Lanthanide shift reagents LSR

Clobazam (54, X = Cl) and related compounds (X = H, X = CF3) exist in the dioxo tautomeric form [80JHC551,87JCS(P2)1071], as do the analogous pyrazolo[3,4-h][l,4]diazepinediones (89JHC949). Tliese conclusions were mainly based on careful NMR studies including the use of lanthanide shift reagents (LSR). 

Table T. Lanthanide Shift Reagents (LSRs) used in NMR Spectroscopy...

The fundamentals and applications of NMR spectroscopy in the presence of lanthanide shift reagents (LSR) have been reviewed in a number of articles237-244. Hence, only a brief introduction is warranted in this section. 

In the determination of the relative configurations in acyclic diastereomers with the aid of lanthanide shift reagents (LSR), conformation plays a major role and the equilibrium of rotamers in the complex may differ from that of the pure solute. 

A variety of techniques including specific deuteration, selective proton decoupling, and lanthanide shift reagents (LSR), Pr(dpm)3, was used by Haines et al. (138) to assign completely the H and Si NMR spectra of methyl-2,3,4,6-tetra-O-trimethylsilyl-oc-D-glucopyranoside. This paper also presents the first LSR study in Si NMR. It shows that the magnitudes of the LSR induced shifts in the spectra are of the same order as those found for the Si resonances. (138)... 

Chiral lanthanide shift reagents (LSR) also promote asymmetric hetero Diels-Alder reactions (Figure 45) 131-134 Lack of rigorous structural information regarding LSR/carbonyl interactions, however, does not allow meaningful speculation on the source of chirality transfer. 

The use of lanthanide shift reagents (LSR) to simplify NMR spectra and for structure determinations was recognized in the early 1970s (43). Although it was shown that contact contributions are generally not negligible in 13C NMR (44, 45), this complication can be overcome by using ytterbium... 

Smith et have prepared 11 chiral calix[4]arenes, calix[4]resorcarenes, and anionic cyclodextrin derivatives and investigated the properties of their lanthanide (Yb, Dy +) complexes as chiral lanthanide shift reagents (LSR). Baldovini et alP report an application of a camphor-derived chiral complex, Yb(hfc)3 (hfc = tris[3-(heptafluoropropylhydroxymethylene)-(- -)-camphorate]) to differentiate the C NMR spectra of enantiomers of bornyl acetate. 

Paramagnetic complexes, mainly the lanthanide shift reagents (LSR) bringing about the lanthanide induced shifts (LIS) [1, 2, 7]. 

Two-dimensional NMR has been extensively used and the technique is fairly routine for many applications. One consequence of the increasing use of 2-dimensional NMR is a revival of interest in H-NMR for polymer studies. Yet, a major drawback of H-NMR remains its limited chemical shift range, such that in complex molecules the resonances of different protons are frequently overlapped or incompletely resolved. Cheng and Lee [67] showed that the use of lanthanide shift reagents (LSR) combined with two-dimensional NMR can alleviate this difficulty in suitable cases. 

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