Catalytic dendronized polymers

Abstract Enantioselection in a stoichiometric or catalytic reaction is governed by small increments of free enthalpy of activation, and such transformations are thus in principle suited to assessing dendrimer effects which result from the immobilization of molecular catalysts. Chiral dendrimer catalysts, which possess a high level of structural regularity, molecular monodispersity and well-defined catalytic sites, have been generated either by attachment of achiral complexes to chiral dendrimer structures or by immobilization of chiral catalysts to non-chiral dendrimers. As monodispersed macromolecular supports they provide ideal model systems for less regularly structured but commercially more viable supports such as hyperbranched polymers, and have been successfully employed in continuous-flow membrane reactors. The combination of an efficient control over the environment of the active sites of multi-functional catalysts and their immobilization on an insoluble macromolecular support has resulted in the synthesis of catalytic dendronized polymers. In these, the catalysts are attached in a well-defined way to the dendritic sections, thus ensuring a well-defined microenvironment which is similar to that of the soluble molecular species or at least closely related to the dendrimer catalysts themselves. 

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)... 

Despite the advantage of their easy separation, the use of conventional insoluble polymer-supported catalysts often suffered from a reduced catalytic activity and stereoselectivity, due either to diffusion problems or to a change of the preferred conformations within the chiral pocket created by the ligand around the metal center. In order to circumvent these problems, a new class of crosslinked macromolecule-namely dendronized polymers-has been developed and employed as catalyst supports. In general, two types of such solid-supported dendrimer have been reported (i) with the dendrimer as a hnker of the polymer support and (ii) with dendrons attached to the polymer support [12, 113]. 

Today, dendronized supports are well-established tools in the arsenal of synthetic, catalytic, and polymer chemists and their uses are extending to the areas of bio- and materials sciences. One may hope that new and fascinating synthetic schemes and applications will follow. 

Hu et al. [31] reported a new type of macro molecular chiral catalysts for asymmetric catalysis using Suzuki coupling polymerization and obtained optically active ephedrine-bearing dendronized polymers. Their finding showed that the optically active dendronized polymers have characteristics joined features like huge numbers of catalytic sites, more solubility and nanoscopic dimensions towards more acceptable in comparison to its existing chiral catalysts of linear polymeric and dendritic nature. 

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... 

Dendrimers have been used as large compartmented hosts for drug delivery, as carriers for catalytically active sites in flow reactors, and for charge or energy transfer purposes. New types of such dendrimers were prepared starting from functionalized poly-p-phcnylcncs as polymeric cores whose functional groups were further used to anchor dendrons [165]. The polymers were synthesized from dibromide 117 and diboronic acid 118 via a Suzuki crosscoupling the dendrons were attached via a Williamson ether synthesis [165]. 

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