Dendritic chirality

Other efforts based on the macromonomer approach to homopolymers having dendritic side chains, include the work of Draheim and Ritter on acrylate and methacrylate derived structures having dendritic chiral side chains based on L-aspartic esters [17a], and of Xi and coworkers with poly(methacrylate) structures containing very small benzyl ether dendritic side-chains [17b]. Unfortunately, both of these approaches met with limited success due to a significant drop in degree of polymerization (DP) when the size of the dendron used as pendant group in the macromonomers increased from G-l to G-2. 

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). 

Recently, dendrimers, which are hyperbranched macromolecules, were found to be an appropriate support for polymer catalysts, because chiral sites can be designed at the peripheral region of the dendrimers (Scheme 5). Seebach synthesized chiral dendrimer 14, which has TADDOLs on its periphery and used an efficient chiral ligand in the Ti(IV)-promoted enantioselective alkylation [21]. We developed chiral hyperbranched hydrocarbon chain 15 which has six p-ami-no alcohols [22], It catalyzes the enantioselective addition of diethylzinc to aldehydes. We also reported dendritic chiral catalysts with flexible carbosilane backbones [23]. 

Following up this work, the same group recently published the synthesis of hybrid dendritic ligands containing a combination of dendritic chiral DPEN and Frechet polyether dendrons (Scheme 7) [50]. The solubility of these hybrid dendrimers was found to be controlled by the polyether den-dron. Compared with the simple core-functionalized systems displayed in Fig. 21, the hybrid dendrimers showed similar catalytic activity but reduced recyclability. 

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. 

Sato and coworkers reported on addition reactions of diethylzinc to aldehydes. They used dendrimers with rigid backbones and therefore isolated catalytic centres [131] as well as dendrimers with flexible backbones, where the catalytic centres can interact [ 132 ]. They concluded that flexibihty of the skeleton of the dendritic chiral catalyst enables high enantioselectivity. 

Bohn and co-workers also studied diethylzinc addition to benzaldehyde with soluble polymeric catalysts [10]. Dendritic chiral catalysts consisting of poly(benzyl ethers) and chiral pyridyl alcohols (3) were used as organocatalysts for the asymmetric C-C linkage reaction. The enantiocontrol by the dendritic systems was slightly lower than that of the parent pyridyl alcohols (2-3% drop in ee) but the conversion toward the chiral secondary alcohol was actually slightly higher for the largest dendritic catalyst (84% versus 80% yield after 3 h of reaction time). In more... 

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). 

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. 

H. Wang, J.-L. Wang, S.-C. Yuan, P. Jian and W.-W. Pei, Novel highly fluorescent dendritic chiral amines synthesis and optical properties. Tetrahedron, 61, 8465-8474 (2005). 

Figure 4.22 Modular chiral dendritic chiral monodentate phosphoramidite ligands for the Rh-catalyzed asymmetric hydrogenation of a-dehydroamino acid esters.

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