Chiral support

Several chromatographic modes will be reviewed in this respect, and most will make use of a chiral support in order to bring about a separation, differing only in the technology employed. Only countercurrent chromatography is based on a liquid-liquid separation. 

The analytical capability of these matrices has been demonstrated for chiral amines [12, 13]. The procedure is illustrated in Fig. 8-4 for the separation of NapEtNH " CIO . Concentrated methanol/dichloromethane solutions of the racemic mixture were placed on a column containing the chiral macrocycle host. The enantiomers of the ammonium salts were resolved chromatographically with mixtures of methanol and dichloromethane as the mobile phase. The amounts of R and S salts in each fraction were determined by polarimetry. Because the chiral supported macrocycle interacts more strongly with S salts, the R salt passes through the column first and the S salt last, as seen in Fig. 8-4. 

Partially successful attempts towards chiral electrochemical synthesis have involved chiral supporting electrolytes chiral solvents and chiral adsorbates, mostly alkaloids With the latter method enantiometric excess values >40% have... 

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). 

The synthesis of technologically interesting ROMP materials using (233) includes the preparation of molecular wires,701 liquid-crystal polymers,702,703 chiral supports for catalysis,704 redox-active macromolecules,705 photochromic materials706 and embedded clusters of CdSe.707 Polymers... 

Asymmetric hydrogenations catalyzed by supported transition metal complexes have included use of both chiral support materials (poly-imines, polysaccharides, and polyalcohols), and bonded chiral phosphines, although there have been only a few reports in this area. 

Before 1968, attempts to perform enantioselective hydrogenations had either used a chiral auxiliary attached to the substrate [1] or a heterogeneous catalyst that was on a chiral support, usually derived from Nature [2]. Since the disclosure of chiral phosphine ligands to bring about enantioselective induction in a hydrogenation, many systems have been developed, as evidenced in this book. The evolution of these transition-metal catalysts has been discussed in a number of reviews [3-12]. 

Since the electroreduction of ketones shown in Scheme 29 has been well established [1-3, 12, 62-65], one more recent interest in the electroreduction of carbonyl compounds is focused on the stereo-selective reduction of ketones. For example, the diastereo-selective cathodic coupling of aromatic ketones has been reported. In the presence of a chiral-supporting electrolyte, a low degree of enantioselectivity has been found [66] (Scheme 30). 

Scheme 30 Diastereoselective cathodic coupling of aromatic ketones in the presence of chiral supporting electrolyte, yield 91%, 25% ee.

Gigante, B. Corma, A. Garcia, H. Sabater, M. J. (2000) Assessment of the negative factors responsible for the decrease in the enantioselectivity for the ring opening of epoxides catalyzed by chiral supported Cr(III)-salen complexes Catalysis Lett. 68 113-119. 

The use of a chiral polymer instead of the achiral polymers in XXXIX and XXXX allows an asymmetric synthesis. An example is the stereoselective reduction of acetophenone to (I )-l-phenylethanol in 76-97% enantiomeric excess by using the indicated chiral support (Eq. 9-69) [Itsuno et al., 1985] ... 

Resolution using chiral supports in HPLC turned out to be successful for the separation of chiral catenates [135, 136], Unfortunately, this technique seemed to be inappropriate to the resolution of the knot. 

The first successful experiments were reported by Schwab [16] Cu, Ni and Pt on quartz HI were used to dehydrogenate racemic 2-butanol 23. At low conversions, a measurable optical rotation of the reaction solution indicated that one enantiomer of 23 had reacted preferentially (eeright-handed quartz gave the opposite optical rotation it was deduced that the chiral arrangement of the crystal was indeed responsible for this kinetic resolution (for a review see [8]). Later, natural fibres like silk fibroin H5 (Akabori [21]), polysaccharides H8 (Balandin [23]) and cellulose H12 (Harada [29]) were employed as chiral carriers or as protective polymer for several metals. With the exception of Pd/silk fibroin HS, where ee s up to 66% were reported, the optical yields observed for catalysts from natural or synthetic (H8, Hll. H13) chiral supports were very low and it was later found that the results observed with HS were not reproducible [4],... 

The asymmetric photooxygenation of alkenes in chiral-supported zeolites was also reported [144,145] ... 

Strategies to induce chirality in a prochiral substrate included modification of existing heterogeneous catalysts by addition of a naturally occurring chiral molecules, such as tartaric acid, natural amino acids, or alkaloids, and the implementation of chiral supports, which include quartz or natural fibers, for metallic catalysts. Both strategies have been successful on a limited basis. 

An overcrowded PAH, 9,10,ll,20,21,22-hexaphenyltetrabenzo[a,c,I,n]pentacene (55), showed an interesting screw-type helicity (Fig. 15.21) [97]. An end-to-end twist of 144° was estimated from the X-ray structure of 55. Pentacene 55 was prepared by the reaction of l,3-diphenylphenanthro[9,10-c]furan 54 with the bisaryne equivalent generated from l,2,4,5-tetrabromo-3,6-diphenylbenzene in the presence of n-butyllithium, followed by deoxygenation of the double adduct with low-valent titanium. Pentacene 55 could be resolved by chromatography on a chiral support, but it racemized slowly at room temperature (t1/2 9 h at 25 °C). 

Another interesting feature of polymer-supported catalysts containing quaternary ammonium salts involves the development of enantioselective catalysis using salts derived from cinchonia or ephedra alkaloids.11341 The first application of such chiral supported catalysts in the Michael reaction between methyl 1-oxoindan-2-carboxylate and methyl vinyl ketone revealed a high chemical yield in condensation product (60-100 %) although the enantioselectivities were only moderate (ee <27 %). 

The generally low-percent asymmetric synthesis in asymmetric heterogeneous hydrogenations may be due, in part, to a nonuniform distribution of chiral modifying agents over the catalytic surfaces. In the case of silk fibroin, metal clumping on the chiral support or dissociation of the metal from the fibroin may allow some reduction to occur in an achiral local environment. 

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