Lanthanide-induced shift Lanthanides, determination

New piperidine amide 154 was isolated from Achillea falcata, along with several known amides. Its structure was established using MS, UV, IR and and 13C NMR [419]. 13C NMR spectra were obtained, and H NMR-lanthanide induced shifts were determined for the amides [419],... 

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. 

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. 

The angular and distance information provided by the lanthanide induced shift has found widespread application from the determination of solution structures of Ln chelates [18,19] to gaining structural information on proteins, nucleotides and amino acids [19], More recently anion binding to coordinatively unsaturated lanthanide complexes has been effectively signalled as the observed lanthanide induced shift has been directly correlated to the nature of the donor atom in the axial position [8,20,21], It is the polarisability of the axial donor that ranks the second order crystal field coefficient, B02, and hence determines the magnitude of the observed shift. Values of the mean shift of the four most-shifted axial protons of the 12-Nq ring for [Yb.la]3+ are collated in Table 2. 

It follows from the above discussion that the systems of 1 2 stoichiometry are complex and that 1 1 systems are preferable for lanthanide induced shifts in the determination of structures of organic molecules. The 1 1 stoichiometry is encountered in the case of Ln(dpm)3 in organic solvents [33], Ln(fod)3 in CHCI3 [32], [Ln(EDTA)]- in aqueous solution [34]. 

Lanthanide shift reagents have been used to study dynamic NMR processes. One of the classic applications involves the study of rotational barriers of various amides " thioamides ", oximes " and amide oximes ". Lanthanide-induced shifts enabled the distinction of cis- and trans- or syn- and awri -isomers, and variable-temperature studies in the presence of the chelate were performed to determine the barrier to rotation. 

The conpling constant of the AA BB portion of bisphenol A-neopentyl glycoltereph-thalate copolyesters (20) was determined in the presence of Pr(fod)3". The compositional tetrads of a 3,3 -dimethylbisphenol A-phenolphthalein copolyterephthalate (21) were resolved in the presence of En(fod)3 . The meso and racemic dyads of a poly(/l-snbstitnted- S-propiolactone) were distinguishable in the NMR in the presence of Eu(dpm)3". The H and NMR spectra of ethylene-vinyl acetate copolymers were assigned on the basis of two-dimensional NMR methods and lanthanide-induced shift data in the presence of Eu(fod)3". ... 

This method of resolution of polyolefins has been extensively studied for cyclooctatetraene systems where excellent enantiomeric excesses are normally observed. Lanthanide-induced shifting can be used to determine the diastereoisomeric composition of the urazoles. Alternate means for the resolution of polyenes based on kinetic resolution using (+)-tetra-2-pinany Iborane have been described, but this reagent consumes valuable substrate. Chiral platinum complexes can also be used but at prohibitive cost on a large scale and with poor regioselec-tivity when several coordination sites are present. 

A new piperideide (155) was isolated from Achillea millefolium, and its structure was determined using MS, UV, and H NMR [420]. Several previously known piperidine amides were also isolated from this plant, including the dominant 156. The stereochemistry of amides 157 and 158 was determined using the lanthanide induced shift method [420],... 

Physical Properties of 1,2,3-Thiadiazoles. Lanthanide-induced shifts have been used for elucidation of the structure of thermally generated monoxides of 1,2,3-thiadiazoles ( H and C). A calculation has been developed for the evaluation of lanthanide-induced shifts according to the McConnell-Robertson equation. The o and Or parameters of triazoles were also determined by n.m.r. spectroscopy. ... 

Solution structure determination essentially relies on NMR data, via either a classical analysis of the number of signals and of their chemical shifts with the help of two-dimensional COSY, NOESY, and NOEDIF measurements, or the more sophisticated investigation of both lanthanide-induced shifts (LIS) and relaxation times (LIR). When the major species in solution is the heterobimetallic helicate, analysis of the... 

Determination of Absolute Configuration and Solution Conformation. Abraham et have employed H and C lanthanide induced shifts produced by Yb(fod)3 as an experimental probe of the solution conformations of cyclopen-tanol and cis- and trans-cyclopentane-l,2-diol. Analysis of the LIS was undertaken in conjunction with molecular mechanics and ab initio calculations of structure. 

Solution structures may be determined on the basis of lanthanide-induced shifts. This is, however, a difficult task the exact origin of the induced NMR shifts is unknown, a great number of lanthanide complexes have a low symmetry and they often present a high lability, e.g., between conformers. The separation of the various contributions to the NMR shifts may be achieved by several procedures under favourable conditions, or can be avoided by selecting the Yb(III) ion as paramagnetic center, since this ion induces shifts that are essentially dipolar in origin. The dipolar shift is expressed by ... 

Reactions.—A new method for the determination of absolute configuration in chiral secondary alcohols is based on the changes in chemical shifts observed in their n.m.r. spectra on glycoside formation. Characteristic shift changes for the glycosidic carbon of the sugar, and for C-a and C-/3 of the alcohol, have been summarized as rules. Measurement of lanthanide-induced shifts for the CF3 resonance in the n.m.r. spectra of a-methoxy-a-trifluoromethylphenyl-acetate (MTPA) esters of secondary alcohols has been reported as an... 

Stensen, W., Svendsen, J.S., Hofer, O., and Sydnes, L.K., Photochemical [2 + 2] cycloadditions. LIII. Addition of 4-substituted 2-cyclopentenones to aUene configuration determination by lanthanide-induced shift studies, Acta Chem. Scand. Ser. B, 42, 259, 1988. 

The determination of ee with chiral lanthanide shift reagents is normally performed in organic solvents. However, the development of chiral reagents capable of inducing shift nonequivalence in water, which is the appropriate solvent for many natural products, is of great importance. The europium(III) (R)-propylenediaminetetraacetate ion (Table 1) has been proposed as a chiral reagent suitable for use in aqueous solution90. 

The crystal field does, however, have a dramatic effect on the magnetic anisotropy of lanthanide complexes. For complexes of less than cubic symmetry the three principal values of the susceptibility tensor are unequal. For uniaxial symmetry, Xx — Xy Xz and for biaxial symmetry Xx Xy Xz- Very extensive studies632-640 have been carried out on the single crystal susceptibilities of the D3d lanthanide hexakis(antipyrene) triiodides over the temperature range 80-300 K, and crystal field parameters were obtained. This crystal field-induced anisotropy is responsible for the effectiveness of lanthanide complexes as NMR shift reagents, and single crystal anisotropies of lanthanide complexes have been determined in this connection also.563... 

Information on the hydration state of the Gd(III) chelate in solution is indispensable for the analysis of its proton relaxivity Several methods exist to determine q, though they are mostly applicable for other lanthanides than Gd(III). In the case of Eu(III) and Tb(III) complexes, the difference of the luminescence lifetimes measured in D20 and H20 can be related to the hydration number [15, 16]. For Dy(III) chelates, the lanthanide induced 170 chemical shift of the bulk water is proportional to the hydration number [17]. Different hydration states of the same chelate may also coexist in solution giving rise to a hydration equilibrium. Such an equilibrium can be assessed by UV-Vis measurements on the Eu(III) complex [18-20]. These techniques have been recently discussed [21]. 

The use of aqueous chiral lanthanide complexes in the determination of the enantiomeric purity of chiral a-hydroxy acids has also been assessed by H NMR [21], Large lanthanide induced shifts, chemical shift non-equivalence and an apparent absence of kinetic resolution in complex formation is observed upon addition of racemic lactate to [Yb.3a]3+ (Figure 1). The lactate CH3 resonances are clearly resolved for the... 

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