Chirality/Chiral matrices

A fourth method is a chromatographic resolution of a racemic mixture of organotin compounds for instance on a chiral matrix such as microcrystalline cellulose triacetate. 

Racemic amino acids have been resolved via stereoselective coordination to the square planar chiral matrix complex (178).584 The bisamidobispyridyl ligand (179) forms a square planar Ni11 complex with considerable tetrahedral twist due to repulsion of the ortho protons of the pyridyl rings.585... 

Enantioselective catalytic surfaces may be conceived of being synthesized three basic ways (Fig. 3.3) (1) attach a catalyst to, or embed a catalyst in, a chiral matrix (2) create a chiral arrangement of active sites on a catalytic surface and (3) adsorb or attach chiral molecules onto a catalytic surface. We discuss each in turn. 

Attach a Catalyst to, or Embed a Catalyst in, a Chiral Matrix... 

One way to overcome the problem of chirality existing only at the metal-matrix interface is to encase the metal particle inside the chiral matrix. In that case, all of the metal surface atoms should be close to a chiral center however, this approach has some problems too. For example, access to the metal surface may be inhibited by the encasing matrix. In spite of this, several attempts have produced moderately successful catalysts by creating metal—polymer catalysts. Pd has been deposited on poly-(5)-leucine (Scheme 3.4) and Pd and Pt colloids have been encased in a polysaccharide to produce catalysts that enanti-oselectively hydrogenated prochiral C=C and C=N bonds (Scheme 3.5).7... 

Wie bereits bei der Strecker-Reaktion beobachtet (s.S. 558), fiihrt die Verwendung von 2,3,4,6-Tetra-0-(2,2-dimethyl-propanoyl)-/J-D-galactopyranosylamin12 als chiraler Matrix auch bei der 4-Komponenten-Kondensation in Gegenwart aquimolarer Mengen einer... 

The synthetic procedures available to the carbohydrate chemist have been largely dominated by standard reactions proceeding by heterolytic processes within a chiral matrix. The preparative utility of radical-mediated reactions has, however, been amply demonstrated in recent years. The chapter contributed here by L. Somsak (Debrecen) and R. J. Ferrier (Wellington), on bromination reactions of carbohydrates proceeding by radical processes integrates the literature related to Ferrier s pioneering work in this area and underscores its excellent potential in synthesis. 

Racemate separation by stereoselective ligand exchange occurs when a chiral matrix complex has additional coordination sites which are capable of readily exchanging a racemic substrate ligand. The chiral induction, i. e., the efficiency of die matrix complex, is related to the product distribution, which depends on the stability of the complexes with the two enantiomers of the racemic substrate (Fig. 7.1). The problem to be solved in the design of effective chiral matrix complexes for specific racemic substrates is therefore related to conformational analyses of the type discussed above. 

An important condition for chiral matrices is that they need to form labile interactions with the substrate in order to facilitate both the recovery of the enantioselec-tively coordinated ligand and the recycling of the chiral matrix. Usually copper(II) complexes have been used[48]. Due to the problems involved in the modeling of Jahn-Teller distorted copper(II) complexes (see Chapter 11 for a detailed discussion on... 

As expected, the effectiveness of a chiral matrix is related to the substrate to be resolved, i. e., a specific matrix complex might be effective in the resolution of a certain substrate while it does not efficiently resolve another one. 

It should not therefore be surprising that a chiral matrix such as a phospholipid membrane can also act as a specific receptor and lead to a change in conformation of a ligand molecule. 

Chiral matrix a) Racemic substrate a> Calculated (%) Observed (%) Reference... 

There are several alternative methods for the synthesis of optically active polymers from achiral or racemic monomers that do not involve polymerization catalysts. Optically active polymers have been formed from achiral dienes immobilized in a chiral host lattices [ 106]. In these reactions, the chiral matrix serves as a catalyst and can be recovered following the reaction. For example, 1,3-penta-dienes have been polymerized in perhydrotriphenylene and apochoUc acid hosts, where asymmetric induction occurs via through-space interactions between the chiral host and the monomer [107,108]. The resultant polymers are optically active, and the optical purities of the ozonolysis products are as high as 36%. In addition, achiral monomers have been found to pack in chiral crystals with the orientations necessary for topochemical soHd-state polymerization [109]. In these reactions, the scientist is the enantioselective catalyst who separates the enantiomeric crystals. The oligomers, formed by a [27H-27i] asymmetric photopolymerization, can be obtained in the enantiomeric pure form [110]. 

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