Racemate-forming enantiomer-differentiating

Note 2 A polymerization in which, starting from the racemate of a chiral monomer, two types of polymer molecules, each containing monomeric units derived from one of the enantiomers, form in equal amounts is termed racemate-forming enantiomer-differentiating polymerization . The resulting polymer is optically inactive (see Note 4 of Definition 2). 

The isoselective polymerization of a racemic mixture of monomers can proceed in two ways depending on initiator, monomer, and reaction conditions. Racemate-forming enantiomer-differentiating polymerization involves both the R and 5 monomers polymerizing at the same rate hut without any cross-propagation [Hatada et al., 2002]. A racemic monomer mixture polymerizes to a racemic mixture of all-5 and all-5 polymer molecules [Pino, 1965 ... 

In the literature concerning the ROP of heterocyclic monomers, the asymmetric, enantiomer-differentiating polymerization and racemate-forming enantiomer-differentiating polymerization are informally named... 

Stereoselective (IDRAG racemate-forming enantiomer-differentiating polymerization)... 

The presence of asymmetric C atoms in a molecule may, of course, be indicated by diastereotopic shifts and absolute configurations may, as already shown, be determined empirically by comparison of diastereotopic shifts However, enantiomers are not differentiated in the NMR spectrum. The spectrum gives no indication as to whether a chiral compound exists in a racemic form or as a pure enantiomer. 

In the meso-form 1, the absolute configurations are different, the molecules are identical, and there is at least one conformation with a symmetry plane, whereas the other conformations possess an enantiomeric equivalent. In contrast, molecules 2 and 3, both with groups of identical absolute configuration, are enantiomers, i.e., d,/-forms, often racemic mixtures. They are chiral and there are conformers with a twofold rotation axis. Thus, a differentiation between the meso- and the racemic form, simply by counting the number of 13C signals, is not possible. For each isomer the 13C-NMR spectrum contains a halved set of resonances, i.e., one signal for each pair of corresponding carbons in the two parts of the molecule. 

If during the hydroformylation of a racemic olefin more than one aldehyde is formed, an enantiomer-differentiating synthesis with formation of two (or more) optically active aldehydes can be obtained 14), even if the conversion is complete (Scheme 1, reaction 4, and Scheme 2). 

If racemic sec alcohols (e.g., butan-2-ol, 52) undergo partial acylation with an acid anhydride in the presence of an optically active amine, an enantiomer differentiation occurs in the example shown, the (S)-alcohol reacts preferentially forming but-2-yl acetate, 53, which is levorotatory, and leaving behind unchanged alcohol which has been optically enriched [68]. This is a process under kinetic control. For further examples of kinetic resolutions, see [69]. 

In a useful extension of this methodology for enantioselection in intramolecular cyclopropanation, Doyle s group have used chiral rhodium (II) carbox-amidates to effect enantiomer differentiation in reactions of racemic secondary allylic diazoacetates [47]. The catalyst-enantiomer matching approach has also been applied very successfully to intramolecular C-H insertion reactions vide infra). The (R)- and (S)-enantiomers, (10) and (11), respectively, of cyclohex-2-en-1 -yl diazoacetate are displayed in Scheme 7. On exposure to Rh2(4i -MEOX)4 the (R)-enantiomer (10) undergoes cyclopropanation to form tricyclic ketone... 

A broad structural range of racemic secondary mono-, bi- and tricyclic acylated alcohols are substrates in lipase-catalyzed enantiomer-differentiating hydrolysis as the examples 1-90 of Table 11.1-16 reveal. A large number of cis- and trans-cycloalkanols bearing a functional group in 2-position (1-20, 25, 26, 58-62) is thereby available in enantiomerically pure form. Enantiomer selectivity in the case of cyclic allylic alcohols where the double bond bears no other substituent in the exposition is frequently low. Through a temporary substrate modification such as mono- or dibromination, enantiomerically pure cyclic allylic alcohols may also be obtained in these cases (51, 52). 

A kinetic resolution is a chemical reaction in which one enantiomer of a racemate reacts faster than the other. Most kinetic resolutions of pharmaceutical compounds are catalyzed processes. Catalysts used in a kinetic resolution must be chiral. Binding of a chiral catalyst with a racemic material can form two different diastereomeric complexes. Since the complexes are diastereomers, they have different properties different rates of formation, stabilities, and rates of reaction. The products form from the diastereomeric substrate-catalyst complexes at different rates. Therefore, a chiral catalyst is theoretically able to separate enantiomers by reacting with one enantiomer faster than the other. The catalysts used in kinetic resolutions are often enzymes. Enzymes are constructed from chiral amino acids and often differentiate between enantiomeric substrates. 

An equimolar mixture of two enantiomers is called a racemate. The separation of two enantiomers that constitute a racemate is called optical resolution or resolution. Their crystalline forms best characterize types of racemates. A racemic mixture is a crystal where two enantiomers are present in equal amounts. A conglomerate is a case where each enantiomer has its own crystalline form. Sometimes their crystals have so-called hemihedral faces, which differentiate left and right crystals. For over a hundred years, crystallization processes have been used for the separation and purification of isomers and optical resolution, both in the laboratory and on an industrial scale. 

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