Zinc ketone enolates

An alternative method for the preparation of a kinetic zinc ketone enolate (123) from an arene thiol ester 121 and bis(iodozincio)methane (122) in the presence of a palladium(O) catalyst was developed by Matsubara and coworkers (equation 36) . The modest reactivity of the zinc reagent 122 makes this transformation highly chemo- and regioselective neither isomerization of the kinetic enolate 123 nor a palladium-catalyzed coupling with the thiol ester 121 could be observed. Thus, treatment of zinc enolate 123 with various aldehydes or ketones led regioselectively to one aldol product 124. The method provides access to reactive functionalized zinc enolates which are otherwise hard to obtain. 

Recently, Shchepin and coworkers described Michael addition reactions with zinc ketone enolates (144) for the preparation of chroman-2-one derivatives 146. The enolates were generated in situ from a-bromo ketones 143 by addition of excess zinc (equation 40), and subsequent conjugate addition to a, S-unsaturated ketones (145) (equation 40 ) provided chroman-2-ones (146a, b) in yields up to 82% . 

Although zinc ketone enolates are generally considered to exist with an oxygen-bound rather than a carbon-bound metal, there is still some controversy. Boersma assumes that they contain both zinc-carbon and zinc-oxygen bonds (Scheme 58). One indication that they contain only an oxygen-metal bond in solution is that the NMR data for zinc enolates are similar to the data for alkali metal enolates. ... 

Zinc ester enolates may also be obtained by the addition of ZnX2 to lithium or sodium enolates as first described by Hauser and Puterbaugh (equation 6)P This approach has so far received little attention but similar reactions have been used to obtain zinc ketone enolates. In this regard, it should be noted that Heathcock and coworkers have shown that deprotonation reactions of ketones with zinc dialkylamide bases reach equilibrium at only about 50% conversion (equation 7). This result implies that attempts to prepare zinc enolates from solutions of amide-generated lithium enolates will be successful only when the lithium enolate is made amine-free. 

No crystallographic structure has been reported for zinc ketone enolates. Although carbon-bonded structures analogous to (5) have been proposed, spectral data obtained for the bromozinc and ethylzinc... 

Chelated structures analogous to (19) and (20) were first proposed by House and coworkers to explain the increased anti selectivity observed for lithium ketone enolates following addition of ZnCh (equation 30). Heathcock and coworkers determined the rate of equilibration as well as the equilibrium composition for a number of aldolates derived from benzaldehyde and zinc ketone enolates (equation 31). Again, the preference for anti aldolates is in accord with zinc-chelated structures. 

The preference of zinc ester enolates for carbon-bonded structures and zinc ketone enolates for oxygen-bonded structures is reminiscent of the situation with silicon. A carbon-bonded structure (9) is the thermodynamically more stable form for the trimethylsilyl derivatives of esters, while the oxygen-bonded structure (10) is the more stable form for ketone derivatives. This has been attributed to the greater resonance stability of ester compared to ketone carbonyls.32... 

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