Enediol dianion

The solution12 is to carry out the reaction in the presence of M SiCl. This does two things. The more obvious is that the enediol dianion 51 is trapped as the silyl enol ether 56, a useful intermediate, but the more important thing is the removal of the basic ethoxide ions as the neutral silyl ether EtOSiMe3. 

On quenching the reaction with acid, this dianion is protonated twice to give the enol of an a-hydroxy-ketone, and it is this a-hydroxy-ketone that is the final product of the acyloin reaction. The yield in this example is a quite respectable 70%. However, in many other cases, this usefulness of the acyloin reaction is hampered by the formation of by-products that arise because of the reactivity of the enediolate dianion. It is, of course, quite nucleophilic, and is likely to be formed in the presence... 

These side reactions can be minimized by adding trimethylsilyl chloride to the reaction mixture as an alkoxide scavenger. This traps the enediolate dianion as a bis-silyl enol ether, and traps the sodium or potassium alkoxides, which are catalysts for the Dieckmann ring closure, as neutral silyl ethers." The resultant bis-siloxy cycloalkenes are either isolated or converted in situ to a-hydroxy ketones by alcoholysis or by acid hydrolysis. ... 

Robinson, J. M., Flynn, E. T., McMahan, T. L., Simpson, S. L., Trisler, J. C., Conn, K. B. Benzoin enediol dianion and hydroxide ion in DMSO a single electron transfer reduction system driven by the irreversible benzilic acid rearrangement. J. Org. Chem. 1991, 56, 6709-6712. 

The products of an acyloin coupling reaction are an enediolate dianion and alkoxide ions. In the presence of the alkoxide the enediolate may be oxidized to the corresponding diketone by molecular oxygen. It is therefore important that, even when the coupling is complete, all reaction mixtures should be worked up in an oxygen-free atmosphere so long as the mixture remains alkaline. For reactions performed in liquid ammonia, all the ammonia should be allowed to evaporate before the reaction is worked up. 

In the presence of TMS-Cl the enediolate dianion and, importantly, the alkoxide ions, are trapped as their neutral silyl ethers (Scheme 5). This leads to much improved yields of the coupled product the acyloin is isolated in the form of its silyl enediol ether (3). Work-up is much easier. It is only necessary to filter the solution, evaporate the solvent, and isolate the product by distillation or chromatography. The TMS-Cl should be purified by distillation from calcium hydride, under a nitrogen or argon atmosphere, before use. A convenient procedure when using an organic solvent is to add the ester and the TMS-Cl together, dropwise, to the alkali metal finely dispersed in the solvent, at a rate sufficient to maintain the reaction. An explosion has been reported where this procedure was not followed. For a reaction conducted in liquid ammonia the TMS-Cl is added at the end of the reaction and after all the ammonia has been allowed to evaporate. Particularly in cases where sodium-potassium alloy has been used, a pyrophoric residue may have formed, so that the filtration must be carried out under an inert atmosphere. 

In principle the acyloin coupling of acyl chlorides, in which the enediolate dianion is trapped by unreacted acyl chloride, might be an effective alternative to trapping with TMS-Cl, but the method has rarely been used. Good yields of the enediol diesters (6) were obtained with several fatty acid chlorides using sodium in ether, and the enediol diesters could easily be hydrolyzed to give the acyloins. ... 

Russell has reported the preparation of bicyclo[3,2,0]hept-2-en-6,7-semidione (582). Reaction of the bicyclo[2,2,l]heptene acyloin (583) with potassium t-butoxide in DMSO gives (582). The rearrangement may occur via the enediol dianion, as the bicyclo[2,2,l]heptene semidione itself, generated by an alternative route, is stable. The methylated bicyclo[2,2,l]heptene acyloins (584a) and (584b) both yielded mixtures of the possible bicydo[3,2,0]heptene semidiones (585) and (586) on treatment with butoxide in DMSO. The rearrangement was shown to be reversible, since the same mixture of semidiones was formed from l-methylbicyclo[3,2,0]hept-2-en-6,7-acyloin (587). 

Semidiones exist in equilibrium with the a-dione and the enediol dianion, equation 2. Low temperatures and ion-pairing favor the diamagnetic components of... 

The presence of the carhoethoxy group in facilitates the rearrangement. This is quite reasonable if the rearrangement proceeds via the enediol dianion. 

Bicyclic—monocyclic valence isomerization of a radical dianion in the bicyclo[3 1.0]hexanesemidione system has been demonstrated. Symmetrical 1,4-semldiones formally derived from cycloheptatriene and cyclobctatetraene perfer to exist in the bicyclic (4.1.0 and 4.2.0) structures. Bicyclic—monocyclic valence isomerization in the bicyclo[4.1.0]hept-3-ene-2,5-dione system occurs more readily for the dianion than for the radical anion. Several radical anions derived from the (CH)6 io annulenes are reported. In the case of the 1,2-oxygenated derivatives of (CH)io the dianions or radical anions are stable, but the diones undergo valence isomerization and under oxida-time conditions are converted to 4-hydroxynaphthalene-1,2-semiquinone. Enediol dianions in the bicyclo [2.2 l]hepta-2,5-dione and l-carbQalkoxybicyclo[3 2.1] octa-2,6-diene systems have been observed to undergo... 

The pH-rate profile for unbuffered hydrolysis of glyceraldehyde-3-phosphate (6-3-P) has been attributed to hydrolysis of the monoanion of the phosphate monoester at pH < 4, spontaneous formation of glyceraldehyde from the phosphate dianion at pH 7-8, and, at higher pH, hydroxide-catalysed methylglyoxal formation. Reaction of the dianion is not subject to a solvent isotope effect and is believed to occur by the irreversible ElcB mechanism whereby an enediolate intermediate, formed on rate-determining C(2) deprotonation, subsequently expels phosphate trianion by C—0 bond breaking. The diethylacetal and 2-methyl-G-3-P do not hydrolyse under the same conditions.5... 

Ivanov Reagents are carboxylate enolates (enediolates, or carboxyl 0 acid dianions) derived from phenyl acetic acid or... 

The so-called acyloin condensation consists of the reduction of esters—and the reduction of diesters in particular—with sodium in xylene. The reaction mechanism of this condensation is shown in rows 2-4 of Figure 14.51. Only the first of these intermediates, radical anion C, occurs as an intermediate in the Bouveault-Blanc reduction as well. In xylene, of course, the radical anion C cannot be protonated. As a consequence, it persists until the second ester also has taken up an electron while forming the bis(radical anion) F. The two radical centers of F combine in the next step to give the sodium glycolate G. Compound G, the dianion of a bis(hemiacetal), is converted into the 1,2-diketone J by elimination of two equivalents of sodium alkoxide. This diketone is converted by two successive electron transfer reactions into the enediolate I, which is stable in xylene until it is converted into the enediol H during acidic aqueous workup. This enediol tautomerizes subsequently to furnish the a-hydroxyketone—or... 

In the initial step of the reductive coupling, electron transfer from Na or K to the diester leads to a di(radical anion) species (two ketyl radical anions), which then combine. Elimination of alkoxide produces the 1,2-diketone, which is further reduced to an enediolate by transfer of two electrons. Protonation of the dianion with dilute aqueous acid leads to the enediol, which tautomerizes to the a-hydroxy ketone. ... 

There are currently two proposed mechanisms for the acyloin ester condensation reaction. In mechanism A the sodium reacts with the ester in a single electron transfer (SET) process to give a radical anion species, which can dimerize to a dialkoxy dianion. Elimination of two alkoxide anions gives a diketone. Further reduction (electron transfer from the sodium metal to the diketone) leads to a new dianion, which upon acidic work-up yields an enediol that tautomerizes to an acyloin. In mechanism B an epoxide intermediate is proposed. ... 

The kinetic product of an acyloin coupling is the enediolate, which may be formed in (Z)- and (El-forms, and is accompanied by some of the thermodynamic product, the corresponding a-hydroxy enol-ate. Each of these dianions is trapped by the TMS-Cl. The products, and the proportions in which they are formed, for the reactions of ethyl ethanoate and diethyl hexane-1,6-dioate are shown in Scheme 6. Use of dichlorodimethylsilane in conjunction with the reduction of ethyl ethanoate gave the acyclic... 

The studies of Ban and Wakamatsu culminated in the preparation of three natural compounds from a single synthetic route (Scheme 1.15). The enediol bis silyl ether 63 was converted to the dianion and immediately alkylated with l-iodo-3-butanol to give glycol 64 as a mixture of diastereomers in 87% yield. Diol fragmentation with lead tetraacetate afforded keto lactone 65 in quantitative yield. Formation of the dithioketal and subsequent Raney nickel desulfurization then gave 66 (81%). Macrocyclic lactone 66 is the simple natural product... 

D-isomer to be monitored by enzyme-linked oxidation of NADH. The same technique was used to monitor the formation of dihydroxyacetone phosphate. The major reaction on formation of the enediolate, however, is loss of phosphate to give the enol of methylglyoxal. The formation of the enediolate is independent of pH between pH 6 and 10 because of intramolecular proton abstraction by the phosphate dianion. Above pH 10 and below pH 6, the pH-rate profile has a gradient of +1.0, in the former case a reflection of direct attack by OH on the phosphate dianion, in the latter a reflection of the proportion of reactive dianionic form of the substrate present. 

A theoretical study in 2000 concluded that while the oxocarbon dianions C 0 (n = 3-5) all favor ) /, symmetry, the QOg" ground state has C2 symmetry, and all isomeric structures are very close in energy, e.g. the energy difference is 0.34 kcal mor between the and nonplanar C2 forms [47]. Notably the measured dimensions of the relatively unstable CeOg" species in 24 and 25 conform to idealized Dch and C2v molecular symmetry, corresponding to respective valence tautomeric structures that manifest nonbenzenoid aromatic and enediolate character (Figure 8.53). 

The resulting dianion, known as an enediolate (ene = double bond, diol = double alcohol, ate = anion), is protonated during work-up to give an a-hydroxyketone as the final product ... 

Regardless of the details of the mechanism, the product prior to neutralization is the dianion of the final a-hydroxyketone, namely, a enediolate. It has been found... 

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