin-bearing mannose ligands.

SCHEME 40. Mannosylated dendrons having an 8-hydroxyquinoline ligand at the focal point for self-assembly.278... 

This value corresponded to an eightfold enhancement as compared to corresponding monovalent lactoside derivative under the same conditions. However, an increase in the branching of the dendron (namely valency) provided only a modest increase in the potency of the ligand, corresponding to a rather constant relative potency per lactose, regardless of the number of peripheral epitopes. 

The research group of Van Leeuwen has focused on catalysis at the core of a carbosilane dendrimer in an effort to be able to control stereoselectivity [10]. To this end, a ferrocenyl diphosphine backbone was functionalized with different generations of carbosilane dendrons producing a series of dendrimer phosphine ligands with an increasing steric demand (see 7 for an example, Scheme 6). In situ... 

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). 

Brunner et al. [26] synthesized and applied so-called dendrizymes in enan-tioselective catalysis. These catalysts are based on dendrimers which have a functionalized periphery that carries chiral subunits, (e.g. dendrons functionalized with chiral menthol or borneol ligands). The core phosphine donor atoms can be complexed to (transition) metal salts. The resultant dendron-enlarged 1,2-diphosphino-ethane (e.g. 16, see Scheme 17) Rh1 complexes were used as catalysts in the hydrogenation of acetamidocinnamic acid to yield iV-acetyl-phenylalanine (Scheme 17) [26]. A small retardation of the hydrogenation of the substrate was encountered, pointing to an effect of the meta-positioned dendron substituents. No significantly enantiomerically enriched products were isolated. However, a somewhat improved enantioselectivity (up to 10-11% e.e.) was... 

In a more recent paper, dendrons with a 3,4-dioxybenzoate focal point were used as ligands for Tb3+, apparently with improved luminescence yield [40]. The discussion... 

By taking advantage of the labile chloride ligands of the central ( inner ) metal, the above-described trinuclear dendrons allowed to obtain higher nuclearity systems, such as the prototype decanuclear [Ru (2,3-dpp)Ru[(2,3-dpp)Ru(bpy)2]2 3]20 +... 

Initial attempts to replicate the idea of using dendronized liquid crystal ligands to induce mesophase formation in magnetic nanoparticles failed for core-shell... 

Fig. 22 Nanoparticles decorated with pro-mesogenic dendronized or bent-core liquid crystal ligands nematic Fe304 mixed monolayer nanoparticles capped with dendronized cyanobiphenyl ligands and oleic acid (17) [132], and mixed monolayer, non-mesogenic gold nanoparticles decorated with bent-core liquid crystal and hexane thiolates (18) [547]...

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