Lanthanoid shift reagents

N.M.R. Spectroscopy. The effects of lanthanoid shift reagents on the H and C n.m.r. spectra of canthaxanthin (90) have been studied in detail. Chemical shifts and signal broadening showed that the metal binds the carbonyl group at two sites. A computer programme was used to study the conformation of canthaxanthin. Similar studies" have been performed with a -trans-, 9-cis-, 11-CW-, and 13-c/s-retinal and 8 -apo-/3-caroten-8 -al (165), A high-resolution... 

Chemical shift assignment in the H NMR spectra is conveniently aided by the use of lanthanoid shift reagents, provided the electron-rich complexation site of a molecule by the metal ion can be predicted. This technique has been used for a betaine (129) and analogues <81JCR(S)107,82H(l8)loi>. 

Non-bonded interactions [632-634] were included in the same way as was used first for the modeling of cobalt(III) complexes [130] and more recently for a wide range of transition metal compoimds [95, 104, 125]. That is, standard models with M-L harmonic bond-stretching terms were used, but the L-M-L harmonic terms were deleted and L... L non-bonded interactions were used in their place. In the first of these studies, on lanthanoid shift reagents, two seven-coordinate europium(III) and ytterbium(III) complexes, [Eu(dipivaloylmethanato)3(quinuchdine)] and [Yb (acetylacetonato)3(H20)] (Figure 15.5), were modeled [632]. The structures of these complexes were well reproduced. In a subsequent study, these complexes and [Eu (dipivaloylmethanato)3(DMSO)] were studied with the same force field, while the random incremental pulse search method was used to locate all potential coordination geometries [633]. 

The first report of the use of enantiomerically pure lanthanoid complexes as chiral Lewis acids for Diels-Alder-type reactions appeared in 1983.90 As shown in Figure 37, in the presence of (+)-Eu(hfc)3(tris(3-(heptafluoropropylhydroxy-methylene)-(+)-camphorato)europium(III)), usually utilized as a chiral shift reagent for NMR study, the hetero-Diels-Alder reaction of benzaldehyde (54) with compound 167 has been found to give 168 in 50% ee. 

The coordination and organometallic chemistries of the lanthanoid metals are rapidly growing areas of research. Since most complexes are paramagnetic, routine characterization by NMR spectroscopic methods is not usually possible. Thus, compound characterization tends to rely on X-ray diffraction studies. The paramagnetic nature of lanthanoid complexes has, however, been turned to advantage in their application as NMR shift reagents (Box 25.4) and MRI contrast agents (Box 3.6). 

Complexes containing diastereotopic methylene protons have occasionally been described in which the anisochronous NMR-behaviour is notably augmented by the paramagnetism of a central f-metal ion. Two such examples are adducts of a shift-reagent like lanthanoid complex and suitable aliphatic amines (see Table 19), while another early example is represented by the uranocene derivative (ri -C8H7PEt2)U(ri "C8H8) [2]. 

The 9-coordinate complexes [Ln(DPA)3] (Ln is a general lanthanoid ion and H3DPA is pyridine-2,6-dicarboxylic acid) bind effectively to proteins and can be used as paramagnetic shift reagents for protein NMR spectroscopic studies. The structure of the [La(DPA)3] ion is shown below. 

Kahuto K, Sasaki K, Sasaki Y. Absolute configuration of aldonic acids and lanthanoid induced shift hy the chiral shift reagent propylenediauiinetetraacetatoeuropium(III) in aqueous solution. Tetrahedron Asymm. 1992 3 1357-1360. 

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