A-enantiomer

Unreacted material is enriched in 5-enantiomer Unreacted material is enriched in A-enantiomer ... 

An overall efficiency of TRISPHAT 8 and BINPHAT 15 anions as NMR chiral shift agents for chiral cations has been demonstrated over the last few years. Additions of ammonium salts of the A or A enantiomers of 8 and 15 to solutions of racemic or enantioenriched chiral cationic substrates have generally led to efficient NMR enantiodifferentiations [112-121]. Well-separated signals are usually observed on the spectra of the diastereomeric salts generated in situ. 

The first resolution of an octahedral complex into its enantiomers was achieved in 1911 by A. Werner, who got the Nobel Prize in 1913, with the complex [Co(ethylenediamine)(Cl)(NH3)] [10]. Obviously, resolution is to be considered only in the case of kinetically inert complexes whose enantiomers do not racemize quickly after separation. This is a very important remark since, as noted above, the interesting complexes are those containing exchangeable sites required for catalytic activity and thus more sensitive to racemization. We will not discuss here the very rare cases of spontaneous resolution during which a racemic mixture of complexes forms a conglomerate (the A and A enantiomers crystallize in separate crystals) [11,12]. 

Jacques, J., Collet, A., Wilen, S. A. Enantiomers, Racemates, and Resolutions, New York, Wiley-Interscience 1981... 

The diphosphines 29-31 induced low asymmetries (8-15% ee) irrespective of the reaction conditions. The more electron-rich ligand (R, R)-32 gave (R)- 1-phenylethanol with a much-improved 65% ee (entry 1). Conversely, the application of the more electron-poor diphosphine (R, R)-33 resulted in the (A)-enantiomer, but with the same level of enantioselection (entry 2), highlighting the importance of the electron density of the phosphorus center on the... 

Most optically active polysilanes owe their optical activity to induced main-chain chirality, as outlined above. However, backbone silicon atoms with two different side-chain substituents are chiral. Long-chain catenates, however, are effectively internally racemized by the random stereochemistry at silicon, and inherent main-chain chirality is not observed. For oligosilanes, however, inherent main-chain chirality has been demonstrated. A series of 2,3-disubstituted tetrasilanes, H3Si[Si(H)X]2SiH3 (where X = Ph, Cl, or Br), were obtained from octaphenylcyclote-trasilane and contain two chiral main-chain silicon atoms, 6.16 These give rise to four diastereoisomers the optically active S,S and R,R forms, the activity of which is equal but opposite, resulting in a racemic (and consequently optically inactive) mixture and the two meso-forms, S,R and R,S, which are optically inactive by internal compensation. It is reported that the diastereoisomers could be distinguished in NMR and GC/MS experiments. For the case of 2-phenyltetrasilane, a racemic mixture of (R)- and (A)-enantiomers was obtained. 

NMR studies (XH, 13C, 29Si) have shown that the zwitterions 3 and 27-38 also exist in solution (3, 27-29, 31-38, [D6]DMSO 30, CDC13). The NMR data, in context with the results of ab initio studies of 39, can be interpreted in terms of a rapid Berry-type pseudorotation at room temperature, leading to an interconversion of the (A)- and (A)-enantiomers. The 29Si chemical shifts of the zwitterions 3 and 27-38 are listed in Table IV. In most cases, these values are very similar to the isotropic 29Si chemical shifts obtained for the respective crystalline compounds by solid-state 29Si CP/MAS experiments. However, the 29Si chemical shifts observed for 31 (8 -131.9) and 32 (8 -123.0) in solution differ substantially from those determined for... 

As illustrated for compounds 77 and 78 in Scheme 18, different methods were applied for the syntheses of 77-79 (79 was obtained analogously to 78 according to method a). The racemic products 77a 0.7CH3CN, 78 CH3CN, and 79 were isolated as crystalline solids. In addition, crystals of the racemic compound 77b (an isomer of 77a) were obtained. For the solvent-free compound 78 formation of enantiomorphic crystals was observed. The crystals studied by X-ray diffraction contained (just by accident) the (A)-enantiomer. 

The /-coordination polyhedra of the chiral zwitterions 81-84 in the crystal were found to be distorted trigonal bipyramids, with the two carbon-linked oxygen atoms in the axial positions. This is illustrated for 81 in Fig. 10. The crystals of 81-84 are built up by pairs of (A)- and (A)-enantiomers. Selected geometric parameters for these compounds are listed in Table XI. As can be seen from these data, the axial Si-O distances [1.749(2)-1.801(4) A] are significantly longer than the equatorial ones [1.683(2)-1.7182(14) A]. The Si-C distances are 1.866(6)-1.897(2) A. 

As demonstrated by single-crystal X-ray diffraction, the /-coordination polyhedra of 85-87 are distorted trigonal bipyramids, with each of the axial positions occupied by the oxygen atoms. This is shown for compound 86 in Fig. 11. In all cases, the crystals are formed from pairs of (A)- and (A)-enantiomers. Selected geometric parameters for 85-87 are listed in Table XIII. As can be seen from the Si-O [1.8004(10)-1.829(6) A], Si-N [1.741(7)-1.764(6) A], and Si-C distances [1.867(8)-1.915(2) A], the A/02N2C frameworks of 85-87 are built up by five normal covalent bonds and do not involve a bonding system in the sense of the 4+1 coordination usually observed for pentacoordinate silicon species with Si-N bonds. 

Ultrapurification of 50 mmolL DNB-D,L-Leucine in a Cascade of Five Stages with Two Modules and Two Enantiomeric Carrier (Quinine and Quinidine Derivative with 5 vol% Polysiloxane-supported Carrier) Transmembrane Material Stream J, Enantioselectivity a, Enantiomer Excess ee, Purity, and Yield of DNB-D-Leucine... 

Abbreviations A enantiomer A A enantiomer /I rac racemic H----- indicates that the observation is not in favor nor against intercalation LD linear... 

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