Acetophenone zinc enolate

Zinc enolate chemistry is rich and was recently reviewed However, it is dominated by species with a-alkoxy groups, i.e. the enolate of carboxylic acid esters, and so these studies are not relevant in the current context. Acetophenone, 2-acetylthiophene and 3-acetylthiophene form enolate complexes with Zn and The degree of... 

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]. 

Trost and coworkers developed a chiral zinc phenoxide for the asymmetric aldol reaction of acetophenone or hydroxyacetophenone with aldehydes (equations 62 and 63) . This method does not involve the prior activation of the carbonyls to silyl enol ethers as in the Mukaiyama aldol reactions. Shibasaki and coworkers employed titanium phenoxide derived from a phenoxy sugar for the asymmetric cyanosilylation of ketones (equation 64). 2-Hydroxy-2 -amino-l,l -binaphthyl was employed in the asymmetric carbonyl addition of diethylzinc , and a 2 -mercapto derivative in the asymmetric reduction of ketones and carbonyl allylation using allyltin ° . ... 

The optimum approach to kinetic stereoselection in the Reformatsky reaction would appear to be the use of two-stage procedures, which allows the zinc aldolates to be formed at the lowest possible temperature. Gaudemar-Bardone and Gaudemar prepared a variety of zinc ester enolates in dimethoxymethane at 40 C which were then reacted at lower temperatures with benzaldehyde or with acetophenone (equation 38). Selected data from their study are shown in Table 5. If these data are the result of total kinetic control, as concluded by the authors, it is clear that the reactions exhibit only a modest kinetic stereoselectivity. 

---