Zinc ketone enolates structured

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... 

But also simple ketone enolates without a chelating a-substituent were found to exist under the form of the O-bound tautomers. This is clearly evidenced by crystal structures of bis(amidoamine)-complexed zinc enolates 49 and 50 derived of diisopropyl ketone and acetophenone, respectively. They feature carbon-carbon double bonds, and their core unit is formed by a square or parallelogramshaped Zu202 skeleton. NMR studies revealed related structures for zinc enolates of isobutyrophenone, cyclohexanone, and 2,2-dimethylcyclopentanone (Scheme 3.16) [83]. The mononuclear enolate 51 of methyl mesityl ketone wherein zinc is chelated by TMEDA has the O-bound structure [84a] as well as mixed alkali zinc enolates derived of the same ketone [84]. 

Kimura and co-workers have synthesized a series of alkoxide complexes with the alcohol functionality as a pendent arm.447 674 737 A zinc complex of l-(4-bromophenacyl)-l, 4,7,10-tetraaza-cyclododecane was also synthesized by the same workers to mimic the active site of class II aldolases. The X-ray structure shows a six-coordinate zinc center with five donors from the ligand and a water molecule bound. The ketone is bound with a Zn—O distance of 2.159(3) A (Figure 12). Potentiometric titration indicated formation of a mixture of the hydroxide and the enolate. Enolate formation was also independently carried out by reaction with sodium methoxide, allowing full characterization.738... 

Tetranortriterpenoids.—The structure of sendanin (72), from the bark of a Japanese variety of Melia azedarach, has been confirmed by AT-ray analysis. The novel enol-ether (73) has been isolated from the heartwood of Khaya anthotheca along with 11/3- and lla-acetoxyazadirone (74) and (75). Zinc-copper couple is a very convenient reagent for reduction of epoxides, a,/S-epoxy-lactones, a/S-unsaturated ketones, and a-ketols. The full details of the X-ray analysis of prieurianin have appeared. " ... 

Huorinated etiolates are generally difficult to form. Ishihara and coworkers used fluorovinyl phosphates, which can be prepared from a-fluoro ketones and sodium diethyl phosphite. Reaction of these fluorinated enol phosphates with a reagent prepared from lithium aluminum hydride (LiAIH4> and cop-per(II) bromide, zinc(II) chloride, tin(II) chloride or bromine afforded the enolate (Scheme 34).The reaction of the enol phosphate with the reagents mentioned above suggests that the metal cation of the enolate is an aluminum species, though its actual structure is not known at present. 

Compound 136 can be transformed to the 3,4-trans isomer 138, which has the same configuration as ( + ) thienamycin, according to the procedure described by Chiba and coworkers in related structures [74]. The method involved oxidation of the hydroxyl group in 136 with activated manganese dioxide with concomitant isomerization at C-3, followed by K-selectride reduction of the resulting methyl ketone 137. Zinc enolates of iV,iV-disubstituted a-aminoacid esters have also been recently utilized for the synthesis of 3-amino P-lactams [75]. 

Under the conditions used for the generation of silyl enol ethers of symmetrical ketones, unsymmetrical ketones give mixtures of structurally isomeric enol ethers, with the predominant product being the more substituted enol ether (eq 20). Highly hindered bases, such as lithium diisopropylamide (LDA), favor formation of the kinetic, less substituted silyl enol ether, whereas bro-momagnesium diisopropylamide (BMDA) generates the more substituted, thermodynamic silyl enol ether. A comhination of TMSCl/sodium iodide has also been used to form silyl enol ethers of simple aldehydes and ketones as well as from a,p-unsaturated aldehydes and ketones. Additionally, treatment of a-halo ketones with zinc, TMSCl, and TMEDA in ether provides... 

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