Core of a dendrimer

Remarkably, alkylation of the core of a dendrimer of generation 6 (two P = N-P(S) fragments on the core, 64 for the fifth generation, and an upper generation incorporating 256 terminal aldehyde groups) can be performed. This clearly demonstrates that the core of this sixth generation dendrimer is available for certain reactions. 

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). 

Gandhi P, Huang B, Gallucci JC, Parquette JR. Effect of terminal group sterics and dendron packing on chirality transfer fi om the central core of a dendrimer. Org Lett 2001 3 3129-3132. 

Compared to polymers, dendrimer architectures offer favourable conditions for fixation of catalytically active moieties thanks to their monodispersity, variability, structural regularity of the molecular scaffold, and numerous functionalisation possibilities. Catalytic units can be fixed - multiply if required - on the periphery, in the core of a dendrimer, or at the focal point of a dendron. If the dendrimers are suitably functionalised at the periphery, appropriate metal complexes can be directly attached to the surface of the molecule. In contrast, dendrimers functionalised in the core or at the focal point shield the catalytically active site through their shell structure in a targeted manner, for example to attain substrate selectivity in the case of reactants of different sizes [1]. The corresponding concepts of exodendral and endodendral fixation of catalysts were inttoduced in the context of functionalistion of carbosilane, polyether, and polyester dendrimers [2]. Exodendral fixation refers to attachment of the catalytic units to the... 

Fig. 2 Schematic representations of metallodendritic architectures according to the metal (catalyst) location A at the periphery of a dendrimer or of a dendron B at the core of a dendrimer or at the focal point of a dendron C at branching points of a dendrimer or of a dendron D dendrimer-encapsulated metal nanoparticles (DEMNs)...

Encapsulation of a Ru atom into a caged [109, 110] or hemicaged [299] tris-bipyridine ligand extends the MLCT excited state lifetime and improves photostability relative to [Ru(bpy)3] ", while retaining the fast (diffusion controlled) bimolecular excited state electron transfer reactivity. In contrast, the [Ru(bpy)3] + in the core of a dendrimer [248] has about the same inherent lifetime as the free complex but the rate of electron transfer quenching rapidly decreases with increasing the number and size of dendrimer branches. 

Metal centers can typically be included in the core or as the core of a dendrimer via metal-ligand coordination bonds. Complexes (M(L) ) of metals (M) containing ligands (L) that can readily be replaced are generally employed for synthesizing these metallodendrimers. Due to its remarkable ability to form stable coordination compounds, Ru(II) metal ion is an excellent candidate for such a build-up process. One of the first examples of Ru(II)-based metallodendrimers was prepared by reacting... 

Dendrimers are a class of macromolecules with a precisely controllable branched structure, consisting of three structural units a core, a hyperbranched scaffold and an external surface [16]. Dendrimers have been shown to possess unusual physical and chemical properties that differ significantly from those of linear oligomers and polymers. By using a fluorescent chromophore as the core of a dendrimer, one can apply fluorescence spectroscopy to study stmctural aspects and the conformational mobility of dendrimers in solution [17, 18]. At the same time, the dendritic shell provides a unique nanometer-sized environment for the spatial isolation of the chromophore, making them interesting materials for investigations by SMS. The synthesis of dendrimers with fluorescent chromophores attached to the rim serves as an efficient way to obtain a weU-defined number of chromophores in a confined volume [19-25]. Not only can the number of chromophores be easily controlled. 

Both energy and electron transfer quenchers have been employed to show that the quenching rates of the fullerene triplet state are decreased as a function of the size of the dendrimer shell [36]. These results further demonstrate that fullerene is an excellent functional group to probe the accessibility of a dendrimer core by external molecules. 

Dendrimers are complex but well-defined chemical compounds, with a treelike structure, a high degree of order, and the possibility of containing selected chemical units in predetermined sites of their structure [4]. Dendrimer chemistry is a rapidly expanding field for both basic and applicative reasons [5]. From a topological viewpoint, dendrimers contain three different regions core, branches, and surface. Luminescent units can be incorporated in different regions of a dendritic structure and can also be noncovalently hosted in the cavities of a dendrimer or associated at the dendrimer surface as schematically shown in Fig. 1 [6]. 

Dendrimer 1 + is a classical example of a dendrimer containing a luminescent metal complex core. In this dendrimer the 2,2 -bipyridine (bpy) ligands of the [Ru(bpy)3] +-type core carry branches containing 1,2-dimethoxybenzene- and 2-naphthyl-type chromophoric units [15]. 

To catalyze asymmetric transformations, catalytically active sites can be incorporated in different areas of a dendrimer a) chiral sites at the periphery, b) chiral sites in cavities or at the core, c) achiral sites which are surrounded by chiral branches in the interior of the dendrimer. 

When the only metal complex of a dendrimer is that constituting the core of the structure (Fig. la), the most interesting problem is whether and, if so, how much the electrochemical properties (potential value, kinetic reversibility) of the metal-based core are modified by the surrounding branches. 

The synthesis and structure of a dendrimer can be illustrated by the well-known poly (amidoamine) type (called PAMAM), which describes the monomers making up the complete polymer. The synthesis starts from a core diamine (or ammonia) molecule. The diamine can be of various lengths and spacer arm properties and even contain cleavable components. Typically, the core... 

Figure 11. The implications for the total yield, assuming an idealized dend-rimer functionalization for the example of a dendrimer with AB2-branching and bifunctional core unit (G = generation).

A similar approach was taken by Moore [27], utilizing poly(phenyl acetylene) dendrimers with a dimethylbenzene moiety attached at the core of the dendrimer. An anomalous shift (41 nm) in the fluorescence spectra of the probe in various nonpolar hydrocarbon solvents was observed for G5 and G6, but not for G1 to G4. This observation confirmed significant in the size and shape changes for these dendrimers between G4 and G5. 

Metallodendrimers can be constructed via binding of groups with suitable donor atoms (e.g., polydentate ligands) on either the periphery or the core of the dendrimer and the subsequent complexation/coordination of these ligands to an appropriate metal salt. Ultimately, this binding can involve the formation of a direct a bond linkage (i.e., a M-C bond). This chapter describes various... 

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... 

Its design versatility, as generic dendrons may be prepared to be used later as building blocks in conjunction with other reactive molecules, or coupled to a multifunctional core to afford functional dendrimers, dendritic-linear hybrids, dendronized polymers, etc. This may be a particularly significant advantage if the coupled reactive or core molecule is itself sensitive to the reaction conditions used in the multiple steps of the iterative synthesis of a dendrimer. 

Functional groups can be introduced at different locations in the dendrimer. Placement in the core of the dendrimer results in a topological isolation and protection of the functional group from the environment [48,49]. Furthermore, single shells of the dendrimer corresponding to different generations can be endowed with functionalities [50]. Thus particles with functions having a defined depth under the surface can be realized. The decoration of the dendrimer sur-... 

Earlier we found that the addition of alkyl-modified poly(propylene imine) dendrimers to polypropylene leads to fibers which can be dyed in conventional acid or disperse dyeing processes [3]. The alkyl chains make the additive compatible with the polypropylene matrix, while the polar core of the dendrimer can act as a receptor for the dye molecules. This host-guest behavior is analogous to the principle of the dendritic box as described by Meijer et al. [30] and elaborated by Baars et al. for dye extraction processes [31]. 

The functionality at the central core of the dendritic macromolecule can also be exploited as a unique reactive site for a variety of purposes. Perhaps the most dramatic example is the introduction of self-assembling units at the focal point to give dendritic fragments, 36, which undergo spontaneous self-assembly to give larger dendrimers [68] (Fig- 9). In this case the central core of the dendrimer is a self-as-... 

Reaction of 217 with Cjq leads to the amino-protected porphyrin-fulleropyrroli-dine, which can easily be deprotected to the corresponding amine [229, 277]. By further functionalization via amide coupling an easy access to extended donor-acceptor systems is possible. A carotene-porphyrin-fullerene triad was prepared by reaction of the amine with the appropriate carotene acid chloride. The motivation for the synthesis of all these donor-acceptor systems is the attempt to understand and imitate the photosynthetic process. On that score, a model for an artificial photosynthetic antenna-reaction center complex has been achieved by attaching five porphyrin cores in a dendrimer-like fashion to the fullerene [242]. 

Dendrimers, among other applications, are generating interest as soluble supports thanks to the following intrinsic characteristics (i) the well-defined molecular composition of a dendrimer provides a support with a precisely defined arrangement of the reactive sites, (ii) a high loading of reactive sites is achieved on the dendrimer surface and (iii) nanofiltration techniques are available to separate the dendritic support from products. Dendrimer 143, based on a carbosilane core, possesses 12 ester functionalities on... 

---