Structure enolate anions

The CPop intermediate is the j5-cuprio ketone intermediate widely debated in mechanistic discussions of conjugate addition (cf. Scheme 10.3). On the basis of recent theoretical analysis, two limiting structures for CPop may now be considered these are shown in the bottom box in Scheme 10.5. The reason for the exceptional stability of CPop as a trialkylcopper(III) species can be readily understood in terms of the j5-cuprio(III) enolate structure, with the internal enolate anion acting as a strong stabilizing ligand for the Cu state [82]. 

This is an equilibrium reaction, and it raises a couple of points. First, there are two a-positions in the ketone, so what about the COCH3-derived enolate anion The answer is that it is formed, but since the CH3 group is not chiral, proton removal and reprotonation have no consequence. Racemization only occurs where we have a chiral a-carbon carrying a hydrogen substituent. Second, the enolate anion resonance structure with charge on carbon is not planar, but roughly tetrahedral. If we reprotonate this, it must occur from just one side. Yes, but both enantiomeric forms of the carbanion will be produced, so we shall still get the racemic mixture. 

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. ... 

Enolate Ion Formation This mechanism suggests that the reaction begins when the sodium methoxide (catalyst) attacks the acidic hydrogen from the carbon a to the carbonyl carbon to produce an enolate structure (Figure 4). This reaction produces a carbanion, which is a strong nucleophile. The enolate anion then reacts... 

Now let s look at the problem space that we just navigated in Figure 3.6 to see all the branches and where else we could have gone under basic conditions. Two new enol structures from the protonation of the oxygen anion are very reasonable dead ends for this problem. A Af/ calculation will show that the enol is uphill from the reactant. The decisions and operations that we went through to navigate this example proton transfer mechanism problem space are flowcharted in Figure 3.7. 

Recent general spectral stndies of such nitriles include the vibration spectra of 2-pyrazinecarbonitrile and a (> 99%)"N-isotopic version " the mass spectra of 2,3-pyrazinedicarbonitrile, its 5,6-diphenyl derivative, and 2,3,5,6-pyrazmetetracarboni-trile for comparison with those of analogous heterocyclic nitriles and the C NMR spectra of 2-pyrazinecarbonitrile and the like for correlation with their reactivities toward acetone enolate anions. The structure-activity relationship of pyrazinecarbonitriles as herbicides has been reported. ... 

The nucleophiles that can be considered as behaving in a similar way to stabilized enolates have the general structure 45. In general, the anion (enolate) is preformed by addition of base, often sodium hydride, to the pronucleophile 44. If the pK of the pronucleophile is sufficiently low, then the acetate leaving gronp can act as an in situ base.f Another conamon procedure is the use of bistrimethylsilyl acetamide (BSA) as a convenient method for deprotonation of the pronucleophile when allylic acetates are being nsed. The liberated acetate generates the base 47, which deprotonates the pronucleophile (Scheme 9). It is normal to add a small amount of acetate at the outset in order to start the process off. 

The crystal structure of the following complex reveals the Pd-C bond lengths shown. The bond lengths to carbons C and D are nearly identical, so this alkene has a symmetrical rj -structure. However, the bond lengths to carbons A and B are significantly different. Nucleophilic attack by alkoxide anions, enolates, and amines occurs at carbon A only, supporting the notion that slipping the alkene enhances its susceptibility to nucleophilic attack. 

The synthesis of a series of Cu(I) enolate complexes supported by 1,10-phenanthroline has been reported. Their structures consist of an unusual combination of one cationic Cu(I) centre ligated by two 1,10-phenanthroline ligands and one free anionic enolate unit. The reactivity of these complexes and the mechanism of a-arylation reactions through a Cu(in) enolate intermediate have been described. 

The alkylation reactions of enolate anions of both ketones and esters have been extensively utilized in synthesis. Both very stable enolates, such as those derived from (i-ketoesters, / -diketones, and malonate esters, as well as less stable enolates of monofunctional ketones, esters, nitriles, etc., are reactive. Many aspects of the relationships between reactivity, stereochemistry, and mechanism have been clarified. A starting point for the discussion of these reactions is the structure of the enolates. Because of the delocalized nature of enolates, an electrophile can attack either at oxygen or at carbon. 

Reduction of a conjugated enone to a saturated ketone requires the addition of two electrons and two protons. As in the case of the Birch reduction of aromatic compounds, the exact order of these additions has been the subject of study and speculation. Barton proposed that two electrons add initially giving a dicarbanion of the structure (49) which then is protonated rapidly at the / -position by ammonia, forming the enolate salt (50) of the saturated ketone. Stork later suggested that the radical-anion (51), a one electron... 

How many different enolates may arise from deprotonation of 2,4-pentanedione Draw Lewis structures for each, and predict which is likely to be the most stable. Check your conclusions by examining the energies of the different possible enolates (enolate A, B...). Is the most stable enolate that derived from deprotonation of the most electron-poor hydrogen Compare the electrostatic potential maps of the anions with each other and with your Lewis structures. Revise your drawings to be consistent with the maps. Why is one of the enolates preferred over the others ... 

Tire tautomerism and ionization of xanthosine (21), a 9-substituted xanthine, have been studied by IR spectroscopy in aqueous solution [83MI(2)231].Tlie diketo structure 21 was shown to exist below pH 5, and the 2-enolate anion 22 at neutral and slightly basic pH. 

Only in 1961 did Woodward and Olofson succeed in elucidating the true mechanism of this interesting reaction by making an extensive use of spectroscopic methods. The difficulty was that the reaction proceeds in many stages. The isomeric compounds formed thereby are extremely labile, readily interconvertible, and can be identified only spectroscopically. The authors found that the attack by the anion eliminates the proton at C-3 (147) subsequent cleavage of the N—0 bond yields a -oxoketene imine (148) whose formation was established for the first time. The oxoketene imine spontaneously adds acetic acid and is converted via two intermediates (149, 150) to an enol acetate (151) whose structure was determined by UV spectra. Finally the enol acetate readily yields the W-acyl derivative (152). 

Q Base removes an acidic hydrogen from the a position of the carbonyl compound, yielding an enolate anion that has two resonance structures. 

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