Dendrimer wedges

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... 

Fig. 26 Dendrimer wedges and first-generation dendrimers that have been used as catalysts with hydrogen peroxide in thiolperoxidase- and haloperoxidase-like reactions.

Table 5 Thiolperoxidase-like behavior of tnonotellurides 60, dendrimer wedges 61-63, and dendrimers 64 toward the oxidation of thiophenol with hydrogen peroxide in methanol...

Dendrimer wedges, 111 Di-4-aminophenyltelluroxide hydration/dehydration, 103/ Di-4-chlorophenyltellurium(IV) diiodide, 98 Di-4-hydroxyphenyltelluroxide hydration/dehydration, 103/ Di-4-methoxyphenyltelluride, 81 Di-4-methoxyphenyltellurium(IV) diiodide, 98 Di-4-methylphenyltellurium(IV) diiodide, 98 Di-n-hexyltelluride, 93 Dialkylselenides... 

Dendrimers are attractive nanosize model compounds because of their globular architecture and their highly functionalized surface. These hyperbranched compounds are synthesized in a repetitive reaction sequence of nearly quantitative reactions. The synthetic route can either be divergent, starting from the nucleus toward the surface, or convergent, where dendrons or wedges are covalently linked to a polyfunctional nucleus. The number of metallodendrimers is still limited.493-506... 

Dendrimers are taking their space in the tool box of the modern synthetic chemist. Dendritization might offer solutions to problems yet unsolved. Dendritic wedges, i. e. dendryl substituents of well-chosen size and generation allow us to tune molecular properties like solubility, steric accessability of reactive sites, redox behaviour, and other features. Easy-to-make dendrimers and dendrons will thus become extremely helpful for any chemist in the covalent as well as in the supramolecular world . 

Practically all dendrimers [ 1 ] known today have cores with a few, typically three to six functional groups to which the corresponding number of dendrons (dendritic wedges) are attached. The fact that these dendrons are connected to one another by a small, almost dot-like molecule results in considerable steric congestion in the space around the core. This congestion is a unique structural feature and has one led to view dendrimers, specifically those of high genera-... 

Attachment of dendritic wedges of either the carbosilane or benzylphenyl ether type to the para-hydroxy aryl site in [2,6-(ArN=CMe)2C5H3N (1 R = Me, Ar = 2-Me-4-OHC6H3), has been shown to proceed in good yield [162], Complexation with iron(II) chloride allows access to dendrimer-supported precatalyst 42 (Scheme 13). Using MAO as a co-catalyst, it was shown that 42 are active in the oligomerisation of ethylene the activity of these new catalysts is not, however, related to the type of dendritic wedge employed. 

Figure 16.14 To investigate hydrogen bonding in dendrimers Zimmerman and Moore [187] prepared dendritic wedges (A-B = CH20 or C=C) functionalized with anthypyridine units at the focal point that function as hosts for benzamidinium guest molecules...

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). 

The physical properties of dendrimers such as solubility, arising from their hyperbranched globular shapes and the peripheral groups, can be modified by end-group modification. In core-functionalized dendrimers, the immiscibility of the wedges with a solvent enables precipitation and subsequent separation by filtration. 

BINAP core-functionalized dendrimers were synthesized by Fan et al. (36), via condensation of Frechet s polybenzyl ether dendritic wedges to 5,5 -diamino-BINAP (26—28). The various generations of BINAP core-functionalized dendrimers were tested in the ruthenium-catalyzed asymmetric hydrogenation of 2-[p-(2-methyl-propyl)phenyl]acrylic acid in the presence of 80 bar H2 pressure and in a 1 1 (v/v) methanol/toluene mixture. As later generations of the in situ prepared cymeneruthe-nium chloride dendritic catalysts were used, higher activities were observed (TOF values were 6.5, 8.3, and 214 h respectively). Relative to those of the BINAP... 

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