Frechet dendron

In 2008, these authors reported a new strategy to attach chiral trans-l-arenesulfonylamino-2-isoborneolsulfonylaminocyclohexane to an achiral Frechet dendron (polyether having a repeated 3,5-dioxybenzyl structure) by a radical approach.The dendrimers obtained were successfully used in the enantioselective nucleophilic alkylation and arylation of ketones, providing... 

Access to a fourth-generation Frechet dendron can thus be shortened by one stage, since no first-generation dendron can be formed by this method - i.e. by doubling of the numbers of generations. 

Repetitive two-stage synthesis of Frechet dendrons is based in the first stage on the reaction of a 3,5-dihydroxybenzyl alcohol with benzyl bromide. Transformation of the lengthened benzyl alcohols into the corresponding lengthened benzyl bromide permits iteration of the reaction with the monomer [42]. 

The first dendrimers with chiral cores for studies on the influence of the stereo-genic centres of a core unit on the chiroptical properties of the overall molecule were presented by Seebach s group [18]. These workers first synthesised dendrimers based on a chiral tris(hydroxymethyl)methane core unit. To these were attached zeroth- to second-generation Frechet dendrons, either directly or separated from the core by an aliphatic (n-propyl) or an aromatic spacer (p-xylylene) (Fig. 4.62). Remarkably, the dendrimers with aliphatic spacer showed no significant optical activity. This loss of chiral information was attributed to a dilution effecf, resulting from linkage of the achiral dendron to the chiral core unit,... 

The protonated dendrimer exhibits a far more intense excimer band than corresponding Frechet dendrons without a cyclam core. One possible reason for this behaviour is that excimer formation is facilitated by folding of the - flexible - benzyl ether framework. Whereas a change in emission intensity is observed in the course of protonation of the cyclam dendrimer, a reference substance containing no cyclam and benzene moieties (Fig. 5.17) shows a linear increase... 

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... 

Related ligands for catalysis, namely BINAP ligands substituted with Frechet dendrons (5, Fig. 6.34) were prepared by Chan et al. [51]. They form rutheniu-m(II) complexes in situ, whose activity in the stereoselective hydrogenation of 2-[p-(2-methylpropyl)phenyl]acrylic acid was investigated. 

The amines 3 reported by Morao and Cossio also constitute neutral dendritic catalysts without metal sites they consist of a simple amine core functionalised with Frechet dendrons (Fig. 6.44) [74]. Such amines can catalyse the nitroaldol or Henry reaction [75] between aromatic aldehydes and nitroalkanes. Whereas neither the yield nor the stereoselectivity (syn/anti 1 1) of the reaction of p-nitro-benzaldehyde with nitroethane was found to change on use of different generations of dendritic catalysts, a distinctly negative dendritic effect was observed in the reaction of benzaldehyde with 3-nitro-l-propanol. Catalysts 3 a and 3 b gave... 

In addition, sulfonamides bear more acidic, compared to carboxamides, protons that can be selectively A-alkylatcd or V-acylatcd. Thus, alkylation of sulfonamide catenane and rotaxane with Frechet-dendrons of 2nd generation led to the first representatives of the dendrocatenane 15 and dendrorotaxane 16 [46], respectively. These compounds were of especial interest, since they allowed for the first time to study chiral induction of topologically chiral cores on appended dendrons and to compare to the analogous centrochiral dendrimers for which phenomenon of the crypto-optical activity was postulated [47],... 

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. 

FuUerodendrimers also allow an evaluation of the accessibility of the Cgo core unit by studying bimolecular deactivation of its excited states by external quenchers. Recently Ito, Komatsu, and co-workers have used this approach to investigate a series of fuUerodendrimers (9-11) in which Frechet-type dendrons have been connected to a fullerene moiety via an acetylene linker (Fig. 5) [35]. 

Fig.1. Rotaxane dendrimers from crown ether type macrocycle ring, bipyridinimn derivatives rod component and Frechet-type dendron stoppers...

JMJ Frechet, DA Tomalia (2001) Dendrimers and other Dendritic Polymers. Wiley, Chichester, UK. GR Newkome, CN Moorefield, F VBgtle (2001) Dendrimers and Dendrons. Wiley-VCH,Weinheim, Germany. 

Delivery of physically encapsulated small molecules at targeted sites by dendrimers has been superbly envisioned in a recent report by McGrath and Junge [95]. Second generation Frechet type dendrons linked to a central azo-benzene-derivative (32, Fig. 15) underwent reversible cis/trans isomerization of... 

In their efforts to construct stimuli-responsive, supramolecular amphiphiles, Frechet et al. [126-129] reported the synthesis of a novel series of AB and ABA block copolymers via the Williamson ether synthesis (e.g., 47, Fig. 21). Polyethylene glycols (PEGs) of different lengths were used as the linear hydrophilic B block while polyaryl ether dendrons of different sizes were used as the hydro-phobic A block. These copolymers were characterized by optical microscopy,... 

In a recent report [141] Stoddart et al. reported a new class of rotaxanes with dendritic stoppers by using a so-called threading approach (Fig. 25). Alkylation of bipyridinium based units with Frechet s third tier branched aryl ethereal dendron, in the presence of BPP34C10 afforded 58 as one of the products. Variable temperature H-NMR spectroscopy in different NMR solvents helped determine the novel shuttling process of BPP34C10 from one bipyridinium unit to the other in 58. The dendritic framework of 58 assists in its solubility in a wide range of solvents. 

Scheme 3. Frechet-type dendrons of generations one to three (2a-c) with a functional group (X) at the focal point... 

Grayson, S., and Frechet, J.M.J. (2001) Convergent dendrons and dendrimers from synthesis to applications. Chem. Rev. 101, 3819-3868. 

Recently, Bo and coworkers [317] reported on dendronized PF with carbazole end groups in the peripheral Frechet-type dendrons 214a-c. Polymers emitted blue light with high < pL of 86-96% in solution for all three polymers 214a-c (G0-G2). In the solid state, < PL depended on the size of the attached dendron (29, 55, and 64% for GO, Gl, and G2, respectively) (Chart 2.50). 

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). 

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