Enolates, ionic

Tokunaga and coworkers reported the enantioselective hydrolysis of enol esters (111) in the presence of catalyst 8b under phase-transfer conditions with aqueous KOH. The proposed mechanism of this reaction has the protonation of the ammonium-enolate ionic complex as the enantioselective step. Their achievement of the first nonbiomimetic asymmetric hydrolysis of esters catalysed by organocatalysts with high catalytic efficiency in buffer-free conditions has considerable potential to replace enzymatic resolutions in industrial processes (Scheme 16.41). ... 

The initial reaction between a ketene and an enamine is apparently a 1,2 cycloaddition to form an aminocyclobutanone adduct (58) (68-76a). This reaction probably occurs by way of an ionic zwitterion intermediate (75). The thermal stability of this adduct depends upon the nature of substituents Rj, R2, R3, and R. The enolic forms of 58 can exist only if Rj and/or R4 are hydrogens. If the enamine involved in the reaction is an aldehydic enamine with no 3 hydrogens and the ketene involved is di-substituted (i.e., R, R2, R3, and R4 are not hydrogens), then the cyclo-butanone adduct is thermally stable. For example, the reaction of dimethyl-ketene (61) with N,N-dimethylaminoisobutene (10) in isopropyl acetate... 

Whether or not the highly electropositive alkali metals or magnesium form an ionic instead of a covalent bond to the oxygen of the enolate is less important. Even if there is a contact ion pair of the metal cation and the oxygen anion, the geometry of the six-membered chair transition state, as outlined above, will be maintained. 

The tris(diethylainino)sulfoiiium difluorotrimethylsiliconate induced aldol addition of enolsi-lancs, which delivers predominantly syw-aldols independent of the cnolate geometry (sec p 1608), calls for another mechanistic model. A.n open transition state model has been proposed which assumes that the naked" ionic oxygens are as far apart as possible28. For (Z)-enolates, one transition state is favored over the diastereomeric orientation due to the avoidance of a repulsive R /CHj interaction. 

When a substituent is able to resonantly stabilize the positive charge of the ionic intermediate, there is no bromine bridging and the intermediate is an open P-bromocarbocation. These carbocations have been shown to occur in the bromination of a-methylstilbenes (ref. 9), 1 and 2, and of a variety of enol ethers (ref. 10) and acetates (ref. 11). 

A number of other methods exist for the a halogenation of carboxylic acids or their derivatives. Acyl halides can be a brominated or chlorinated by use of NBS or NCS and HBr or HCl. The latter is an ionic, not a free-radical halogenation (see 14-2). Direct iodination of carboxylic acids has been achieved with I2—Cu acetate in HOAc. " ° Acyl chlorides can be a iodinated with I2 and a trace of HI. Carboxylic esters can be a halogenated by conversion to their enolate ions with lithium A-isopropylcyclohexylamide in THF and treatment of this solution at -78°C with... 

It is believed that this reaction involves the formation of the a-stannyl ester. Metals such as lithium that form ionic enolates would be more likely to reverse the addition step. 

Cyclobutanes can also be formed by nonconcerted processes involving zwitter-ionic intermediates. The combination of an electron-rich alkene (enamine, enol ether) and an electrophilic one (nitro- or polycyanoalkene) is required for such processes. 

At the first step, the insertion of MMA to the lanthanide-alkyl bond gave the enolate complex. The Michael addition of MMA to the enolate complex via the 8-membered transition state results in stereoselective C-C bond formation, giving a new chelating enolate complex with two MMA units one of them is enolate and the other is coordinated to Sm via its carbonyl group. The successive insertion of MMA afforded a syndiotactic polymer. The activity of the polymerization increased with an increase in the ionic radius of the metal (Sm > Y > Yb > Lu). Furthermore, these complexes become precursors for the block co-polymerization of ethylene with polar monomers such as MMA and lactones [215, 217]. 

The determination of the lifetime of the ionic intermediates using the azide-clock method has been however useful in showing that electrophilic addition of Br2 can occur, even through a fully concerted mechanism, definable as SN2-like. Bromination of cyclic enol ethers (glycals) 8-10 in methanol in the presence of... 

The main conseqnences of the solvation are fonnd to be the increment in bond lengths between the enolate oxygen atom and the lithinm in the mono and the disolvated a) enolates, together with the increment in the Li-Osolvent bond. However, the trend con-tinnes np to trisolvated species 3b (Fignre 4), where the Li-O distance is fonnd to be less than that in isolated species. These characteristics of larger Li-dimethyl ether distance (dne to the steric hindrance) and the absence of coordination to the double bonds snggest an ionic interaction of Li with enolate oxygen. 

In the proposed mechanism (Scheme 9), the rate-determining step is the reaction between aldehyde and enolate. In the absence of a solvent, a major issue with this reaction is the typical low rate and the need for a high concentration of catalyst (usually DABCO). It was reported recently that, under basic conditions, the ionic liquid [BDMIM][PF6] is inert and that the Baylis Hillman reaction in [BDMIMjPFg proceeds smoothly with better yields than in [BMIMjPFg (163). 

The plausible mechanism of the reaction is shown in Fig. 25. The reaction probably proceeds through the activation of imine (formed in situ from the o-hydroxy benzaldehyde and the aromatic amine) by the catalyst followed by the addition and subsequent cyclization of the enol ether, resulting in the formation of the fused acetal. Ionic liquids are stable enough with amines and water and also effectively activate the imines to undergo cyclization. The recovered ionic liquid can be re-used with gradual decrease in the efficiency of the method. The hydro-phobic nature of the ionic liquid also helps in recovery of the catalyst. 

One of the most important factors for successful diastereoselection in chiral amide enolate alkylation reactions is the presence of strongly chelated ionic intermediates1 3. The chelation serves the purpose of locking the chiral auxiliary in a fixed position relative to the enolate. The metal counterion is chelated between the enolate oxygen and an additional polar group, anionic, carbonyl or ether oxygen attached to the chiral auxiliary. 

Smietana, M., and Mioskowski, C., Preparation of silyl enol ethers using (bistrimethylsilyl)acetamide in ionic liquids, Org. Lett, 7,1037-1039,2001. 

Similarly, siloxycyclopropanes (e.g., 3) react smoothly with mercuric acetate in methanol at 20 °C to give 3-mercurio ketones (e.g., 4) [8]. The attack of the metal occurs in such a way that the less alkyl-substituted bond is cleaved. Thus, starting from the enol silyl ether, the overall sequence represents a-mercurio-methylation of the parent ketone. The reaction is likely to proceed via an ionic intermediate Eq. (10). 

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