Tautomerism 1,3-diketones

Why must the diketone tautomerize to the keto-enol before the addition of EtOH can occur By the principle of microscopic reversibility, both enol ether formation and enol ether hydrolysis must proceed by the same sequence of steps (only in reverse of one another). Protonation of the enol ether, the first step in its hydrolysis, would certainly occur on the carbonyl O, not on C, so enol ether formation must proceed through the same intermediate. 

The enediol-monoesters initally formed by 1,4-addition of aldehydes to a-diketones tautomerize to the more stable keto-form so that the observed products are a-acyloxyketones. In reaction of camphorqui-none 15>130)j the acyloxy group had the more stable, endo configuration... 

Why must the diketone tautomerize to the keto-enol before the addition of EtOH can occur Actually, it doesn t have to do, but remember that the rate of a reaction is directly related to the stability of the carbocation intermediate. The carbocation derived from the keto-enol is much lower in energy than the carbocation derived from the diketone, so it is more likely to survive long enough for the EtOH to add. 

Cyclic 1,2-diketones demonstrate enolic tautomerism, with solvent polarity affecting tautomeric equilibrium ... 

PurpurogaHin (5), a red-brown to black mordant dye, forms from electrolytic and other mild oxidations of pyrogaHol (1). The reaction is beHeved to proceed through 3-hydroxy-(9-benzoquinone (2) and 3-hydroxy-6-(3,4,5-trihydroxyphenyl)-(9-benzoquinone (3). The last, in the form of its tautomeric triketonic stmcture, represents the vinylogue of a P-diketone. Acid hydrolysis leads to the formation of (4), foHowed by cyclization and loss of formic acid... 

The mechanism of this reaction is generally understood to consist of subsequent Claisen condensation reactions to produce an intermediate diketone 12, which readily tautomerizes to the fully conjugated dihydroxythiophene 13. ... 

The carbanions derived from acylthiophenes have been condensed with aldehydes,and, through the Claisen condensation with esters, thienylsubstituted -diketones have been obtained. 2-Thenoyl trifluoroacetone, first prepared by Reid and Calvin through the Claisen condensation of 2-acetylthiophene with ethyl trifluoracetate, has become an extremely useful chelating agent for the extraction of numerous elements from strongly acidic solutions, The tautomeric form which dominates in aqueous solution is the ketone hy-drate. Other thiophenes have also proved useful for analytical purposes. ... 

Syntheses from Phthalimidines Syntheses from Ortho-Disubstituted Benzenes Condensation of 1,4-Diketones with Amines and Pyrroles Other Reactions and Rearrangements Leading to Isoindoles IV. Properties of the Isoindole System A. Tautomerism... 

Bis-acceptor-substituted diazomethanes are most conveniently prepared by diazo group transfer to CH acidic compounds either with sulfonyl azides under basic conditions [949,950] or with l-alkyl-2-azidopyridinium salts [951] under neutral or acidic conditions [952-954]. Diazo group transfer with both types of reagents usually proceeds in high yield with malonic acid derivatives, 3-keto esters and amides, 1,3-diketones, or p, y-unsaturated carbonyl compounds [955,956]. Cyano-, sulfonyl, or nitrodiazomethanes, which can be unstable or sensitive to bases, can often only be prepared with 2-azidopyridinium salts, which accomplish diazo group transfer under neutral or slightly acidic reaction conditions. Other problematic substrates include amides of the type Z-CHj-CONHR and P-imino esters or the tautomeric 3-amino-2-propenoic esters, which upon diazo group transfer cyclize to 1,2,3-triazoles [957-959]. 

Both D-[l- C]xylose and D-[5- C]arabinose were exposed to a concentrated phosphate buffer solution (pH 6.7). 1-Hydroxy-2-propanone (ace-tol) was distilled from the heated solution. Radioassay indicated that similar labeling [3- C] occurred in the acetol from both pentoses, with loss of the configurational difference thus, a 3-ketopentose or its enediol was suggested as an intermediate. Further work with 3-0- and 6-0-methyl-D-glucose and with 1-0-methyl-D-fructose indicated that /3-elimination from a 3-ketose or, in the case of a hexose, from a 3-ketose or a 4-ketose, or both, tautomerization of the resulting a-diketone to a /3-diketone, and hydrolytic cleavage are essential steps in the formation of acetol. 

Diketones usually exist as mixtures of tautomeric keto and enol forms. The enolic form does not show the normal absorption of conjugated ketones. Instead, a broad band appears in the 1640-1580 cm-1 region, many times more intense than normal carbonyl absorption. The intense and displaced absorption results from intramolecular hydrogen bonding, the bonded structure being stabilized by resonance. 

The constant for reaction (31) for /3-diketones in tautomeric keto (HK) and enol (HE) forms, can be partitioned between Kk and KE (reactions 32 and 33). At equilibrium, equations (34) and (35) hold, and values obtained are given in Table 19. Kinetics showed that the reaction occurs through the enol form by parallel acid-independent and inverse-acid paths (Scheme 15).514 K... 

S-Diketones, by virtue of their keto-enol tautomerism (Scheme 21), are weak acids and in most cases bind nickel(II) in the deprotonated enol form, acting as uninegative chelating ligands. 

Triketones are homologues of 1,3-diketones and in these, too, keto-enol tautomerism has been probed by H NMR spectroscopy.567,568 Triketones and tetraketones may coordinate to one or more metal ions per molecule. In the latter case the metal centers are held in such close proximity that, in some cases, interesting magnetic effects may be observed. Structural and magnetic properties of polynuclear transition metal jS-polyketonates have been thoroughly reviewed.569,570... 

Unsymmetrical /i-dikctoncs can form two /i-kcto—cnol tautomers, (90a), (90b). The corresponding N//-pyrazolcs—readily synthesized from the diketones—exhibit annular tautomerism, (91a), (91b). These tautomerisms have been probed via AMI semiempirical calculations that show that the two phenomena are related 135 in each case the position of equilibrium is strongly influenced by whether or not the CC double bond is part of (another) ring system (the Mills-Nixon effect). 

Diketones are easily reduced electrochemically because of the mutual electron-withdrawing effect of one carbonyl group on the other. Two-electron reduction usually takes place to afford an enediolate (62), which can then react with an electrophile E present in the medium (equation 31 see also Schemes 10 and 13) an alpha-hydroxy ketone is formed by subsequent tautomerization when E = H1,85,86. An unusual exception to the generalization that these substances undergo two-electron reduction is the electrochemical behavior of acenaphthenequinone (63), which is converted to the expected enediol dibenzoate (64) in the presence of aroyl chlorides but affords the dimeric substance 65 (shown to be the meso diastereomer by X-ray crystallography) in the presence of acetic... 

Finally, [Method (c)], treatment of an internal alkyne with potassium permanganate under PTC conditions leads to a good yield of a 1,2-diketone.146 The reaction probably proceeds via hydroxylation of the triple bond, tautomeric rearrangement to the a-ketol, and oxidation to the diketone. The reaction is illustrated by the conversion of 1-phenylpent-l-yne to l-phenylpentane-1,2-dione (Expt 5.101). 

---