Cerium complexes aldol reaction

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...

Steric effects of the hexacarbonyldicobalt moiety may be responsible for altered reactivity, notably enhanced stereoselectivity in reactions of adjacent substituents, e.g. formyl groups in aldol condensations. Thus, condensation of trimethylsilylpropy-nal with the silyl enol ether of cyclopentanone gives a 90% yield of the aldol product (eq 50) as a 40 60 erythro threo mixture, whereas reaction of the Co2(CO)6-complexed aldehyde followed by cerium(IV) oxidation gives the same total yield, but in an 87 13 diastereomeric ratio. ... 

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