Shift chiral

The silver ion coordinates with the soft Lewis base and the lanthanide induces the shifts. Chiral binuclear reagents have been applied to chiral olefins and aromatics. Binuclear reagents formed using the dcm ligand are not effective for chiral resolution, presumably because this ligand is too sterically hindered to permit formation of the tetrakis chelate anion. Experimentation is often needed to find the correct combination of lanthanide and silver P-diketonate to cause enantiomeric resolution. 

Enantiotopic protons can have different chemical shifts in a chiral solvent Because the customary solvent (CDCI3) used in NMR measurements is achiral this phenomenon is not observed in routine work... 

Chiral liquid crystals Chiral recognition Chiral separation Chiral separations Chiral shift reagents... 

Diastereomeric derivathation of a chiral alcohol (111) with an enantiopure compound such as Mosher s reagent [20445-33-4] (a-ttifluoromethyl-a-methoxy-a-phenylacetjichloride) (112) (91) results in two distinct compounds (113) and (114) with nonequivalent chemical shifts in the H-nmr spectmm (92). 

NMR spectra of oxaziridine enantiomers may be different from each other in chiral media. In the presence of chiral arylperfluoroalkylcarbinols, shift differences of up to 0.35 p.p.m. are observed, which may be used for discrimination of enantiomers (77JOC3217). 

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. 

Figure 2.24, Determination of the enantiomeric excess of 1-phenylethanol [30, 0.1 mmol in 0.3 ml CDCI3, 25 °C] by addition of the chiral praseodymium chelate 29b (0.1 mmol), (a, b) H NMR spectra (400 MHz), (a) without and (b) with the shift reagent 29b. (c, d) C NMR spectra (100 MHz), (c) without and (d) with the shift reagent 29b. In the C NMR spectrum (d) only the C-a atoms of enantiomers 30R and 30S are resolved. The H and C signals of the phenyl residues are not shifted these are not shown for reasons of space. The evaluation of the integrals gives 73 % R and 27 % S, i.e. an enantiomeric excess (ee) of 46 %...

If the amount of the sample is sufficient, then the carbon skeleton is best traced out from the two-dimensional INADEQUATE experiment. If the absolute configuration of particular C atoms is needed, the empirical applications of diastereotopism and chiral shift reagents are useful (Section 2.4). Anisotropic and ring current effects supply information about conformation and aromaticity (Section 2.5), and pH effects can indicate the site of protonation (problem 24). Temperature-dependent NMR spectra and C spin-lattice relaxation times (Section 2.6) provide insight into molecular dynamics (problems 13 and 14). 

Fig. 2.7. NMR spectrum of 1-phenyIethyIamine in the presence of a chiral shift reagent, showing differential chemical shift of methine and methyl signals and indicating ratio of R- to iS-enantio-mers. [Reproduced from J. Am. Chem. Soc. 93 5914 (1971) by permission of the American Chemical Society.]...

Chiral organoselenenyl halides may also be stabilized by intramolecular Se N interactions Se NMR chemical shifts indicate that these interactions are maintained in solution. ... 

Thermal cyclization of 2-vinyl-N,N-dialkylanilines 138 afforded 139 with creation of a new chiral center in 98% purity (89JOC199). In case of pyrrolidine with methyl or methoxymethyl substituent, cyclization with ZnCl2 occurs via an irreversible 1,5-hydrogen shift in boiling acetonitrile (87JA3136) or BuOH (91RTC115) to afford the diastereoisomers 140 (33%), 141 (35%) and 142 (6%) (87JA3136) (Scheme 27). 

The fragmentation/cyclization ratio is determined by the relative orientation of the respective molecular orbitals, and thus by the conformation of diradical species 2. The quantum yield with respect to formation of the above products is generally low the photochemically initiated 1,5-hydrogen shift from the y-carbon to the carbonyl oxygen is a reversible process, and may as well proceed back to the starting material. This has been shown to be the case with optically active ketones 7, containing a chiral y-carbon center an optically active ketone 7 racemizes upon irradiation to a mixture of 7 and 9 ... 

---