Enolate ions dimerization

The absence of dimerization in S l reactions indicates that it is not an important termination step (equation 8), which may be related to the low concentration of the radical R in the chain reaction. However, 17% of the dimerization product 1, l -biadamantyl was found in the reaction of 1-iodoadamantane with the less reactive carbanionic nucleophiles, such as acetone enolate ion in DMSO22. Likewise, under appropriate experimental conditions, the dimerization product 4,4 -dicyanobiphenyl (39%) was the principal product of the termination step in the electrochemical induced reaction of 4-chloroben-zonitrile with 2-pyridinethiolate ions in liquid ammonia23. 

Carbanions, such as the enolate ions of acetone, acetylmorpholine and others were un-reactive toward 1-AdI in liquid ammonia under photostimulation, and only the reduction product AdH and the dimer (1-Ad)2 were formed. It has been proposed that even though the 1-Ad radicals are formed under photostimulation, they do not couple with carbanions to give the S l product, at least at a competitive rate with other reactions121. 

A further evidence for the extreme reactivity of acetaldehyde with its enolate ion is provided by the fact that, when the aldol condensation is run in D2O, there is no substitution of D atoms on the a carbon positions. This is interpreted as indicating a much more rapid reaction of the enolate ion with acetaldehyde than with D2O, as might have been already anticipated from the relative slowness of the reaction with BH+. In contrast, the base-catalyzed exchange of acetone with D20 is much faster than the rate of dimerization of acetone. 

The enolate ion is nucleophilic at the alpha carbon. Enolates prepared from aldehydes are difficult to control, since aldehydes are also very good electrophiles and a dimerization reaction often occurs (self-aldol condensation). However, the enolate of a ketone is a versatile synthetic tool since it can react with a wide variety of electrophiles. For example, when treated with an unhindered alkyl halide (RX), an enolate will act as a nucleophile in an Sn2 mechanism that adds an alkyl group to the alpha carbon. This two-step a-alkylation process begins by deprotonation of a ketone with a strong base, such as lithium diisopropylamide (LDA) at -78°C, followed by the addition of an alkyl halide. Since the enolate nucleophile is also strongly basic, the alkyl halide must be unhindered to avoid the competing E2 elimination (ideal RX for Sn2 = 1°, ally lie, benzylic). 

An important general reaction of enolate ions involves nucleophilic addition to the electrophilic carbonyl carbon atom of the aldehyde or ketone from which the enolate is derived. A dimeric anion 18 results, which may then be neutralized by abstraction of a proton to produce a p-hydroxycarbonyl compound, 19 (Eq. 18.10). If the reaction is performed in hydroxylic solvents such as water or an alcohol, the source of the proton may be the solvent, whose deprotonation will regenerate the base required for forming the enolate ion. Thus, the overall process is catalytic in the base that is used. 

No significant difference in the rate of the polymerization was observed in the presence and in the absence of TMEDA at -20 ° C. It was assumed that the chelation only replaces the THF molecules in the dimeric enolate ion pair retaining the peripheral coordination with lithimn during propagation (Scheme 8). 

Vlhen the chiral methylation is carried out with 30% aqueous NaOH the indanone is deprotonated at the interface but does not precipitate as the sodium enolate (Figure 11). In this system there are 3 to 4 molecules of H2O per molecule of catalyst available while in the 50% NaOH reactions the toluene is very dry with only 1 molecule of H2O available per catalyst molecule thus forcing the formation of tight ion pairs. Solvation of the ion pairs in the toluene/30% NaOH system should decrease the ee which we indeed observe with an optimum 78% versus 94% in the 50% NaOH reaction. In the 30% NaOH reactions the ee decreases from 78% to 55% as the catalyst concentration increases from 1 mM to 16 mM (80 mM 5, 560 mM CH3CI, 20 C). Based on these ee s rates of formation of (-h)-enantiomer and racemic product can be calculated. When the log of these rates are plotted versus the log of catalyst concentrations (Figure 13) we find an order of about 0.5 in the catalyst for the chiral process similar to that found using 50% NaOH consistent with a dimer-monomer pre-equilibrium. The order in catalyst for the... 

The enzyme is a hexamer, actually a dimer of trimers made up of 291-residue polypeptide chains.28 Aceto-acetyl-CoA is a competitive inhibitor which binds into the active site and locates it. From the X-ray structure of the enzyme-inhibitor complex it can be deduced that the carboxylate group of E144 abstracts a proton from a water molecule to provide the hydroxyl ion that binds to the P position (Eq. 13-6, step a) and that the E164 carboxyl group donates a proton to the intermediate enolate anion in step b.28 The hydroxyl group... 

An ion-pair derived from the substrate and solid NaOH forms a cation-assisted dimeric hydrophobic complex with catalyst 39c, and the deprotonated substrate occupies the apical coordination site of one of the Cu(II) ions of the complexes. Alkylation proceeds preferentially on the re-face of the enolate to produce amino acid derivatives with high enantioselectivity. However, amino ester enolates derived from amino acids other than glycine and alanine with R1 side chains are likely to hinder the re-face of enolate, resulting in a diminishing reaction rate and enantioselectivity (Table 7.5). The salen-Cu(II) complex helps to transfer the ion-pair in organic solvents, and at the same time fixes the orientation of the coordinated carbanion in the transition state which, on alkylation, releases the catalyst to continue the cycle. 

This is caused by the two enantiomers of Nobin forming a very stable - but catalytically inactive - heterochiral dimer, thus removing the minor enantiomer of Nobin from the catalytic cycle and leaving the major enantiomer of Nobin to form a complex with the sodium ion and the enolate of substrate 11a. From a practical perspective, the non-linear effect means that amino adds with essentially identical enantiomeric excesses are obtained whether the Nobin catalyst is enantiomerically pure or has an enantiomeric excess of just 30%. Since Nobin is prepared as a racemate [26,27] and must be resolved [27,28] prior to use, this is a major synthetic... 

The even more strained alkylidene cycloproparenes gave rise to the same kind of G-complex intermediate with silver ion. In the presence of alcohol, trapping of this intermediate occurred, leading to alkoxystyrene derivatives. Water could also act in the same way, yielding arylmethylketones after keto-enol equilibration. However, if a proton was present on the alkylidene moiety, H shift occurred, leading to an arylalkyne. No dimerization was observed in this case, probably due to steric constraints in such a process (Scheme 3.17).31... 

The structure of mixed aggregates involving ester enolates is also of major interest to macromolecular chemists, since ionic additives are often introduced in the polymerization medium. The more stable arrangement between lithium 2-methoxyethoxide and MIB lithium enolate was thus calculated (at the DFT level) to be a 5 1 hexagonal complex with similar O—Li lateral coordinations212. The same team has recently extended this study to complexes formed between the same enolate in THF and a-ligands such as TMEDA, DME, 12-crown-4 and cryptand-2,1,1213. Only in the case of the latter ligand could a separate ion pair [(MIB-Li-MIB),2 THF]-, Li(2,l,l)+ be found as stable, still at the DFT level, as the THF solvated dimer [(MIB-Li)2,4 THF]. 

Dissociation of protonated dimers of acetaldehyde/ketone (homogenous or heterogenous clusters) leads to water loss [144,145], which is also the result of the corresponding ion molecule reactions [130]. Rearrangement of at least one of the reaction partners to the (protonated) enol seems likely. No comparable rates or barriers can be inferred from these data. 

The enolate and iminate ions of tetra-K-butylammonium salts of carbonyl compounds e.g. malonates) and nitriles e.g. 2-phenylpropionitrile) exhibit special dimeric molecular structures in the solid state and in solution (benzene), held together by multi-... 

Trimethylacetophenone forms a dimeric sodium enolate with two chelating TMEDA ligands (6). There is significant jt-electron interaction of the metal ion with the olefinic carbons but no evidence of jt-bonding with the aromatic rings. ... 

Both monomeric and aggregated species (e.g., open dimers in Bu OMe and triple ions in HMPA/ THF) are reactive. In related work in which aggregate formation was maximized, it was shown that the rates of enolization in the presence of the mixed aggregates are much lower and solvent dependent. The autoinhibition correlates with the relative stabilities of the mixed aggregates the stabilities do not, however, correlate in a straightforward marmer with the ligating properties of the solvent. 

Aliphatic amines are mainly converted to a-substituted products [99,100], whereby especially the a-methoxylation leads to valuable reagents for synthesis. The intermediate iminium salts can be directly trapped by silyl enol ethers to form Mannich bases [108]. If the a-position is blocked or steric conditions favor it, N,N coupling to hydrazo or azo compounds occurs (Table 5, numbers 17-19). 1,1-Disubstituted hydrazines are dimerized to tetrazenes in fair to excellent yields (Table 5, numbers 20-24). The intermediate diaze-nium ions can attack enolizable carbonyl compounds to form aza-Mannich bases [109]. Arylazonaphthols undergo anodic oxidation, producing radical cations. These couple to biphenylbisazo compounds (up to 34%) or can be trapped by anisidine to form azodiphe-nylamines (up to 74%) [110a]. 

Satisfactory to good yields of adducts have been found for styrenes [Eq. (21a), Y = phenyl], conjugated dienes (Y = vinyl), enamines (Y = NR2), and enol ethers (Y = alkoxy), particularly if they are unsubstituted at the 6-carbon atom to Y. Nonactivated alkenes react less satisfactorily. In the oxidation of anionized 1,3-dicarbonyl compounds (Table 11, numbers 1-8) at potentials between 0.6 and 1.4 V (SCE) and in the presence of butadiene, only the additive dimer LXII is obtained in the presence of ethyl vinyl ether only the disubstituted monomers LXVI or LXVII arise, but with styrene both types of products LXII and LXVI are formed. This result indicates that the primary adduct LXIII is oxidized rapidly between 0.6 to 1.4 V to the carbenium ion in the case of an ethoxymethyl radical (Y = OEt), and slowly in the case of an allyl radical (Y = vinyl). 

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