Enantiomeric unit

From the copolymer composition dependence of the molar ratio of the d- and L-enantiomeric units of 15e in the copolymer, the rate of reaction of the growing chain end of 21 with the D-enantiomer of 15e was estimated to be about four times faster than that with the L-enantiomer. Such asymmetric selection is mainly ascribable to the steric and electronic interactions between the asymmetric environment created by the bulky terminal unit of 21 and the rigid bicyclic monomer having three asymmetric centers and a polar bulky bromine substituent (Scheme 6, [23]). 

The cyclic dimer 43 is a me so compound consisting of a pair of different enantiomeric units of 41. [45] All four substituents attached to the two tetra-hydropyran rings are located in the axial positions. This compound has a center of symmetry and is readily crystallized. The cyclic tetramer 44 is a racemic mixture of optically active enantiomers, that is, 44R and its enantiomer 44S. [46] Similarly, the cyclic pentamer 45 is a racemic mixture of 45R and its enantiomer 45S. [47] In these cyclic oligomers, every exo-cyclic acetal oxygen occupies the axial position and every carbonyl carbon occupies the equatorial position of the tetrahydropyran ring. The molecules of these oligomers are chiral but not asymmetric (gyrochiral... 

ECH elastomers have more than 97% of head-to-tail sequences, they are amorphous (no cristallinity detected by DSC or x-ray analysis) and with a stereorandom distribution of enantiomeric units. These polymers are thus atactic. In ECH/EO copolymer the mole ratio of both monomers was kept to approximately I. In the terpolymer the amount of AGE is small (around 2.5%). 

X-ray analysis of some of the cyclic oligomers described above has been achieved so far. The cyclic dimer derived from the racemic monomer is composed of a pair of different enantiomeric units and all of the four substituents attached to the two tetrahydropyran rings are oriented axially [11]. Such a structure of the cyclic dimer is compatible with the fact that no cyclic dimer was produced from the optically active monomer. 

A "stereoelective" (17) or "asymmetric-selective" (3) polymerization is a process in which a single stereoisomer oT a mixture is polymerized giving macromolecules containing one type of configurational base units. For example an optically active catalyst will choose one enantiomer from a racemic mixture and form a macromolecule containing only one type of enantiomeric units. Such an ideal reaction should stop at 50 % yield after consumption of the corresponding stereoisomer. 

In 1962, Inoue et al [140, 141], first succeeded in carrying out an asymmetric selective polymerization of /-propylene oxide (J,/-PO) using dialkylzinc-optically active (+)-borneol or (-)-menthol catalysts with a conversion of 30% (Scheme LVIII) the polymer was found to be optically active, levorotatory in benzene solution and dextrorotatory in chloroform, like the polymer obtained by Price [92] from the purified /-enantiomer monomer. There were no absorption bands corresponding to the original alcohol detected in the IR spectrum and the optical activity of the polymer must, therefore, be attributed to the prevalence of one of the two enantiomeric units. 

In the case of chiral cyclic monomers the prepared polymer is isotactic, while this is not a necessity in principle in the case of chiral olefins. In most of the cases however, the choice is not as perfect as defined and, generally, one speaks of stereoelective process when there is a preferential polymerization of one type of enantiomer from a mixture. Moreover the sites controlling the polymerization could be more or less stereospecific, so that heterotactic polymers could in principle be obtained with the predominance of one type of enantiomeric unit. 

It was pointed out that polypropylene oxide prepared with stereoselective catalysts can be fractionated into crystaUine and amorphous fractions. The crystalline fraction has a high melting point (close to 72—73°) which differs somewhat depending on the catalyst used. These differences could be explained by differences in sequential length of enantiomeric units of one type and by the presence of irregularities of head-to-head type. We have found, that crystalline fractions of polypropylene oxide prepared by the stereoelective route have a m.p. always close to 72° [13]. 

The 2,3-epoxy alcohols are often obtained in high optical purity (90% enantiomeric excess or higher), and are useful intermediates for further transformations. For example by nucleophilic ring opening the epoxide unit may be converted into an alcohol, a /3-hydroxy ether or a vicinal diol. 

The high diffusivity and low viscosity of sub- and supercritical fluids make them particularly attractive eluents for enantiomeric separations. Mourier et al. first exploited sub- and supercritical eluents for the separation of phosphine oxides on a brush-type chiral stationary phase [28]. They compared analysis time and resolution per unit time for separations performed by LC and SFC. Although selectivity (a) was comparable in LC and SFC for the compounds studied, resolution was consistently... 

Chiral, nonracemic allylboron reagents 1-7 with stereocenters at Cl of the allyl or 2-butenyl unit have been described. Although these optically active a-substituted allylboron reagents are generally less convenient to synthesize than those with conventional auxiliaries (Section 1.3.3.3.3.1.4.), this disadvantage is compensated for by the fact that their reactions with aldehydes often occur with almost 100% asymmetric induction. Thus, the enantiomeric purity as well as the ease of preparation of these chiral a-substituted allylboron reagents are important variables that determine their utility in enantioselective allylboration reactions with achiral aldehydes, and in double asymmetric reactions with chiral aldehydes (Section 1.3.3.3.3.2.4.). 

Optically active (Z)-l-substituted-2-alkenylsilanes are also available by asymmetric cross coupling, and similarly react with aldehydes in the presence of titanium(IV) chloride by an SE process in which the electrophile attacks the allylsilane double bond unit with respect to the leaving silyl group to form ( )-s)vr-products. However the enantiomeric excesses of these (Z)-allylsilanes tend to be lower than those of their ( )-isomers, and their reactions with aldehydes tend to be less stereoselective with more of the (E)-anti products being obtained74. 

Cyclodextrins, toroidal molecules composed of 6, 7 and 8 D-glucose units, are now commercially available at reasonable cost. They form inclusion compounds with a variety of molecules and often differentially include sulfoxide enantiomers29,30. This property has been used to partially resolve some benzyl alkyl, phenyl alkyl and p-tolyl alkyl sulfoxides. The enantiomeric purities after one inclusion process ranged from 1.1 % for t-butyl p-tolyl sulfoxide to 14.5% for benzyl r-butyl sulfoxide. Repeating the process on methyl p-tolyl sulfoxide (10) increased its enantiomeric purity from 8.1% to 11.4% four recrystallizations raised the value to 71.5%. The use of cyclodextrins in asymmetric oxidations is discussed in Section II.C.l and in the resolution of sulfmate esters in Section II.B.l. 

The use of chiral ruthenium catalysts can hydrogenate ketones asymmetrically in water. The introduction of surfactants into a water-soluble Ru(II)-catalyzed asymmetric transfer hydrogenation of ketones led to an increase of the catalytic activity and reusability compared to the catalytic systems without surfactants.8 Water-soluble chiral ruthenium complexes with a (i-cyclodextrin unit can catalyze the reduction of aliphatic ketones with high enantiomeric excess and in good-to-excellent yields in the presence of sodium formate (Eq. 8.3).9 The high level of enantioselectivity observed was attributed to the preorganization of the substrates in the hydrophobic cavity of (t-cyclodextrin. 

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