Polystyrene-supported chiral

The soluble polymer-supported catalysts have also been used for asymmetrically catalyzed reactions Following a procedure for the preparation of insoluble polymeric chiral catalysts a soluble linear polystyrene-supported chiral rhodium catalyst has been prepared. This catalyst displays high enantiomeric selectivity compared to the low molecular weight catalyst. Thus, hydroformylation of styrene using this catalyst produces aldehydes in high yields. The branched chiral hy drotropaldehy de is formed in 95% selectivity. 

Week [215] has also described polystyrene-supported chiral salen ligands 349 and 350 synthesized respectively by radical homopolymerization or copolymerization of an... 

Structure of the polymer support. Again using polystyrene-supported chiral aminoalcohols in this reaction they have deliberately used reaction conditions where the enantiomeric excess is not maximised, and then varied the detailed morphology of the support to probe the influence of the latter. Despite examining many complex polymeric structures including species with very high pore volumes high surface area matri-... 

Pericas et al. prepared polystyrene-supported chiral phosphinooxazoline (PHOX) ligands having a 1,2,3-triazole tether, which was constructed by a 1,3 dipolar cycloaddition. The catalyst (50) gave allylamine from racemic allyl acetate in high yield with excellent enantioselectivity under microwave-assisted continuous-flow conditions (Scheme 7.37) [141]. Although the enantioselectivity was not changed, the catalytic activity of the polymer catalyst was decreased after 3 h. 

Benzyl ethers are amongst the easiest to cleave by Lewis acids (20). Significant clipping of such bonds, with consequent loss of functionality, resulted during attempted HCl-catalyzed hydrolyses of polystyrene-supported oxazoline intermediates (21, 22) and chiral supports (23, 24). 

When considering the easy recovery and reuse of chiral catalysts, or simple separation process of the product from chiral catalyst, polymer-supported catalysts are very attractive [1,3]. For the enantioselective ethylation using dialkylz-inc, Frechet and Itsuno s group and our group developed polystyrene-supported amino alcohols [1]. 

Asymmetric hydrosilylation of styrene with HSiCl3 catalyzed by a palladium complex of a chiral ferrocenylphosphine attached to cross-linked polystyrene support at 70 °C gives PhMeC HSiCl3 in quantitative yield with only 15.2% ee65. 

With a polymer-bound chiral selenium bromide reagent, the stereoselective activation of olefins has been accomplished (entry 19) 24 Uehlin and Wirth24 found that both mesoporous silica and polystyrene-supported... 

The first report of a polymer-supported oxazaborolidine appeared in 1985 [37]. The polymer-supported chiral ligand amino alcohol (27) was prepared by reaction of chlor-omethylated polystyrene resin and enantiopure amino alcohol 26 with a phenolic hydroxyl group (Eq. 10). Borane reduction of ketones by use of polymer-supported oxazaborolidines proceeded very smoothly to give the corresponding chiral alcohol in quantitative yield. For example, the reduction of butyl phenyl ketone afforded 1-phe-nylpentan-l-ol in 97 % ee (27, Eq. 11). This is somewhat higher than that obtained by... 

The above mentioned polymer-supported oxazaborolidines are prepared from polymeric amino alcohols and borane. Another preparation of polymer-supported oxazaborolidines is based on the reaction of polymeric boronic acid with chiral amino alcohol. This type of polymer can be prepared only by chemical modification. Lithiation of the polymeric bromide then successive treatment with trimethyl borate and hydrochloric acid furnished polymer beads containing arylboronic acid residues 31. Treatment of this polymer with (li ,2S)-(-)-norephedrine and removal of the water produced gave the polymer-supported oxazaborolidine 32 (Eq. 14) [41 3]. If a,a-diphenyl-2-pyrrolidinemetha-nol was used instead of norephedrine the oxazaborolidine polymer 33 was obtained. The 2-vinylthiophene-styrene-divinylbenzene copolymer, 34, has been used as an alternative to the polystyrene support, because the thiophene moiety is easily lithiated with n-butyl-lithium and can be further functionalized. The oxazaborolidinone polymer 37 was then obtained as shown in Sch. 2. Enantioselectivities obtained by use of these polymeric oxazaborolidines were similar to those obtained by use of the low-molecular-weight counterpart in solution. For instance, acetophenone was reduced enantioselectively to 1-phe-nylethanol with 98 % ee in the presence of 0.6 equiv. polymer 33. Partial elimination of... 

Not only polystyrene supports, also other polymer supports were used in the preparation of polymeric amino alcohol ligands for dialkylzinc alkylation. For example, a vinylferrocene derivative with A,N -disubstituted norephedrine was copolymerized with vinylferrocene [60]. This polymeric chiral ligand (53) was used in the ethylation of aldehydes with moderate activity. Brown has reported that chiral oxazaborolidines have catalytic activity in the addition of diethyl zinc to aldehydes [61]. Polymers bearing chiral oxazaborolidines 37 were also active in the reaction and result on moderate enantioselectivity (<58 % ee) [62]. Enantiopure a,a -diphenyl-L-prolinol coupled to a copolymer prepared from 2-hydroxyethylmethacrylate and octadecyl methacrylate... 

The first successful examples of enantioselective Diels-Alder reactions catalyzed by chirally modified Lewis acids were reported by Koga [85]. The catalysts were prepared from menthol and AlEt2Cl [86]. Alumina-supported chiral menthoxy aluminum derivatives (64, 65, 66, 67) have been prepared by simple mixing of (-)-menthol, AlEt2Cl, and alumina in toluene under reflux. The reaction of methacrolein with cyclopentadiene (Eq. 20) was conducted with 67 as catalyst at -50 °C and afforded 81 % conversion with 31 % ee [87] Koga reported 57 % ee at -78 °C by use of an homogeneous catalyst [85]. Solid catalyst 69, prepared from silica gel-supported proli-nol 68 and AlEt2Cl (Eq. 21) is also an active catalyst in the same reaction, but with low enantioselectivity [87]. When the same catalyst was attached to crosslinked polystyrene (70) the ee in the reaction was lower [88]. 

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... 

The asymmetric alkylation of glycine derivatives is one of the most simple methods by which to obtain optically active a-amino acids [31]. The enantioselective alkylation of glycine Schiff base 52 under phase-transfer catalysis (PTC) conditions and catalyzed by a quaternary cinchona alkaloid, as pioneered by O Donnell [32], allowed impressive degrees of enantioselection to be achieved using only a very simple procedure. Some examples of polymer-supported cinchona alkaloids are shown in Scheme 3.14. Polymer-supported chiral quaternary ammonium salts 48 have been easily prepared from crosslinked chloromethylated polystyrene (Merrifield resin) with an excess of cinchona alkaloid in refluxing toluene [33]. The use of these polymer-supported quaternary ammonium salts allowed high enantioselectivities (up to 90% ee) to be obtained. 

The first solid-phase synthesis of chiral pyridine-2,6-bis(oxazoline) (Pybox) ligands has been cleanly and efficiently performed in quite satisfactory overall yields and purity on polystyrene support, via a five-step synthetic sequence <05JOC4556>. 

The polymer-supported chiral a-amino alcohol is obtained easily by two methods. One method involves attaching the enantiopure a-amino alcohol to the partially chloromethylated crosslinked polystyrene through a benzyl ether linkage [71,105] (Scheme 13). 

Polymer-supported chiral amine reagents represent an attractive extension of this methodology, since these catalysts can readily be recovered (Fig. 5). Succi-nated polystyrene-divinylbenzene (17) attached to 1 promoted the addition of 6 to 7 at a slower rate than the homogeneous reaction [17]. Although the absolute configuration of the adduct (S)-8 was identical, the enantiomeric excess was low-... 

Kawana et al. used xylofuranose derivates as chiral auxiliaries [7, 13], Through its primary hydroxy function, the auxiliary was loaded to the polystyrene resin. Esterification of the immobilized auxiliary 8 gave a-keto esters. Subsequent nucleophilic additions of Grignard reagents afforded resin-bound a-hydroxy esters. Subsequent saponification afforded the chiral a-hydroxy acids 9 (Scheme 12.5) in 18-84% yields and 36-65% enantiomeric excesses. The recovered polymer supported chiral auxiliaries could be reused without decrease of enantioselectivity. 

L-proline was reduced to prolinol, and conversion with benzyl chloride into N-benzyl substituted prolinol 17 followed. Subsequent complex formation was achieved by the usage of Cr(CO)6. Irradiabon of complex 18 in the presence of polystyrene-diphenylphosphine gave the polymer-supported chiral auxiliary 19 (Scheme 12.10). 

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