Lanthanide-induce shifts

A study195 based on the NMR lanthanide-induced shifts (LIS) for a series of cis- and trans-3-substituted, and 3,3-disubstituted thietane oxides concluded that all cis-substituted oxides (5c R = CH3, t-Bu and aryl) exist exclusively in the diequatorial conformation. The trans-3-substituted isomers (185) prefer the equatorial oxygen conformation (R = CH3,86% t-Bu, 65-75% aryl, 75%), which means an axial preference for the substituents (e.g. 185d), at least when they are bound to a shift reagent (equation 75). 

Computer simulation of lanthanide-induced shifts in the 2-formyl and 2-acetyl derivatives of furan, thiophene, selenophene, and tellurophene108 indicate a nearly equipopulated mixture of trans and cis conformers of the furan, and a preponderance of the cis for the thiophene, selenophene, and tellurophene derivatives. This difference is due to an interaction between the ring heteroatom and the carbonyl oxygen lone pair electrons. 

Hofer, O., The Lanthanide Induced Shift Technique Applications in Conformational Analysis, 9, 111. 

SN NMR spectroscopy is still underexploited in structural analysis of nitrogen-containing heterocycles. It can however be a powerful tool, for instance in tautomerism studies <2002JP2126>. Some 1SN NMR investigations of lanthanide induced shift have been performed on bicycles 50-52, showing that the complexing site of these 4,5-dihydro-l/f-l,2,3-triazoles is located on N-3. 

Once assignments of polysaccharide signals are known, they may be used as a basis in determination of the position of substitution by such groups as acetate, malonate, phosphate, and sulfate, whose a- and /3-shifts may be estimated by referral to suitable monosaccharide models. For phosphate, the phosphated carbon atoms and adjacent resonances may be identified, as they give coupled signals and are subject to lanthanide-induced shifts. These data are described in Section VI, 8. 

The LIS (Lanthanide Induced Shift) NMR technique is useful for such analysis25 and the separation of olefin enantiomers such as limonene, a-camphene and /J-pinene has been performed upon addition of silver salts such as Ag(fod) or Ag(hfc) to the commonly used lanthanide chiral salts such as Ln(tfc )3 or Ln(hfc)3, where fod = 6,6,7,7,8,8,8-heptafluoro-2,2-dimethyloctanedione, hfc = heptafluoro-3-butyrylcam-phorato and tfc = trifluoroacetylcamphorato. 

The recently introduced lanthanide-induced shift (LIS) technique has also found useful application in configurational studies of sulfur compounds. For example, the absolute configurations of a new type of bridged steroidal sulfoxide 203, which is epimeric at sulfur, were... 

Lanthanide-induced Shift Technique - Applications in Conformational Analysis/Hb/er 9 111... 

Conformational Analysis, Applications of the Lanthanide-induced Shift... 

Lanthanide-induced Shift Technique— Applications in Conformational... 

Figure 15. Plots81 of the lanthanide-induced shift (LIS) for the methoxy group hydrogens in the two diastereomeric Mosher esters of 1-phenylethanol versus the molar ratio of Eu(fod)3.

This technique (see Section 4.1.1.4.) is well suited to the exploration of double-bond configurations. This is because these compounds arc often rigid and therefore there are no conformational rearrangements to complicate lanthanide induced shift (LIS) interpretations. However, if present, mobile groups within a molecule must be taken into consideration. 

Diastereomeric MTPA esters of secondary alcohols can be differentiated by their relative methoxy proton signal shifts, e.g., the (7 ,i )-diastereomer 1 shows a much stronger lanthanide induced shift than the (/ ,.S)-isomer417. 

DM, Dipole moments KC, Kerr constants LIS, lanthanide-induced shifts. 

Early work on the experimentally established conformational preferences in solution for a variety of other 2-substituted heterocycles is summarized in Table 30. Most of these conclusions have been deduced either from dipole moment measurements in benzene or by the use of lanthanide induced shifts for chloroform solutions. The aforementioned MO studies correctly predict the preferred conformations, (63, R = H) or (64, R = H), of pyrrole-2-carbaldehyde, thiophene-2-carbaldehyde and furfural in the gas phase. 

Optically active lanthanide shift reagents such as tris-(3-trifluoromethyl-hydroxymethylene-( + )-camphorato) europium (4) are commercially available. They can be used for the direct determination of optical purity and for the measurement of enantiomeric composition. The differences in the lanthanide-induced shift between enantiomers can be as high as 1.8 p.p.m. depending on the geometry of the molecule. 

NMRD nuclear magnetic relaxation dispersion LIS lanthanide induced shifts ... 

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