Dendrimer periphery

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). 

In most cases, metal ion coordination by a dendrimer takes place by units that are present along the dendrimer branches (e.g., amine, imine, or amide groups) or appended at the dendrimer periphery (e.g., terpyridine, cathecolamide ligands). When multiple identical coordinating units are present, dendrimers give rise to metal complexes of variable stoichiometry and unknown structures. Luminescent dendrimers with a well defined metal-coordinating site have been reported so far [16, 17], and the most used coordination site is 1,4,8,11-tetraazacyclotetradecane (cyclam). 

The monomeric catalyst fraction showed similar R[ values as the metathesis products, which complicated the chromatographic separation and recycling procedure. Immobilization of the ruthenium catalyst on a dendrimer was anticipated to facilitate the chromatographic separation. Indeed, the presence of multiple (polar) organometallic sites on the dendrimer periphery resulted in stronger adsorption interactions between the dendritic catalyst and the silica and thus a better separation from the product. Two types of dendritic catalysts were prepared in which... 

Dendrimers are ideal scaffolds to construct these devices since (i) a large number of proper redox units can be placed in the periphery and/or branching points of their structure with the possibility of tuning their distance, (ii) the dendrimer skeleton can be designed to minimize electronic interaction between the redox centers, and (iii) the dendrimer periphery can be optimized to get the desired solubility properties and processability to eventually deposit the dendrimers on a surface. Other possible scaffolds are constituted by polymers11 and nanoparticles,12 which are easier to synthesize, but they do not enable control and tuning of the number, position, and distance of the active units. 

Lee JW, Ko YH, Park SH et al (2001) Novel pseudorotaxane-terminated dendrimers supramolecular modification of dendrimer periphery. Angew Chem Int Ed Engl 40 746-749... 

In the case of surface-block dendrimers, the dendrimer periphery exhibits different functionalities in specific molecular segments. They are formed by coupling of dendrons differing in the nature of their terminal functionalities to a common core unit (cf. Section 3.2.1) [6, 10]. 

While the previous sections have largely addressed variation of the dendrimer periphery by covalent transfunctionalisation, an alternative concept is based on modification of the dendrimer surface by non-covalent interactions [18]. Selective interactions of guest molecules with dendritic hosts depend upon the nature of both the dendrimer core and the dendrimer shell. 

Contrary to expectations, the catalytic activity was found to decrease with increasing generation number. This was explained in terms of the increasing spatial demands of the nickel complex units bound in increasing numbers to the dendrimer periphery and increasingly limiting access to their active sites (see Fig. 6.28). 

Their advantage over other types of dendrimers is their straightforward synthesis and, most importantly, their chemical and thermal stabilities. Two distinct steps characterize their synthesis a) an alkenylation reaction of a chlorosilane compound with an alkenyl Grignard reagent, and b) a Pt-cata-lyzed hydrosilylation reaction of a peripheral alkenyl moiety with an appropriate hydrosilane species. Scheme 2 shows the synthesis of catalysts Go-1 and Gi-1 via this methodology. In this case, the carbosilane synthesis was followed by the introduction of diamino-bromo-aryl groupings as the precursor for the arylnickel catalysts at the dendrimer periphery. The nickel centers of the so-called NCN-pincer nickel complexes were introduced by multiple oxidative addition reactions with Ni(PPh3)4. 

Steric compression and channel/ cavity effect at the dendrimer periphery A" - -... 

C6o [88]. It could thus be concluded that C6o had been reduced to its monoanion, as befits a process that is exergonic by 0.9 V [89]. The [dendr-FeI,]+ units, being very large, must be located at the dendrimer periphery, presumably in rather tight ion pairs, although the number of fullerene layers and overall molecular size are unknown (Figure 1). 

Because of their reversible electrochemical properties, ferrocene and its methyl derivatives are the most common electroactive units used to functionalize dendrimers. A recently reported example of this class of dendrimers is constituted by giant redox dendrimers (see e.g., the 81-Fc second generation compound 11 shown in Fig. 13) with ferrocene and pentamethylferrocene termini up to a theoretical number of 39 tethers (seventh generation), evidencing that lengthening of the tethers is a reliable strategy to overcome the bulk constraint at the dendrimer periphery [66]. 

While retentions of the moleailarly enlarged systems of up to 99.75% were observed, the formation of insoluble purple species occurred under continuous-flow conditions. Addition of [Bu4N]Br prevented catalyst precipitation but a fast decrease in the conversion was detected. After 45 cycles, the activity of the catalyst dropped to almost zero, while the retention of the catalyst under the applied conditions was 98.6% (Figure 3). The authors state that the main decrease in activity was due to formation of inactive Ni(III) species. Furthermore, the carbosdane support plays a pivotal role in the accessibility of the active sites surface congestion can lead to the formation of mixed-valence Ni(II)/Ni(III) complexes on the dendrimer periphery that compete for reactions with substrate radicals. 

One example of a dendrimer as an anticancer drug carrier is the covalent attachment of drug molecules to the dendrimer periphery, which is in a conjugated form. The generation number of the dendrimer determines the drug... 

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