Enantiomorphic selectivity

The clear cut case of enantiomorphic selection has been reported by Vander-berg ° ) in polymerization of trans-2,3-epoxybutanes ... 

As one might predict, there is enantiomorphic selectivity with regard to various chiral ligands. For example, albumins in general have a 100-fold higher affinity for L-tryptophan (King and Spencer, 1970 La-gercrantz et ai, 1981) and stereoselectivity is also known for many pharmaceuticals such as rf-oxazepam hemisuccinate (Muller and Wollert, 1975). 

Anticipating Section IV.D, tacticity can come about because the initiator is enantiomorphic selective or because incoming monomer is controlled by the configuration of the terminal or... 

Control of enantiomorphic selectivity in polymerization of the substituted oxiranes can lead to controlled-structure polymers, the properties of which will range from crystalline thermoplastics to amorphous elastomer precursors such as are used as soft segments in polyurethanes. Crystallizable sequence distributions in highly controlled-structure polymers can lead to thermoplastic elastomers and/or to elastomers that will stress-crystallize that is, crystallize on stretching as does natural rubber (79). 

The resulting polymer can be both crystalline and optically active, suggesting that the polymerization was both stereospecific and enantiomorphic selective. 

Polymerization occurs by cleavage of the oxygen bond attached to the less-substituted carbon atom of the oxirane ring with inversion of the configuration of the secondary carbon atom and formation of a secondary alkoxide polymer molecule while the approaching monomer oxirane complexes with the now vacant aluminum atom site. Stereospecificity of the resulting polymer depends on whether the coordination of monomer is enantiomorphic selectic (97). 

From this view, there is no clear way in which the initiator site would be enantiomorphic selective, unless the activator polyether participated in site selectivity. The polymer formed with propylene oxide and dioxane activator is believed to be largely nonstereoregular. 

While the major thrust in process development for chemicals in the 1980s was to reduce by-product manufacture, to increase process efficiencies, and by these means to control and reduce the impact that chemical processes and products may have on the environment, the next vista may well involve radically new chemical reaction concepts. Enantiomorphic selectivity, which is the ability to control the stereoisomer produced from a complex organic reaction, will be particularly important in the areas of pharmaceuticals and agricultural chemicals, where it is commonly the case that only one of many possible stereoisomers has the desired bioactivity. In 1992, this area of chemical synthesis is still in its true infancy. 

A simple example of enantiomorphic selective synthesis can be taken from polymer chemistry and the polymerization of propylene oxide. Propylene oxide contains an asymmetric carbon atom, C ... 

Examples of this kind of enantiomorphic or chiral selectivity are now being found in organic synthesis. Asymmetric synthesis, for example, has been demonstrated with stereo-controlled Michael addition in the synthesis of beta-lactams using chiral catalysts, where an acyl ligand such as acetyl is bound to cyclo-pentadiene carbonyl triphenylphosphine. Essentially complete enantiomorphic selectivity has been achieved in this Michael addition synthesis. Another case is enantio-morhic ketone reduction in ethylbenzene reduction in the ethylation of benzaldehyde. Using chiral catalysts, 97% selectivity has been achieved. Closely related research involves the making of catalytic antibodies and hybrid enzymes. ... 

Extensive studies of stereoselective polymerization of epoxides were carried out by Tsuruta et al.21 s. Copolymerization of a racemic mixture of propylene oxide with a diethylzinc-methanol catalyst yielded a crystalline polymer, which was resolved into optically active polymers216 217. Asymmetric selective polymerization of d-propylene oxide from a racemic mixture occurs with asymmetric catalysts such as diethyzinc- (+) bomeol218. This reaction is explained by the asymmetric adsorption of monomers onto the enantiomorphic catalyst site219. Furukawa220 compared the selectivities of asymmetric catalysts composed of diethylzinc amino acid combinations and attributed the selectivity to the bulkiness of the substituents in the amino acid. With propylene sulfide, excellent asymmetric selective polymerization was observed with a catalyst consisting of diethylzinc and a tertiary-butyl substituted a-glycol221,222. ... 

Sample II being someWhat more syndiotactic. Thus, the selectivity control agent (and any attendant changes in synthetic procedure) appears to change (1) the amount of polymer made at different sites (wj w2 w3 = 39 42 20), and (2) the nature of the Bemoullian polymer. It appears from this analysis that the nature of the enantiomorphic catalytic sites remains unchanged in the absence of the selectivity control agent. 

Another hypothesis on homochirality involves interaction of biomolecules with minerals, either at rock surfaces or at the sea bottom thus, adsorption processes of biomolecules at chiral mineral surfaces have been studied. Klabunovskii and Thiemann (2000) used a large selection of analytical data, provided by other authors, to study whether natural, optically active quartz could have played a role in the emergence of optical activity on the primeval Earth. Some researchers consider it possible that enantioselective adsorption by one of the quartz species (L or D) could have led to the homochirality of biomolecules. Asymmetric adsorption at enantiomor-phic quartz crystals has been detected L-quartz preferentially adsorbs L-alanine. Asymmetrical hydrogenation using d- or L-quartz as active catalysts is also possible. However, if the information in a large number of publications is averaged out, as Klabunovskii and Thiemann could show, there is no clear preference in nature for one of the two enantiomorphic quartz structures. It is possible that rhomobohedral... 

The enantiomorphic site selectivity of (R)-(—)-(263) allows highly syndiotactic PLA to be prepared via the polymerization of meso-LA (Mn = 12,030, Mn calc= 13,540, Mw/Mn= 1.05).802 The ring opening of meso-LA by (R)-(—)-(263) occurs to produce a syndiotactic propagating chain bound to the metal via an (S)-lactic acid unit.803 Attempts to produce syndiotactic PLA via the ROP of meso-LA using rac-( )-(263) instead afforded heterotactic-biased material. 

Metal complexes which initiate rac-LA ROP with a high degree of stereocontrol are currently an area of major research interest and have the potential to produce a spectrum of different materials [19, 21], Much attention focuses on iso-selectivity as this can enable production of PLA of good thermal resistance (isotactic, stereoblock or even stereocomplex PLA). There are two mechanisms by which an initiator can exert iso-selectivity in rac-LA ROP (1) an enantiomorphic site control mechanism or (2) a chain end control mechanism. Enantiomorphic site control occurs using chiral initiators (Fig. 6) it is the chirality of the metal complex which... 

Arnold and colleagues have reported a series of chiral homoleptic yttrium and lanthanide fra(alkoxide) complexes [49, 50], These initiators (including complex 1) show high degrees of iso-selectivity and rapid rates, even at low temperatures. Thus, using the racemic mixture of the lanthanide initiator, stereoblock PLA was produced with a P, of 0.81 so far, this is the only known type of yttrium initiator able to exert such stereocontrol and a very exciting finding. Analysis of the stereoerrors indicates that an enantiomorphic site control mechanism is responsible for the iso-selectivity. 

To date, no chiral zeolite or molecular sieve has been obtained. However, Newsam et al. (48) have shown that zeolite beta is an intergrowth of two distinct structures polymorph A and B. Polymorph A forms an enantiomorphic pair. Thus, synthesis of one of the enantiomorphs of polymorph A would yield the first chiral zeolite and initiate the possibility of performing intrazeolitic asymmetric catalysis. Shape selective asymmetric catalysis would be the ultimate achievement in shape selective catalysis, and would certainly be a step closer toward truly mimicking enzyme catalysis. 

When two enantiomorphous right- and left-crystals are separately obtained, one can conveniently use each crystal for the seeding of the selective chiral crystallization to either one of the two enantiomorphous crystals. More elegant pseudoseeding, based on utilizing different crystals with similar crystal structure as seed crystals, can enantiocontrol crystallization from solutions of tryptamine and achiral carboxylic acids [35]. 

In historical accuracy, it should be remembered that Fischer s original second convention was applied to the enantiomorphous saccharic acids as the reference substances or standards his formula for natural glucose (XV) followed as a sequel. It has now become customary to select the enantiomorphous forms of glyceraldehyde as the reference substances or "standards, from the stereo-formulas of which there follow... 

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