Cerium enolates

Cerium enolate complexes of type Cl2Ce(OCR=CHR) achieve higher yields in stoichiometric cross-aldol reactions of sterically crowded substrates than the corresponding lithium enolates (Scheme 26). The larger cerium is assumed to be more effective in the inital aldol chelate formation. Formation of oc-bromo-/ -hydroxyketones is also catalyzed [249]. 

Scheme 26. Cross aldol reaction mediated by a cerium enolate complex...

Aldol reaction. Quantitative yields of 1,2-adducts of alkyllithiums to ketones can be obtained at -65° in the presence of Cel,. Cerium enolates, formed by reaction of CeCl, with lithium enolates, also show enhanced reactivity in reactions with carbonyl compounds, particularly ketones. Yields of aldols are increased, but the stereoselectivity remains moderate. ... 

Pale yellow cerium dioxide (ceria, ceric oxide) has the fluorite structure and is used in catalysis" ", solid oxide fuel cells (SOFC)", thin film optical waveguides" , reversible oxygen storage materials for automobile catalysts" and for doping copper oxide superconductors". The diverse cerium enolate precursors and deposition methods used in the formation of cerium oxide thin films are summarized in Table 6, whereby the most common precursor for ceria is Ce(thd)4. 

Cerium enolates are generated by the reaction of lithium enolates with anhydrous cerium chloride in THF. The cerium enolates react readily with various aldehydes and ketones at -78 °C (Scheme 24). The yields are generally higher than in reactions of lithium enolates. This is presumably due tt> the relative stabilities of the adducts, that of the cerium reagent being greater by virtue of coordination to the more oxo lic cerium atom. The stereochemistry of the products is almost the same as in the case of lithium enolates, as shown in Table 4. The reaction is assiuned to proceed through a six-membeied chair-like transition state, as with lithium enolates. 

A synthetic application of this cerium chloride methodology has been reported by Nagasawa et al., as shown in Schenne 25. It is noteworthy that aldol reaction of the cerium enolate proceeds in high yields, even though the acceptor carbonyl group is sterically crowded and is readily enolized by lithium 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... 

Table 7 Aldol Reactions of Ketone Cerium Enolates with Aldehydes and Ketones (Scheme 5) ...

Cerium enolates may also be generated reductively in THF from oi-iodo or a-bromo esters in the presence of an aldehyde or ketone using cerium metal turnings and catalytic HgCk (or cerium amal-gam). This leads to a Reformatsky-type reaction (Volume 2, Chapter 1.7) producing the (3-hydroxy ester (47) as a mixture of diastereomers (equation 16). 

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