0-diketones enol form

FIGURE 5-1 Some common m/ramolecular H bonds. This page o-chlorophenol ethyl acetoacetatc (enol form) /3-diketones (enol form) salicylaldehyde. Facing page 5,8-dihydroxy-a-napthoquinone chloral hydrate oc-acetylamino acetamide. 

A 1 3 arrangement of two carbonyl groups (compounds called P diketones) leads to a situation m which the keto and enol forms are of comparable stability... 

Enolization (Sections 18 4 through 18 6) Aldehydes and ke tones having at least one a hydro gen exist in equilibrium with their enol forms The rate at which equilibrium is achieved is in creased by acidic or basic cata lysts The enol content of simple aldehydes and ketones is quite small p diketones however are extensively enolized... 

Because the stmcture of 1,3-diketones comprise a methylene group between two activating carbonyls, equiUbrium is shifted toward the enol form. The equihbrium distribution varies with stmcture and solvent (303,306) (Table 13). The enol forms are cycHc and acidic and form covalent, colored, soHd chelates with metals ... 

P-Diketone Chelates. P-Diketones, reacting as enols, readily form chelates with titanium alkoxides, Hberating in the process one mole of an alcohol. TYZOR AA [17927-72-9] (6) is the product mixture from TYZOR TPT and two moles of acetylacetone (acac) reacting in the enol form. The isopropyl alcohol is left in the product (87). The dotted bonds of stmcture (6) indicate electron... 

A recent paper by Singh et al. summarized the mechanism of the pyrazole formation via the Knorr reaction between diketones and monosubstituted hydrazines. The diketone is in equilibrium with its enolate forms 28a and 28b and NMR studies have shown the carbonyl group to react faster than its enolate forms.Computational studies were done to show that the product distribution ratio depended on the rates of dehydration of the 3,5-dihydroxy pyrazolidine intermediates of the two isomeric pathways for an unsymmetrical diketone 28. The affect of the hydrazine substituent R on the dehydration of the dihydroxy intermediates 19 and 22 was studied using semi-empirical calculations. ... 

IR spectra of PAN obtained from the CAN-1,3-diketone systems revealed that, besides the characteristic absorption band of CN group at 2244 cm , the characteristic absorption bands at 1727 and 1700 cm (keto form of 1,3-diketone) and 1625 cm (enol form of 1,3-diketone) were observed simultaneously. Therefore, the 1,3-diketone moiety is present as an end group in PAN. 

Thenoyltrifluoroacetone(TTA), C4H3S,CO,CH2,COCF3. This is a crystalline solid, m.p. 43 °C it is, of course, a /1-diketone, and the trifluoromethyl group increases the acidity of the enol form so that extractions at low pH values are feasible. The reactivity of TTA is similar to that of acetylacetone it is generally used as a 0.1-0.5 M solution in benzene or toluene. The difference in extraction behaviour of hafnium and zirconium, and also among lanthanides and actinides, is especially noteworthy. 

Diketones give furans when treated with acids. This is actually an example of an intramolecular addition of an alcohol to a ketone, since it is the enol form that adds ... 

The mild character of the reaction conditions is exemplified effectively here by the preparation of 2-acetyl-2-cyclohexen-l-one from 2-acetyIcyclohexanone. The crude product is initially isolated entirely in the less stable enedione form which is partially converted to the more stable enol form, 2-ace tyl-1,3-cyclohexadien-l-ol, during distillation at 45-55°. A series of a,(3-unsaturated j3-keto esters, -diketones, and a p-keto sulfoxide have also been prepared in the unenolized form by this... 

Mesityl oxide (Section 111,79) (I) condenses with ethyl malonate in the presence of sodium ethoxide to give the sodium derivative of (II) this upon hydrolysis with aqueous potassium hydroxide, followed by acidification, gives the cyclic diketone 5 5-dimethyl-1 3-cyclohexanedione (III), of which the enolic form is 5 5-dimethyldihydroresorcinol (IV) ... 

Many of the compounds that undergo ready base-catalysed keto i enol prototropic changes, e.g. / -keto esters, l,3-(/ -) diketones, aliphatic nitro compounds, etc., form relatively stable carbanions, e.g. (25), that can often be isolated. Thus it is possible to obtain carbanions from the keto forms of the /J-keto ester (23a) and nitromethane (24a) and, under suitable conditions, to protonate them so as to obtain the pure enol forms (23b) and (24i>), respectively. It thus seems extremely probable that their interconversion follows the intermolecular pathway (a). The more acidic the substrate, i.e. the more stable the carbanion to which it gives rise, the greater the chance that prototropic interconversion will involve the carbanion as an intermediate. 

An additional example of cycloamylose-induced catalysis which can probably be attributed to a microsolvent effect is the oxidation of a-hy-droxyketones to a-diketones (Scheme VIII). The rate of this oxidation is accelerated by factors ranging from 2.1 to 8.3 as the structure of the substrate is varied. As noted by Cramer (1953), these accelerations may be attributed to a cycloamylose-induced shift of the keto-enol equilibrium to the more reactive enol form. 

Another class of molecules related to the metalla-/3-diketonates are the metalla-B-diphosphinites, LnM(R2PO)2 (7-10). These anionic complexes can be protonated to give neutral enolic tautomers or coordinated to main group elements or metal ions to give polynuclear complexes. The metalla-/3-diketonate anions form the same types of complexes. 

The 0-diketones extremely rich in enolic form likewise fail to react. This lack of reactivity can be explained independently of the mechanism of reaction, as the great richness in enolic form indicates a high energetic stability and a lower chemical reactivity. 

Now let us look at the ease of forming the enolate anion nucleophiles. Ketones are more acidic than esters (see Section 10.7). Taken together, these factors mean the more favoured product is going to be the P-diketone (acetylacetone), formed from a ketone nucleophile by a Claisen reaction with an ester. This is the reaction observed. 

Conjugate addition can also be carried out by completely forming the nucleophilic enolate under kinetic conditions. Ketone enolates formed by reaction with LDA in THF react with enones to give 1,5-diketones (entries 1 and 2, Scheme 1.12). Esters of 1,5-dicarboxylic acids are obtained by addition of ester enolates to a,/J-unsaturated esters (entry 5, Scheme 1.12). 

Isoniazide, the hydrazide of pyridine-4-carboxylic acid, is still, well over half a century after its discovery, one of the mainstays for the treatment of tuberculosis. Widespread use led to the serendipitous discovery of its antidepressant activity. This latter activity is retained when pyridine is replaced by isoxazole. The requisite ester (45-4) is obtained in a single step by condensation of the diketo ester (45-1), obtained by aldol condensation of acetone with diethyl oxalate, with hydroxylamine. One explanation of the outcome of the reaction assumes the hrst step to consist of conjugate addition-elimination of hydroxylamine to the enolized diketone to afford (45-2) an intermediate probably in equilibrium with the enol form (45-3). An ester-amide interchange of the product with hydrazine then affords the corresponding hydrazide (45-5) reductive alkylation with benzaldehyde completes the synthesis of isocarboxazid (45-6) [47]. 

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