Supported dendritic catalysts

One of the most important applications of the dendronized supports is the preparation of support-bound dendritic catalysts. Such systems, based on outward branching dendrons, were prepared and explored by the groups of Alper and Arya, Reymond, Portnoy, Kawi, Rhee, and Sreekumar. A number of heterogeneous dendritic catalysts based on a different design (e.g., polymerized dendrimers bearing catalytic units) are also known, but are beyond the scope of this book. °  

Subsequently, palladium-based complexes were used for car-bonylation of aryl halides,hydroesterification and hydroamidation of olefins, hydroamidation of alkynes, Heck reaction, and oxidation and hydrogenation of olefins. i°  

In the case of polystyrene-supported catalysts, an additional study explored the influence of the isolation of the catalyst environment on the outcome of the hydroformylation reaction. For this purpose, first- and second-generation dendrons (very similar to those described above) were constructed and the biphosphine-Rh complex was assembled only on 

DENDRITIC MOLECULES ON SOLID SUPPORT SOLID-PHASE SYNTHESIS AND APPLICATIONS 

PAMAM itself was used as a multivalent macromolecular ligand, probably due to its multiple amino groups, in order to complex and immobilize metal ions, complexes, and nanoparticles with catalytic capabilities. Thus, Kawi and coworkers used PAMAM-on-silica and PAMAM-on-alumina templates to immobilize Rh(l) complexes as hydroformy-lation catalysts. Passivation of the silica OH sites outside the pores of SBA-15 silica resulted in a tighter binding of rhodium complexes inside the pores and led to a series of catalysts that displayed a positive dendritic effect up to the second PAMAM generation. Sreekumar and Krishnan used PAMAM on polystyrene to complex Mn(ll) precursors and catalyze the oxidation of secondary alcohols.  

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E.2. Supported Dendritic Catalysts for Carbonylation, Hydroesterification, Oxidation, and Heck Reactions... 

E.3. Supported Dendritic Catalysts for the Asymmetric Addition of Diethylzinc... 

The periphery of convergently synthesized den-drimers has also been modified to allow the assembly of monolayers,494 to support dendritic catalysts,495 to control the intermolecular assembly of porphyrin dendrimers,246 to probe the effect of photo isomerization,319 and to enable cross-linking of the periphery followed by removal of the core.496 These studies in peripheral modification highlight the versatility of the convergent synthesis. In particular, the ability to selectively modify the periphery and focal functionalities of a dendron enables the design of complex macromolecules that involve the interaction between multiple functional components. 

There is much current interest in dendritic molecules, i.e. those with branched arms that diverge from a central core. The supported dendritic catalyst 27.39 can be used in hydroformylation reactions, and shows high selectivity for branched over linear aldehyde products. 

Scheme 5.5 The preparation of supported dendritic catalysts, reported by Blechert.

So far, few data are available which allow the comparison of differences in efficacy and selectivity of one catalytic system attached to different supports. As far as the TADDOLate complexes are concerned, no clear rules can be drawn. Polystyrene-based catalysts derived from (8) and (10) show similar enantioselectivities and reaction rates. Differences appear, however, when comparing them with a polystyrene-embedded dendritic ligand system, generated by co-polymerization from TADDOL-derivative (32) (Scheme 4.18) which is described in Section 4.3.2.1. Re-cydabihty seems to be easier for the dendritic catalyst and the enantioselectivity. 

Ultrafiltration has been used for the separation of dendritic polymeric supports in multi-step syntheses as well as for the separation of dendritic polymer-sup-ported reagents [4, 21]. However, this technique has most frequently been employed for the separation of polymer-supported catalysts (see Section 7.5) [18]. In the latter case, continuous flow UF-systems, so-called membrane reactors, were used for homogeneous catalysis, with catalysts complexed to dendritic ligands [23-27]. A critical issue for dendritic catalysts is the retention of the catalyst by the membrane (Fig. 7.2b, see also Section 7.5). 

Dendritic Polymers as High-Loading Supports for Catalysts I 331... 

Dendritic catalysts can be recycled by using techniques similar to those applied with their monomeric analogues, such as precipitation, two-phase catalysis, and immobilization on insoluble supports. Furthermore, the large size and the globular structure of the dendrimer can be utilized to facilitate catalyst-product separation by means of nanofiltration. Nanofiltration can be performed batch wise or in a continuous-flow membrane reactor (CFMR). The latter offers significant advantages the conditions such as reactant concentrations and reactant residence time can be controlled accurately. These advantages are especially important in reactions in which the product can react further with the catalytically active center to form side products. 

Kragl 13) pioneered the use of membranes to recycle dendritic catalysts. Initially, he used soluble polymeric catalysts in a CFMR for the enantioselective addition of Et2Zn to benzaldehyde. The ligand a,a-diphenyl-(L)-prolinol was coupled to a copolymer prepared from 2-hydroxyethyl methyl acrylate and octadecyl methyl acrylate (molecular weight 96,000 Da). The polymer was retained with a retention factor > 0.998 when a polyaramide ultrafiltration membrane (Hoechst Nadir UF PA20) was used. The enantioselectivity obtained with the polymer-supported catalyst was lower than that obtained with the monomeric ligand (80% ee vs 97% ee), but the activity of the catalyst was similar to that of the monomeric catalyst. This result is in contrast to observations with catalysts in which the ligand was coupled to an insoluble support, which led to a 20% reduction of the catalytic activity. 

In a batch process, all dendritic catalysts showed very high activity. When a substrate-to-Pd molar ratio of 2000 was used, the conversions after 5 min were 49, 55, 45, and 47% when dendrimers with 4, 36, 8, and 24 phosphine ligands were used, respectively. These results show that all the active sites located at the periphery of the dendrimer support acted independently as catalysts. 

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). 

In the Mukaiyama aldol additions of trimethyl-(l-phenyl-propenyloxy)-silane to give benzaldehyde and cinnamaldehyde catalyzed by 7 mol% supported scandium catalyst, a 1 1 mixture of diastereomers was obtained. Again, the dendritic catalyst could be recycled easily without any loss in performance. The scandium cross-linked dendritic material appeared to be an efficient catalyst for the Diels-Alder reaction between methyl vinyl ketone and cyclopentadiene. The Diels-Alder adduct was formed in dichloromethane at 0°C in 79% yield with an endo/exo ratio of 85 15. The material was also used as a Friedel-Crafts acylation catalyst (contain-ing7mol% scandium) for the formation of / -methoxyacetophenone (in a 73% yield) from anisole, acetic acid anhydride, and lithium perchlorate at 50°C in nitromethane. 

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