Diketones complexes with shift reagents

Europium(III), and particularly ytterbium(III) shift reagents, induce downfield proton resonance shifts while the praseodymium(III) analogs cause upfield shifts. Lanthanide chelates of fluorinated /3-diketonates are more soluble in organic solvents, and they form more stable association complexes with donor molecules, than do LSRs with nonfluorinated ligands. Thus Eu(fod)3 is the preferred achiral LSR for weak nucleophiles89. 

Under this section are considered lanthanide complexes of /3-diketones and their adducts, but work specifically in the area of lanthanide shift reagents is dealt with in Section 39.2.9. Of course, the majority of lanthanide shift reagents are lanthanide /3-diketonates, and when they function as shift reagents they do so by forming adducts in solution. Furthermore, interest in shift reagents has directly stimulated a considerable amount of fundamental research on lanthanide /3-diketonates and their adducts. Much of this fundamental work was, therefore, carried out in the early 1970s. AH this means that the division between this section and Section 39.2.9 is not entirely clear cut. 

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). 

Metal /3-diketonates are often coordinatively unsaturated see Coordinative Saturation Unsaturation) and will therefore react with Lewis bases to form complexes. For example, the trimer [Ni(acac)2]3 will react with Lewis bases such as water or pyridine to form monomeric trans-[Ni(acac)2L2]. The formation of weak complexes with lanthanide /3-diketonates is critical in their use as NMR shift reagents. 

Stable metal complexes can be favorably formed when a bidentate metal-binding site is available, such as a- and -diketone moieties which are the tautomeric forms of a- and /3-ketoenols. Some /S-diketonate complexes of paramagnetic lanthanides such as Pr(III), Eu(III) and Yb(III) have been extensively utilized as paramagnetic shift reagents for structural assignment of molecules with complicated stereochemistry prior to 2D techniques in NMR spectroscopy. Their syntheses and application are discussed in separate chapters in this volume. The examples below provide some dynamic and structural basis for better understanding of metal enolates in biomolecules and biochemical processes. 

The utilization of metal /3-diketonate complexes as organic-soluble NMR shift reagents was first explored in the mid-sixties with Ni(II) and Co(II) complexes of 2,4-pentanedione [(Ni(acac)2 and Co(acac)2] . Donor ligands such as triarylphosphines, isonitriles, pyridine V-oxides and picoline V-oxides bind to the metal ion. Relatively small shifts were observed in the NMR spectrum of the bound donor groups. These shifts were the results of contact (through-bond) and pseudocontact (dipolar or through-space) effects. 

The complex formed by extraction of uranium(IV) from an acidic aqueous solution by benzoyltrifluoroacetone in carbon tetrachloride is also reported to be a useful shift reagent, the shifts induced being to lower frequency. (4) Adducts with complexes formed from uranium(IV) and other fluorinated diketones are also mentioned, but no details of shifts are given. 

By the reaction of theenolatc of camphor with carboxylic acid esters or chlorides, 1,3-diketones [better formulated as enols. such as (hydroxymethylene)camphor] are obtained. When trifluo-roacetic acid or heptafluorobutanoic acid are used, the corresponding diketones (abbreviated as tfc or hfc, respectively) have been successfully used as ligands for lanthanides and these are used as chiral shift reagents in NMR spectroscopy12. The complex Eu(hfc)3 [derived from ( + )-camphor)] 3 was used as a chiral catalyst for enantioselective Diels-Alder-type cycloadditions of aldehydes to dienes (Section D.l.6.1.1.1.2.4.). 

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