Support dendritic

Chapman et al. [131] reported the synthesis of poly(ethylene oxide) (PEO) supported dendritic f-BOC-poly(a, c-L-lysines). These dendritic polymers termed as hydramphiphiles formed foams possessing good temporal stability in aqueous solution. Scrimin et al. [132] synthesized a three-directional polypeptide having uses in membrane permeability modulation. Decapeptide fragments were linked to TREN [tris(2-aminoethyl)amine] core. 

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

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

In a subsequent paper, the authors developed another type of silica-supported dendritic chiral catalyst that was anticipated to suppress the background racemic reaction caused by the surface silanol groups, and to diminish the multiple interactions between chiral groups at the periphery of the dendrimer 91). The silica-supported chiral dendrimers were synthesized in four steps (1) grafting of an epoxide linker on a silica support, (2) immobilization of the nth generation PAMAM dendrimer, (3) introduction of a long alkyl spacer, and (4) introduction of chiral auxiliaries at the periphery of the dendrimer with (IR, 2R)-( + )-l-phenyl-propene oxide. Two families of dendritic chiral catalysts with different spacer lengths were prepared (nG-104 and nG-105). 

Ar = polystyrene supported dendritic TADDOL with aromatic tether Ref. 25 ... 

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. 

Figu re 4.1 Commonly encountered chiral catalyst immobilization on dendritic polymer supports (a) core-functionalized chiral dendrimers (b) peripherally modified chiral dendrimers (c) solid-supported dendritic chiral catalysts. 

A dendritic spacer of poly(aryl benzyl ether) was used for incorporation of phosphine ligands onto a PS resin support [116,117]. Thus, the condensation of 4-(diphenylphosphino)benzoic acid with Wang resin bearing poly(aryl benzyl ether) (generation 1-3) gave the PS-supported dendritic phosphine 86. Treatment of the dendritic phosphine 86 with Pd(dba)2 in THF afforded the bisphosphine-palladium complex 87 (Scheme 29) [ 118]. The PS-supported palladium complex exhibited a positive dendritic influence on the Heck reaction of bromobenzene with methyl acrylate. 

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. 

El-Shehawy has reported the synthesis of original polystyrene-supported dendritic chiral ephedrine [142], Chain end-fimctionalized polystyrenes bearing a number of 2, 4, 8 or 16 BnBr residues at their chain-ends, PS(BnBr)2-PS(BnBr)i6 were first isolated (Scheme 86). 

The catalytic activity of these four chiral polymer-supported dendritic aminoalcohols was evaluated in the enantioselective diethylzinc addition to V-diphenylphosphinoylbenzaldimine. The reaction was first carried out in toluene at rt for 48 h using an equimolar amount of supported-ligand (based on the total numbers of ligand unit). 

Table 15. Enantioselective diethylzinc addition to A diphenylphosphinoylbenzaldimine using polymer-supported dendritic aminoalcohols.

Y.M. Chung, H.K. Rhee, Design of Sdica-Supported Dendritic Chiral Catalysts for the Improvement of Enantioselective Addition of Diethylzinc to Benzaldehyde, Catalysis Letters 82, 249, 2002. 

Scheme 5.4 The preparation of supported dendritic cataiysts, reported by Hoveyda.

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

The supported dendritic molecules, described thus far, were all based on carbonyl-containing connecting groups (amides, ureas, esters). These groups are vulnerable to a variety of reaction conditions (reductive, hydrolytic, nucleophilic) and, therefore, are unsuitable for a wide range of synthetic and catalytic applications. Moreover, the coordinating nature of the... 

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