Binuclear lanthanide-silver shift reagents

VII. BINUCLEAR LANTHANIDE-SILVER SHIFT REAGENTS A. General Considerations... 

Binuclear lanthanide-silver chelates have also been used as shift reagents for com-ponnds with weakly basic heteroatoms that are part of jt systems and do not associate strongly with lanthanide ions. With the binuclear reagents, silver association with the Jt electrons leads to larger shifts. Substrates of this type include compounds such as... 

Finally, binuclear lanthanide(III)-silver(I) shift reagents are noteworthy. These form complexes with olefins, aromatic rings, halogenated saturated hydrocarbons, and phosphines. Due to the lack of polar groups, these functionalities do not give significant LIS with common mononuclear LSR. Applications of this binuclear technique have been reviewed261 for example, the Z- and E-isomers of 2-octene can be differentiated. 

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. 

Chiral lanthanide shift reagents for soft Lewis bases have been prepared by mixing a chiral lanthanide tris-P-diketonate such as Ln (facam) 3 or Ln(hfbc)3 with a silver P-diketonate such as Ag(fod) (fod = 2,2-dimethyl-6,6,7,7,8,8,8-heptafluorooctane-3,5 dione). Addition of the two reagents leads to the formation of a binuclear reagent as illustrated in Equation [3]. The binuclear reagent consists of an ion pair between a lanthanide tetrakis chelate anion and a silver cation. 

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 silver ion coordinates with the soft Lewis base and the lanthanide induces the shifts. Chiral binuclear reagents have been applied to chiral olefins and aromatics. Binuclear reagents formed using the dcm ligand are not effective for chiral resolution, presumably because this ligand is too sterically hindered to permit formation of the tetrakis chelate anion. Experimentation is often needed to find the correct combination of lanthanide and silver P-diketonate to cause enantiomeric resolution. 

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