Reagents, lanthanide-induced shifts

The site of Ai-alkylation in pyrazolotropones 143a,b (72TL1925) and imidazotropones 185 and 186 (92JHC1219) was elucidated by the lanthanide-induced shift reagents Eu (dpm)3 and Eu (fod)3, respectively. Likewise, Pr (dpm)3 was used [93JCS(P1)1617]. 

Alternatively, the sulfites can be oxidized to the sulfates 85 in 45 min at 0°C by employing a 350 ppm RUO4 solution in water, but in only 30% yield <1992NJC107>. Diastereotopically labeled 0-sulfates 84 and 85, obtained by oxidation of diastereoisomeric cyclic sulfites with Ru 04, were shown by application of lanthanide-induced shift reagents on the NMR signals to be formed with retention of configuration at sulfur <1983CC1392>. 

Barriers for interconversion between BC-3 and its enantiomer, BC-7, have been calculated. Passage through either a BC-5 or a BC-1 conformation, are calculated to be 5.0 kcal mol and 6.7 kcal mol , respectively. The small barriers allow for a rapid pseudorotation between BC-3 and BC-7, even at low temperatures, and account for the symmetric spectra. NMR results on oxocane-2,2,7,7-d4 have allowed a definitive assignment for the H and C chemical shifts. Lanthanide-induced shift reagents provide compelling proof, that the lowest energy conformations are BC-3 and BC-7. 

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). 

A highly consistent correlation was observed between the absolute configuration of a-amino acids and the shift of their a-proton signals induced by the chiral lanthanide NMR shift reagent l,2-propanediaminetetraacetatoeuropium(III)60. Shift differences at a molar ratio of 0.02-... 

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. 

R and S diastereomers (AA5 = 10 ppm) and experience a large lanthanide induced shift with the CH3 resonating at +21 and +31 ppm respectively for (It)- and (5)-lactate respectively (c.f. +1.3 ppm in the free form). The large shifts exhibited by this unique chiral derivatising agent may be compared to the much smaller AA8 values reported for shift reagents which are generally of the order 1 ppm, and often <0.1 ppm. 

The NMR spectra of most of the lanthanide shift reagents in solution involve rapid chemical exchange. In many situations one has 1 2 and 1 1 adducts and free ligand in solution. The observed lanthanide induced shift, S is the sum of two contributions... 

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. 

Achiral binuclear reagents have been added to mixtures of a chiral crown ether (74) and chloroform-soluble amino acid ester hydrochlorides to enhance the chiral discrimination in the NMR spectrum. The [Ln(fod)4] preferentially associated with the enantiomer in the bulk solution such that the enantiomer with lower association with 74 showed the larger lanthanide-induced shifts. The system also enhanced the chiral discrimination in acetonitrile-fifs, although [Pr(fod)4] was needed because it causes larger shifts than the... 

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. 

Lanthanide Shift Reagents.—The effects of random co-ordinate error in analysis of lanthanide-induced axial pseudocontact shifts have been discussed, " and the contributions of contact and pseudocontact shifts in the n.m.r. spectra of isoquinoline and of endo-norbornenol have been evaluated. An experimental and computational approach to the use of lanthanide-induced shifts as a rigorous method for structure determination has been described. The method was used to predict the lanthanide-induced shift behaviour of a substrate. The recording of experimental data in excellent agreement with the molecular structure was reported. Contact shift contributions to lanthanide isotropic shifts have been found to be important for organic compounds even where the carbon atom is five bonds away from the lanthanide. 

Shift Reagents. In 1970, Williams, elaborating on the preliminary work of Hinckley, discovered that adding trw-jS-diketonate lanthanide complexes to solutions of substances with lone pairs of electrons available for coordination (such as oxygen- and nitrogen-containing organic compounds) resulted in vastly more dispersed NMR spectra. This phenomenon, named lanthanide-induced shift (LIS), arises from the... 

Lanthanide shift reagents have been extensively reviewed by Flockhart and Hofer in the latter many examples of five- and six-membered ring systems are explicitly covered, as are the principles involved and methodology used. The analysis of lanthanide-induced shift data of mixtures has also been considered. ... 

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

Chemists did not follow the caution voiced by Weissman [30] that both contact and pseudocontact (dipolar) shifts are to be expected in molecules coordinated to Europium shift reagents. Numerous workers applied a simplified form of the McConnell Robertston relationship (Equation (3)) for pseudocontact interaction as the sole interpreter of the lanthanide induced shifts in rigid organic molecules. It has been shown that especially when C data are compared to H data [31], that both the dipolar and contact effects operate simultaneously ... 

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