Lanthanide complexes, enantiomeric determination

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... 

Alternatively, complexation with lanthanide shift reagents allow the signals of the MTPA ester to be resolved and used to determine enantiomeric... 

Similar differentiation between enantiomers by means of NMR can also be achieved by the use of chiral lanthanide shift reagents (243). Tris-[3-(heptafluoropropylhydroxymethylene)-d-camphorato] -europium was used for the first time (244) for determining the enantiomeric content of benzyl methyl sulfoxide 34. The enantiomeric composition of the partially resolved methyl p-tolyl sulfoxide 41 was estimated using tris-[3-(r-butylhydroxymethylene)-c -camphorato]-europium (245). Another complex of europium, tris-[3-(trifluoro-methylhydroxymethylene)-c -camphorato] europium (TFMC), in contrast to those mentioned above, was effective in the differentiation of various enantiomeric mixtures of chiral sulfinates (107), thiosul-finates (35), and sulfinamides (246). 

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. 

Progress has been made in the search for new lanthanide /3-diketonates as sources of luminescence, with application in the fabrication of polymer light-emitting diodes for low-cost, full-color, flat-panel displays.16,17 Moreover, some complexes seem promising as chiral NMR reagents for the determination of enantiomeric purity18,19 and prospective catalysts in many organic syntheses.20... 

The major composition of the Gd catalyst prepared from Gd(0-iPr)3 and ligand (7a) in a ratio of 1 2 was determined to be Gd/ligand = 2/3 by ESI-MS (electronspray ionization mass spectrometry) analysis [84a]. The mass value and the isotope distribution pattern matched well with the calculated values. NMR studies supported the formation of lanthanide cyanide. Free ligand (7a) was disilylated when treated with TMSCN. The relationship between the enantiomeric excess of the product and the ratio of Gd/ligand (Figure 13.6) was also consistent with these observations. The postulated mechanism is shown in Scheme 13.29. One of the Gd in the Gd/ligand (7a) =2 3 complex is speculated to work as Lewis acid to activate the ketone, and the other Gd center would work as a nucleophile. The two Gd centers work cooperatively to promote the reaction smoothly. 

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