Dendrimers Frechet-type

A year later, Chou and Shu [316] reported Suzuki coupling of similar alternating fluorene copolymers containing Frechet-type dendrimers as side chains. In contrast to the above... 

SCHEME 2.30 Synthesis of the Frechet-type dendrimer-substituted PFs and fluorene random and alternating copolymers. (From Marsitzky, D., Vestberg, R., Blainey, P., Tang, B.T., Hawker, C.J., and Carter, K.R., J. Am. Chem. Soc., 123, 6965, 2001.)... 

The dendrimer wedges with two, three, and four organotelluride groups and the first-generation Frechet-type dendrimers based on 3,5-dihydroxybenzyl alcohol and l,l,l-tris-4-hydroxyphenylethane shown in Fig. 26 illustrate statistical effects in thiolperoxidase-like reactions. Nearly linear increases in reactivity on a molar basis with increasing numbers of organotelluride groups were observed in the oxidation of... 

Numerous examples of postsynthetic modifications at the dendritic periphery have been explored. In early work, Hawker et al. investigated the hydro-genolysis and subsequent modification of benzyl-protected poly(aryl ester) dendrimers473 and later the saponification of Frechet-type dendrimers with p-methyl benzoate end groups to afford an anionic carboxylated periphery.41 The isophthalate ester end groups proved to be especially versatile (Scheme 28)... 

Figure 4.6 Chiral dendrimer diphosphine ligands bearing Pyrphos at the focal point of the Frechet-type dendrimer.

During the past two decades the homogeneous and heterogeneous catalytic enan-hoselective addition of organozinc compounds to aldehydes has attracted much attention because of its potential in the preparation of optically active secondary alcohols [69]. Chiral amino alcohols (such as prolinol) and titanium complexes of chiral diols (such as TADDOL and BINOL) have proved to be very effective chiral catalysts for such reactions. The important early examples included Bolm s flexible chiral pyridyl alcohol-cored dendrimers [70], Seebach s chiral TADDOL-cored Frechet-type dendrimers [28], Yoshida s BINOL-cored Frechet-type dendrimers [71] and Pu s structurally rigid and optically active BlNOL-functionalized dendrimers [72]. All of these dendrimers were used successfully in the asymmetric addition of diethylzinc (or allyltributylstannane) to aldehydes. 

Very recently, Fan et al. [13] reported on the use of a water-soluble PEO-substitut-ed first- and second-generation Frechet-type dendrimer with a chiral BINOL (1,1 -bi-2-naphthol) unit in catalysis. The enantiomeric excess in asymmetric hydrogenation of 2-[p-(2-methylpropyl)phenyl]acrylic acid with [RuCl(BINAP)(cymene)]Cl in an aqueous system was reported to increase upon addition of the dendritic... 

Styryl-terminated Frechet-type dendrimers have been introduced as novel polymer crosslinkers by Seebach et al. [43-45]. They are constituted of four to 16 peripheral styryl units attached to aryl end branches of dendritic TADDOL, BINOL or Salen ligands and were copolymerised with styrene by suspension polymerisation. The catalytic performance of the polymer-bound catalyst was identical to that of the homogeneous analogues however, the supported catalysts could be used in many consecutive catalytic runs with only small loss in catalytic activity. A major drawback of fixing the catalytic unit in the core of the crosslinker is the poor loading capacity of the final polymer (0.13-0.20 mmol g 0> especially when high amounts of catalysts (10-20 mol%) are needed. 

Dendritic compounds avoid such a reptation due to their branched structure. Thus, the concave pyridine la was incorporated into Frechet-type dendrimers by a convergent synthesis (see Fig. 8b).The resulting products like 28 can be retained by nanofiltration. 

Scheme 7 Chemoselective AB2 + CD2 approach to making a fourth-generation Frechet-type dendrimer in four steps." ...

Ethers are chemically and coordinatively more inert than amides. Indeed, Frechet-type polyether dendrimers prepared in solution demonstrate a broad spectrum of chemical and coordinative compatibilities. Accordingly, polyether dendron synthesis on solid support was developed, initially by Bradley and coworkers and later by Portnoy and Dahan Instead of the 3,5-dihydroxy benzyl alcohol derivatives used in the soluble Frechet-type dendrimers, a more SPOS suitable 3,5-bis(hydroxymethyl)phenol unit was conceived as a building block for the dendron construction. 

Hawker and Wooley later extended this CuAAC approach to prepare azide- or alkyne-terminated Frechet-type dendrimers via a divergent strategy (Joralemon et al., 2005). 

The versatility of the concept was also demonstrated for the synthesis of Frechet-type dendrimers by sequential etherification and CuAAC reactions starting from a triphenol core, using AB2 and CD2 types of monomers (Antoni et al., 2007). Because of the simplicity of this strategy, a wide variety of other macromolecular architectures, including star-shaped polymers and hyperbranched polymers, could be easily synthesized with unique characteristics that are tuned by the nature of the monomers. 

In another elegant study reported by Fan s group in 2008 [51], monodentate phosphoramidite ligand (23) was incorporated into a Frechet-type dendrimer and... 

Deng s group [54] first extended dendritic catalysis into asymmetric transfer hydrogenation. In their report, Noyori s excellent ligand (S,S)-TsDPEN was incorporated into the focal point of the Frechet-type dendrimer (26), and subsequently... 

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