Dendrimer fixation

Dendrimer fixation may be achieved by attachment of catalysts at the periphery of dendrimers (Fig. la) in the way first established by van Koten, van Leeuwen and co-workers in their pioneering work on the Karasch reaction [3,4]. The second possibility is the attachment of one or more dendritic wedges to the catalysts, which are then located at the core of the result-... 

The majority of studies into the catalytic behaviour of dendrimers with chiral catalytic centres at the periphery of the dendritic support have concerned non-phosphine-based catalysts. As has become apparent in these studies, the effect of the dendrimer fixation on the catalytic performance generally depends on the individual system. Factors such as the high local density of catalytic sites, the interaction of functional groups in the dendrimer backbone with the catalysts and the structural rigidity or flexibility of the dendrimers seem to play a role in many cases. 

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. 8.1 Possible modes of fixation of cataly-tically active units (ML) to a dendrimer.

A exodendral fixation to the periphery of the dendrimer B and C endodendral fixation to the core of the dendrimer/dendron. The... 

Van Koterfs group used a chemically inert, lipophilic carbosilane dendrimer scaffold as support material for fixation of up to 12 transition metal complex fragments. The covalently fixed fragments with nickel as catalytic site acceler-... 

Fig-1 Fixation of catalytic metal centres (represented by the black spheres) in exodendral (a) and endodendral (b) positions of dendrimers or on dendronized polymers (c)... 

Fig. 2 Fixation of chiral Josiphos ligands to a first-generation dendrimer containing a cyclotriphosphazene core...

Brunner s concept of attaching dendritic wedges to a catalytically active metal complex represented the first example of asymmetric catalysis with metal complex fragments located at the core of a dendritic structure [5,6]. Important early examples of catalysts in core positions were Seebach s TAD-DOL systems (TADDOL = 2,2-dimethyl-a,a,a/,a/-tetraphenyl-l,3-dioxolane-4,5-dimethanol) [38,39]. In general, the catalytic performance of such systems was either unchanged with respect to the simple mononuclear reference system or significantly lower. In no case has the potential analogy of this core fixation and the existence of efficient reactive pockets in enzymes been vindicated. This may be due to the absence of defined secondary structures in the dendrimers that have been employed to date. 

Topics which have formed the subjects of reviews this year include excited state chemistry within zeolites, photoredox reactions in organic synthesis, selectivity control in one-electron reduction, the photochemistry of fullerenes, photochemical P-450 oxygenation of cyclohexene with water sensitized by dihydroxy-coordinated (tetraphenylporphyrinato)antimony(V) hexafluorophosphate, bio-mimetic radical polycyclisations of isoprenoid polyalkenes initiated by photo-induced electron transfer, photoinduced electron transfer involving C o/CjoJ comparisons between the photoinduced electron transfer reactions of 50 and aromatic carbonyl compounds, recent advances in the chemistry of pyrrolidino-fullerenes, ° photoinduced electron transfer in donor-linked fullerenes," supra-molecular model systems,and within dendrimer architecture,photoinduced electron transfer reactions of homoquinones, amines, and azo compounds, photoinduced reactions of five-membered monoheterocyclic compounds of the indigo group, photochemical and polymerisation reactions in solid Qo, photo- and redox-active [2]rotaxanes and [2]catenanes, ° reactions of sulfides and sulfenic acid derivatives with 02( Ag), photoprocesses of sulfoxides and related compounds, semiconductor photocatalysts,chemical fixation and photoreduction of carbon dioxide by metal phthalocyanines, and multiporphyrins as photosynthetic models. 

Fixation of Chiral Catalysts on Dendrimers and Hyperbranched Polymers... 

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