Lanthanide metal enolates

The use of lanthanide metal enolates in the aldol reaction has, to date, only been developed to a synthetically useful level in the case of cerium (Scheme S and Table 7). Stereoselectivities are no better than those of lithium enolates, but the cerium enolates of ketones woik well in crossed aldol additions to ketones (Table 7, entries 1-7) and sterically hindered aldehydes (Table 7, entries 9 and 10). Such crossed aldol reactions do not often work well with lithium enolates as enolate equilibration, retroaldolization and steric retardation of addition occur. Imamoto et al. have shown that cerium enolates (44), formed from anhydrous CeCb (1.2 equiv.) and the preformed lithium enolates of ketones in THF at -78 C, undergo such aldol reactions to give the corresponding p-hydroxy ketones (46), usually in high yield. The cerium suppresses the retroaldol reaction by efficient chelation of the aldolate (45). A similar effect is known for zinc halide mediated aldol reactions (Volume 2, (Chapter 1.8). The stereoselectivity of the... 

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... 

An interesting novel coupling reaction of allenes with carbonyl compounds mediated by a lanthanide metal species was reported recently [80], The samarium(II) iodide-mediated reaction of various ketones or aldehydes 153 with methoxyallene (56) afforded exclusively y-addition products 4-hydroxy-l-enol ethers 154 in moderate to good yields with low cis/trans selectivity (Scheme 14.39). 

Although several research groups have been interested in transition metal enolates to use the metal centre as a potential site of asymmetry in the design of chiral catalysts, examples of well defined redox reaction involving middle to late transition elements and lanthanides are scarce in the literatnre. Based on Pearson s theory of hard and soft acids and bases", it has been proposed that combining a hard ligand with a soft late transition metal centre may lead to weak metal-heteroatom links, resnlting in reactive late metal-heteroatom bonds. 

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. 

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),... 

Oriented growth of lanthanide oxide thin films of Sm, Eu, Gd, Er and Yb on different substrates is possible by the low-pressure MOCVD technology from the corresponding phenanthroline adducts of rare earth metal enolates Ln(acac)3(phen) as described by Shiv-ashankar and coworkers (Table As expected, the films grown at lower... 

The postulated catalytic cycle of the asymmetric epoxidation reaction is shown in Figure 13.10. A lanthanide metal alkoxide moiety changes to a rare earth metal-peroxide through proton exchange (I). In this step, lanthanide metal alkoxide moiety functions as a Bronsted base. The rare earth metal-BINOL complex also functions as a Lewis acid to activate electron-deficient olefins through monoden-tate coordination (II). Enantioselective 1,4-addition of rare earth metal-peroxide gives intermediate enolate (III), followed by epoxide formation to regenerate the catalyst (IV). 

At the first step, the insertion of MMA to the lanthanide-alkyl bond gave the enolate complex. The Michael addition of MMA to the enolate complex via the 8-membered transition state results in stereoselective C-C bond formation, giving a new chelating enolate complex with two MMA units one of them is enolate and the other is coordinated to Sm via its carbonyl group. The successive insertion of MMA afforded a syndiotactic polymer. The activity of the polymerization increased with an increase in the ionic radius of the metal (Sm > Y > Yb > Lu). Furthermore, these complexes become precursors for the block co-polymerization of ethylene with polar monomers such as MMA and lactones [215, 217]. 

In Chapter 17 you saw epoxides acting as electrophiles in Sn2 reactions. They can be used to alkylate enolates providing epoxide opening is assisted by coordination to a Lewis acidic metal ion in this case the lanthanide yttrium(III). The new C-C bond in the product is coloured black. Note that the ketone starting material is unsymmetrical, but has protons only to one side of the carbonyl group, so there is no question over which enolate will form. The base is one of the LDA variants we showed you on p. 668—LHMDS. 

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