Shift reagent, NMR

Figure 13. Effect of varying concentrations of the NMR shift reagent Eu(fod), on methyl resonances of soyasapogenol B phenyl borate. Euffodjg was dissolved in a minimum amount of acetone-dg and added to a 2 ml solution of soyasapogenol B phenyl borate in the same solvent. Spectra were obtained at 360 MHz.

Several applications of this methodology are known. For the determination of the relative configuration of the stereocenter and the axial chiral unit of 71, the product of a diastereoselective ester enolate Claisen rearrangement of 70, with AgBF4 a cycli-zation to 72 was initiated. Then the carboxylic acid was reduced to alcohol 73 and the position of the substituents was investigated by NMR and by the use of NMR shift-reagents (Scheme 15.16) [32], Control experiments ensured the stereospecifi-city of the cyclization and the reduction step. There are further examples of this strategy [33]. 

The first report of a cycloaddition reaction in the presence of an optically active catalyst13 appeared in 198314a. The dienes 14 add to benzaldehyde in the presence of 1 mol% of the chiral lanthanide NMR shift reagent Eu (hfc)3, i.e. tris[3-(heptafluoropropyl-hydroxymethylene)-(+)-camphorato]-europium(III), to give, after treatment with trifluo-roacetic acid, the dihydro-y-pyrone 15 enriched in the (R)-enantiomer, the degree of... 

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-... 

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... 

Recently the use of mixed lanthanide silver complexes as NMR shift reagents has been assessed. In the original experiments232,233 silver(I) carboxylates such as Ag02CCF3 and Ag02CC3F7 were used, although later studies showed that larger shifts could be induced with silver /3-diketonates (see Section 54.1.4.4). 

R. S. Sievers. NMR Shift Reagents. New York, Academic Press, 1971. 

It has been reported that the chiral NMR shift reagent Eu(DPPM), represented by structure 19, catalyzes the Mukaiyama-type aldol condensation of a ketene silyl acetal with enantiose-lectivity of up to 48% ee (Scheme 8B1.13) [29-32]. The chiral alkoxyaluminum complex 20 [33] and the rhodium-phosphine complex 21 [34] under hydrogen atmosphere are also used in the asymmetric aldol reaction of ketene silyl acetals (Scheme 8BI. 14), although the catalyst TON is quite low for the former complex. 

This standard was obtained in two ways. An asymmetric hydroformylation was run and the aldehyde was oxidized to S-a-acetoxypropionic acid. A comparison of the rotation obtained with the literature (19) for pure S-acid [a] )0 = -49.3°C (7.2, CHC13) was used to arrive at the 34.9°C figure for pure S-aldehyde. This number was checked using an NMR shift reagent. 

Second, in order to calibrate the intensity of each resonance over the spectral region from + 10 to +20 ppm from HDO to the number of protons per heme for each resonance, a reference standard is required. We have found that an NMR shift reagent, tris(6,6,7,7,8,8,8-... 

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