Alkenes lanthanide shift reagents

In 1975, it was reported that while lanthanide shift reagents could not be used directly to simplify the NMR spectra of alkenes, when coupled with silver salts substantial shifts could be induced.232 Since then, a number of studies have reported the use of both chiral and achiral lanthanide(III)-silver(I) binuclear shift reagents,233-237 where the ligands were generally fluorinated /3-diketones. 

Lanthanide shift reagents are hard Lewis acids and normally do not react with alkenes. But Eu(fod)3 in combination with silver heptafluorobutyrate [46] alters the spectrum of alkenes by a type of interaction shown in Fig. 10.19. 

Bimetallic complexes formed using a lanthanide tris (3-diketonate and silver 3-diketonate are useful shift reagents for soft Lewis bases such as alkenes, aromatics, phosphines, and halogenated compounds.Chiral lanthanide tris (3-diketonates of tfc and hfc are used in forming the binuclear reagents. A variety of silver 3-diketonates have been evaluated, but only the silver chelate of 6,6,7,7,8,8,8-heptafluoro-2,2-dimethyl-3,5-octanedione [Ag (fod)] is commercially available. The mixture forms a tetrakis chelate anion to which the silver is ion paired ([Ln (P-dik)4 ]Ag" "). In the bimetallic reagent, the silver binds to the soft Lewis base while the paramagnetic lanthanide ion causes perturbations in the chemical shifts that account for the enantiomeric discrimination. 

The ability of the coordinatively unsaturated tris(/3-diketonato)lanthanide complexes to expand their coordination sphere by adduct formation with Lewis bases is not only of importance for their application as NMR shift reagents (see section 9). This ability can also be nsed to prepare liquid stationary phases for gas chromatography, which are able to separate organic compounds on the basis of their nucleophilicity. Feibush et al. (1972) studied the retention of different alkanes, alkenes, alcohols, ketones and ethers by GC colttmns with solutions of [R(/9-diketonato)3] complexes in squalane as the stationary phase. Brooks and Sievers (1973) investigated by GC the interaction of ketones and alcohols with a stationary phase consisting of the NMR shift reagent [Eu(fod)3] in squalane. The alcohols were fotmd to form more stable complexes with [Eu(fod)3] than ketones. Picker and Sievers (1981) considered the use... 

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