Ammonium enolate

Dimerization of methylketene is catalyzed by an amine, trimethylsilylquinine, to give the P-lactone enantioselectively (Scheme 27) [129]. The catalyst amine attacks the ketene to form an ammonium enolate, an electron donating alkene. The donor is strong enough to react with a ketene across the C=0 bond. That is why the P-lactone is obtained instead of the 1,3-cyclobutandione, the uncatalyzed dimerization product of the monosubstituted ketene. 

Enol sulfonates. Sulfonylation of enolate anions with 1 is markedly affected by the gegenion. Lithium enolates undergo mainly C-sulfonylation. Cesium or quaternary ammonium enolates undergo regioselective O-sulfonylation. The same behavior is observed with nonailuorobutanesulfonyl fluoride.1... 

With regards to studies on the stereoselective functionalization of prochiral glydne Dpm amide derivative 22, Maruoka and coworkers found that chiral ammonium enolate generated from lg and 22 had an ability to recognize the chirality of P-branched primary alkyl halides, which provides impressive levels of kinetic resolution during the alkylation with racemic halide 29, allowing for two a- and y-stereocenters of 30 to be controlled, as exemplified in Scheme 5.16 [22]. 

The Muzart group reported an organocatalytic protonation reaction based on an in situ-formation of the required enolate by photochemical tautomerization of the chiral ammonium enolate 26 as an initial step [21]. The ammonium ion in 26 functions as the chiral proton source. Subsequent esterification affords the desired car-boxylate 20 in up to 65% yield and enantioselectivity in the range 40-85% ee. An example is shown in Scheme 9.8. The best results were obtained by use of the secondary, N-isopropyl-substituted aminobornanol for formation of the chiral ammo-... 

Schiff base 52 in one-pot under mild phase-transfer conditions. For example, the initial treatment of a toluene solution of 52 and (S,S)-32e (1 mol%) with allyl bromide (1 equiv.) and CsOHH20 at —10 °C, and the subsequent reaction with benzyl bromide (1.2 equiv.) at 0 °C, resulted in formation of the double alkylation product 53 in 80% yield with 98% ee after hydrolysis. Notably, in the double alkylation of 52 by the addition of the halides in reverse order, the absolute configuration of the product 53 was confirmed to be opposite, indicating intervention of the chiral ammonium enolate 54 at the second alkylation stage (Scheme 4.17) [50]. 

Similarly, benzyltrimethylammonium fluoride cleaves trimethylsilyl enol ethers to give quaternary ammonium enolates and Me3SiF (Equation Si3.5). 

Starting from /l-ketoesters (Figure 10.4, see Figure 10.22 for a synthetic application), or /l-ketoaldehydes (Figure 10.5), ammonium enolates are formed with a stereostructure ( -enolates) that differs from that of the corresponding alkaline earth metal enolates (Z-enolates). In the latter, the lithium atom forms a bridge between the two neg-... 

Accordingly, trimethylsilyl enol ethers are enolate precursors (Figure 10.16). Fortunately, they can be prepared in many ways. For instance, silyl enol ethers are produced in the silylation of ammonium enolates. Such ammonium enolates can be generated at higher temperature by partial deprotonation of ketones with triethylamine (Figure 10.18). The incompleteness of this reaction makes this deprotonation reversible. Therefore, the regioselectivity of such deprotonations is subject to thermodynamic control and assures the preferential formation of the more stable enolate. Consequently, upon... 

A short O- - -H distance (1.9 A) was found by Boche and coworkers for the weak ammonium enolate (p.Kbh = 24.3 in acetonitrile) derived from r-butyl a-acetoacetate237, and recently, clearly well-shaped crystals of 1,3-cyclohexanedione lithium enolate (LiCHD) solvated by two molecules of methanol or 2,2,2-trifluoroethanol have been isolated by slow evaporation of a methanol solution of LiCHD. In the aggregate pattern, both oxygens have intramolecular contacts at all available syn or anti lone-pair positions (Figure 2)224. 

Removal of the amine moiety in piperazine derivatives in acidic medium " is thought to occur from the enolic form of the ammonium salt, at least at very low pH values. However,. step b has not been interpreted entirely from the mechanistic point of view, as far as the participation of ammonium/enolate structures is concerned. Several cyclic transition states have been envisaged, some of which involve the presence of a hydroxy group or the enolic form of the P-aminoketone. In particular, the participation of OH groups belonging to other molecules, such as carboxyacids present in the reaction medium, cannot be excluded. " By contrast, a nucleophilic attack by an acid anion on the carbonyl C atom has been claimed to be the rate-determining step for deamination in the reaction between p-aminoketones and triethyl phosphite in dimethylformamide. ... 

This chapter is primarily concerned with keto-enol equilibrium and the chemistry of dissociated enols (enoxides) and lithium enolates. The acid-base aspects of the chemistry of other metal enolates (e.g. silyl enol ethers "", boron enol ethers" " " , aluminium , tin " , gallium , bismuth , zinc " ", rhodium , palladium " , manganese ", copper , nickel , magnesium " , titanium " , molybdenum , zirconium" " and ammonium" " enolates) have been reported elsewhere. 

Bandermann and coworkers studied the MMA polymerization initiated by 37 . Whenever the initiator is contaminated by traces of diethyl 2-ethylmalonate, an inhibition period is observed, which was accounted for by a transfer reaction of the propagating enolate to diethyl 2-ethylmalonate. The polymerization cannot start until this contaminant is completely consumed. Consistently, no inhibition is observed when the initiator is highly pure. Finally, a Hoffman elimination in the propagating ammonium enolate (38) could occur with formation of dead chains (39) tri-n-butylamine and 1-butene (equation 43) . 

In 1982, Wynberg and coworkers discovered the cinchona alkaloid catalyzed enantioselective aldol lactonization of ketenes with chloral or trichloroacetone [35], in which the zwitterionic acyl ammonium enolate provides the carbon nucleophile. This work is probably one of the most important early contributions to enantioselective organocatalysis [36], One drawback associated with this process is the severe substrate limitations. The aldehydes should be highly reactive, presumably due to the relatively limited nudeophilicity of ammonium enolates. Nelson and coworkers first addressed the scope and reactivity problems associated with Wynberg s original protocol by combining a cinchona alkaloid derivative (O-trimethylsilylquinine (12) or O-trimethylsilylquinidine (13)) with a metal Lewis acid as a cocatalyst to... 

Lectka and coworkers first demonstrated that the ammonium enolates 18 generated from acid chlorides in the presence of a cinchona alkaloid catalyst (BQn, 19) and... 

Cinchona alkaloids such as 121 possess a nucleophilic quinuclidine structure and can act as versatile Lewis bases to react with ketenes generated in situ from acyl halides in the presence of an add scavenger. By acting as nucleophiles, the resulting ketene enolates can react intermolecularly [53] or intramolecularly [54] with electrophilic C=0 or C=N bonds to deliver formal [2 + 2]-cycloadducts, such as chiral P-lactones or [1-lactams, via aldol (or Mannich)-i intramolecular cydization sequence reactions (Scheme 8.46). The nucleophilic ammonium enolate can also read with energetic... 

The Baylis-Hillmann reaction is another bench-mark reaction in which ionic liquids have been successfully tested. The catalytic cycle of the Baylis-Hillmann reaction is reported in Figure 7. The catalyst is a highly nucleophilic tertiary amine, generally DABCO, or a tertiary phosphine, which adds to the oc,p-unsaturated electrophile in a 1,4 fashion to deliver an enolate which, in turn, adds to the aldehyde. The critical step is now a proton transfer from the enolisable position to the oxygen atom this process is catalysed by an alcohol which plays the role of a proton shuttle between the two foregoing positions. Once a P-ammonium enolate is formed, a rapid P-elimination takes place, delivering the Baylis-Hillmann condensation product. 

One arm of the imidazolium scaffold contains the catalytic centre, a bridgehead nitrogen atom possessing the required nucleophilicity, the second arm contains a Broensted acidic primary alcohol capable to speed up the critical proton transfer step which leads to the P-ammonium enolate intermediate, direct precursor of the final Baylis-Hillmann product. The reaction of RiCHO and CH2=CH-R2 is carried out under solvent free conditions at room temperature, catalyst 10 can be readily recovered from the reaction mixture and reused for at least 6 times without significant loss of catalytic activity. A few results are reported in Table 3. 

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