Lanthanide metal enolates structure

To facilitate an analysis of enolate reactivity and as an aid to the rationalization of the stereochemical outcome of the aldol reaction, a consideration of the enolate structure is necessary.1 For convenience, the following classification of transition metal and lanthanide metal enolates will be used here T)1-0-bound metal enolates (1) of carbonyl compounds V-C-bound metal enolates (2) and T)3-metal enolates (3) of... 

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. 

Thus, lanthanide (Lu and Yb) and yttrium enolates, which were prepared from the lithium enolate of acetaldehyde and the corresponding dimeric dicyclopenta-dienyl metal chlorides [Cp2MCl2]2> were studied by NMR and IR spectroscopy, and clear evidence for the O-bound tautomer was provided in all cases. This was confirmed by a crystal structure of dimeric yttrium enolate 17 with a Y2O2 core unit > [51]. The chelate complex 18 of scandium, on the other hand, has been shown by a crystal structure to be monomeric. Here again, the metal is bound to the enolate oxygen atom (Scheme 3.7) [52]. 

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