Dendritic BINOL ligands

The catalytic properties of the dendritic BINOL ligand (S)-l (Fig. 6.33) already mentioned in Section 4.2.3.2 (Fig. 4.70) in the catalytic stereoselective reaction of benzaldehyde with diethylzinc were compared with those of unsubstituted... 

Liu et al. 43) prepared chiral BINOL ligands bearing dendritic Frechet-type polybenzyl ether wedges ((J )-41-(J )-44), which were assessed in enantioselective Lewis acid-catalyzed addition of Et2Zn to benzaldehyde. 

Fig. 16 BINOL ligands bearing dendritic poly(aryl ether) wedges...

BINOL ligands 41a-c bearing dendritic wedges at the 6,6 positions were used in titanium-catalysed reaction of tributylallyl stannane and benzaldehyde (Scheme 7.33). Whereas the yield was low, the enantioselectivity was similar to the reaction with BINOL (87% enantiomeric excess), whatever the size of the dendritic moieties. However, no attempt to recycle the catalyst or the ligand was described by the authors. 

Recently, Reek et al. published the synthesis of a 9H,9 H- [4,4 ]bicarbazole-3,3r-diol (BICOL)-based chiral monodentate phosphoramidite ligand, which was functionalized with two different third-generation carbosilane dendritic wedges (Fig. 26) [57]. As reference reaction in the catalytic study, the rhodium-catalyzed asymmetric hydrogenation of Z-methyl-a-acetamido-cinnamate was chosen. Using a ligand-to-rhodium ratio of 2.2 led to enantio-selectivities which were comparable to the results obtained using the parent BINOL-derived monodentate phosphoramidite MonoPhos. 

Nonracemic Ti-BINOLate (BINOL = l,l -bi-2-naplilli()l) and Ti-TADDOLate (TADDOL = a,a,a, a -tetraaryl-2,2-dimethyl-l,3-dioxolan-4,5-dimethanol) complexes are also effechve chiral catalysts for the asymmetric alkylation of aldehydes [9-11]. Seebach developed polystyrene beads with dendritically embedded BINOL [9] or TADDOL derivatives 11 [10, 11]. As the chiral ligand is located in the core of the dendritic polymer, less steric congeshon around the catalyhc center was achieved after the treatment with Ti(OiPr)4. This polymer-supported TiTADDOLate 14 was then used for the ZnEt2 addition to benzaldehyde. Chiral 1-phenylpropanol was obtained in quantitahve yield with 96% ee (Scheme 3.3), while the polymeric catalyst could be recycled many times. 

Recently, Fan et al. reported a series of chiral BINOL-cored dendrimers via substitution at the 3,3 -positions of the binaphthyl backbone by different generations of Frechet-type dendrons (Figure 4.18) [73]. The proximity of the dendritic wedges to the catalytic center is expected to induce catalytic properties different from BINOL. In the absence of Ti(Oi Pr)4, the chiral dendrimer ligands showed much... 

Styryl-terminated Frechet-type dendrimers have been introduced as novel polymer crosslinkers by Seebach et al. [43-45]. They are constituted of four to 16 peripheral styryl units attached to aryl end branches of dendritic TADDOL, BINOL or Salen ligands and were copolymerised with styrene by suspension polymerisation. The catalytic performance of the polymer-bound catalyst was identical to that of the homogeneous analogues however, the supported catalysts could be used in many consecutive catalytic runs with only small loss in catalytic activity. A major drawback of fixing the catalytic unit in the core of the crosslinker is the poor loading capacity of the final polymer (0.13-0.20 mmol g 0> especially when high amounts of catalysts (10-20 mol%) are needed. 

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