Polymer-supported BINOL

Polymer-supported BINOLs thus prepared were treated with Zr(Ot-Bu)4 to form polymer-supported zirconium 20. In the presence of 20 mol% of various zirconium 20, the model aza Diels-Alder reactions of imine Id with Danishefsky s diene (7a) were performed results from selected examples are shown in Table 5.8. Whereas the 4-t-butylphenyl group resulted in lower enantiomeric excess (ee), higher ee were obtained when 3,5-xylyl, 4-biphenyl, 4-fluorophenyl, and 3-tri-... 

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. 

Sasai developed miceUe-derived polymer-supported catalysts for a variety of enantioselective reactions, including diethylzinc addition [18]. The surfactant monomer 20 having tetraethylene glycol chains formed micelles in water and, followed by copolymerization with styrene, gave the spherical polymer. A coupling reaction of the polymer with a BINOL derivative and deprotection of the methoxymethyl groups of the BINOL moiety afforded the desired chiral polymer 21, as shown in Scheme 3.5. The catalytic asymmetric ethylation of benzaldehyde was... 

The polymer-supported Zr catalyst (12) is useful for asymmetric aza-Diels-Alder cycloaddition of benzaldehyde imine to Danishefsky diene [9]. The 6-substituted BINOL-Zr(IV) catalyst is useful for the enantioselective anft -preferred aldol reaction of benzaldehyde with ketene silyl thioacetal (15) (Scheme 5.5) [ 10]. The calculated charge densities on the oxygen atoms of the BINOL derivatives revealed that there is a good correlation between the charge density and the reactivity of 6-substituted BINOL [ 10]. 

Hetero-Diels-Alder reaction. The zirconium complexes of 3,3 -diaryl-BINOL are effective chiral catalysts for the smooth condensation of aldimines with a Danishefsky diene. Linkage of the BINOL moiety to polymer support also provides viable catalysts. 

In place of Ti(O Pr)2-BINOLate, (147) Al-BINOLates (148) are easily prepared by changing the metal source (Scheme 19.29). The polymer-supported chiral Al Lewis acid (148) was used for the enantioselective 1,3-dipolar cycloaddition of diphenyl nitron to vinyl ethers [65]. High levels of exoselectivity (>95 5) and enantioselectivity (96% ee) were obtained. 

Since 2000 a few catalysts for asymmetric epoxidation based on polymer-anchored chiral l,l -bi-2-naphthol (BINOL) have been developed. Polystyrene-supported BINOL was prepared by radical copolymerization of styrene with BINOL, bearing 4-vinylbenzyloxy groups in the 3- or 6-position [96]. Immobilization of lanthanum or ytterbium was accomplished by treatment of the polymers... 

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. 

Self-supported titanium complexes with linked bis-BINOL ligands were used as an alternative approach for the immobilisation of catalysts, as shown in enantioselective sulfide oxidation (see Section 7.2.2). The same ligands were used with success in asymmetric carbonyl ene reactions. The chiral metal-bridged polymer 76, derived from ent-lOa, titanium tetraisopropoxide and water (Scheme 7.45), catalysed the ene reaction between 68b and 71, to give R)-72 in 88% yield and 88% enantiomeric excess. The catalyst can be reused at least five times without affecting its efficiency. 

---