Zirconium and Other Metal Enolates

Earlier studies had demonstrated that such enolates would participate in aldol condensations with aldehydes however, the stereochemical aspects of the reaction were not investigated (68). For the cases summarized in Table 25, the zirconium enolates were prepared from the corresponding lithium enolates (eq. [54]). Control experiments indicated that no alteration in enolate geometry accompanies this ligand exchange process, and that the product ratio is kinetically controlled (35). From the cases illustrated, both ( )-enolates (entries A-E) and (Z)-enolates (entries F-H) exhibit predominant kinetic erythro diastereoselection. Although a detailed explanation of these observations is clearly speculative, certain aspects of a probable 

Kinetically Controlled Aldol Condensations of Zirconium Enolates with Benzaldehyde 

Entry Substrate Enolate Ratio Z E Erythro-Threo Product Ratio Yield (%, Zr case) Ref. 

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Combination of achiral enolates vith achiral aldehydes mediated by chiral ligands at the enolate counter-ion opens another route to non-racemic aldol adducts. Again, this concept has been extremely fruitful for boron, tin, titanium, zirconium and other metal enolates. It has, ho vever not been applied very frequently to alkaline and earth alkaline metals. The main, inherent, dra vback in the use of these metals is that the reaction of the corresponding enolate, vhich is not complexed by the chiral ligand, competes vith that of the complexed enolate. Because the former reaction path vay inevitably leads to formation of the racemic product, the chiral ligand must be applied in at least stoichiometric amounts. Thus, any catalytic variant is excluded per se. Among the few approaches based on lithium enolates, early vork revealed that the aldol addition of a variety of lithium enolates in the presence of (S,S)-l,4-(bisdimethylamino)-2,3-dimethoxy butane or (S,S)-1,2,3,4-tetramethoxybutane provides only moderate induced stereoselectivity, typical ee values being 20% [177]. Chelation of the ketone enolate 104 by the chiral lithium amide 103 is more efficient - the j5-hydroxyl ketone syn-105 is obtained in 68% ee and no anti adduct is formed (Eq. (47)) [178]. 

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