Metal enolates spectroscopy

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. 

L. -J. Ming, in Physical Methods in Bioinorganic Chemistry, Spectroscopy and Magnetism, Chap. 8 (Ed. L. Que, Jr.), University Science Books, Sausalito, 2000 T. J. Wenzel and K. A. Provencher, in The Chemistry of Metal Enolates, Chap. 14 (Ed. J. Zabicky), John Wiley Sons, Ltd, Chichester, 2009. 

Metal enolates found varied application in chemical analysis. An outstanding group are certain lanthanide enolates used as shift reagents in NMR spectroscopy. The analytical methods discussed in Section IV are based on formation of a metal enolate for separation, detection, identification and determination of metal ions or the use of a metal enolate as ancillary reagent to improve analytical quality. Of special relevance in analytical chemistry are the metal /3-diketonates, M(dik) , derivatived from deprotonated /3-diketones (dikH),... 

A few comments concerning the crystallization of carbanions are in order. These comments are based upon the personal experience developed in our own laboratory and also upon observations noted in the literature in the course of crystallizing enolate anions. Although alkali metal enolate anions are relatively unstable compounds, they have been prepared in the solid state, isolated, and characterized by IR and UV spectroscopy in the 1970s. Thus the ot-lithiated esters of a number of simple esters of isobutyric acid are prepared by metallation of the esters with lithium diisopropylamide in benzene or toluene solution. The soluble lithiated esters are quite stable at room temperature in aliphatic or aromatic hydrocarbon solvents and are crystallized out of solution at low temperature (e.g. -70 °C.). Alternatively the less soluble enolates tend to precipitate out of solution and are isolated by centrifugation and subsequent removal of the solvent. Recrystallization from a suitable solvent can then be attempted. The thermal stability of the lithiated ester enolates is dramatically decreased in the presence of a solvent with a donor atom such as tetrahydrofuran. 

For the synthesis of 2-519, the amines 2-516 were first treated with AlMe3 in benzene at r.t. and after addition of the enol acetates 2-515, easily accessible from 2-513 and 2-514, heated under reflux. Mechanistic investigations using on-line NMR spectroscopy, reveal that a metalated amide 2-517 is formed first. This then leads to a N-acyliminium ion 2-518 which undergoes an electrophilic substitution. Overall, three new bonds are formed selectively in the domino process, and the alkaloid scaffolds 2-519 are provided in very good yields of 79-89%. Interestingly, use of the keto esters 2-513 instead of 2-515 did not lead to the desired products 2-519. 

Birch reduction of the chiral benzamide 5 generates the amide enolate 6 (Scheme 4). This enolate has been characterized by NMR spectroscopy and by an extensive examination of the effects of changes in alkali metal, solvent, reaction... 

Triketones are homologues of 1,3-diketones and in these, too, keto-enol tautomerism has been probed by H NMR spectroscopy.567,568 Triketones and tetraketones may coordinate to one or more metal ions per molecule. In the latter case the metal centers are held in such close proximity that, in some cases, interesting magnetic effects may be observed. Structural and magnetic properties of polynuclear transition metal jS-polyketonates have been thoroughly reviewed.569,570... 

Free ligands have been studied in order to obtain an insight into their structure, both in solution and in the solid state, and for comparison with their metal complexes. H NMR spectroscopy has been used to investigate the keto-enol equilibrium and the nature of the hydrogen bonds. In the case of optically active Schiff bases UV and CD spectra provided information about structure in solution. The Schiff bases that have been most widely examined are derivatives of acetylacetone, salicyl-aldehyde and hydroxymethylenecamphor, whose prototypes with en are shown in Figure 13. 

The structural feature of the O-metal bond has a substantial consequence that holds for carbonyl compounds with nonidentical substituents in the a-position the configurational isomerism with respect to the carbon-carbon double bond giving rise to cis- or frans-enolates 8 (Scheme 1.3). This diastereomerism was recognized in the early stage of enolate research by NMR spectroscopy [10,11] and later impressively confirmed by crystal structure analyses [12]. Chemists learned to generate cis- or frans-enolates selectively and to handle them under conditions that prohibited them from cis-trans isomerization. In an early, fundamental work... 

Transmetallation of the lithium or potassium enolates is also a reliable method for the preparation of palladium and nickel enolates, as illustrated in Scheme 2.50. Clear evidence for the C-bound structure of enolates 172 and 173 thus prepared was provided by NMR spectroscopy and - for nickel enolate 172 (M = Ni, L = Cp ) - by a crystal structure analysis. The reaction of C-bound nickel and palladium enolates 172 and 173 with aldehydes is much more sluggish and much less uniform than the analogs of that of the polar main-group metals. In addition to P-hydroxyketones or esters, products resulting from a Tishchenko reaction were also observed [164b]. 

The cleavage of silyl enol ethers by Lewis acidic metal salts may also be categorized as a transmetallation. Kuwajima and coworkers have shown that silyl enol ethers react with titanium tetrachloride under formation of the corresponding titanium enolates 174, which were characterized by NMR spectroscopy (Scheme 2.51). Whereas (Z)-enolsilanes of acyclic ketones retain their configuration in the titanium enolates, the ( )-silyl enol ethers lead to a mixture of... 

In both tin(II) and tin(IV) enolates, it is in general assumed that the metal is bound to the oxygen atom [44], based on NMR spectroscopy and the stereochemical outcome of their aldol additions. However, it should be pointed to early studies of Pereyre et al. [45] who identified mixtures of O- and C-bound tautomers in the case of tin(IV)-derived ketone enolates. Their ratio depends on the individual substituents and conditions. For carboxylic esters, the C-bound tautomers even predominate as a-stannyl esters [46]. These early experimental results were more recently confirmed by ab initio calculations that resulted in only very small energy difference between the tautomers in case of the enolate of acetophenone, whereas the equilibrium is distinctly on the side of the C-bound tautomer for the methyl acetate enolate (Scheme 3.6) [47]. As typical for tin(IV) compounds, the metal... 

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