Carbonyl compounds tautomerization

Protonation of the oxygen end of the enolate forms an enol. Recall from Section 11.9 that enols are unstable and tautomerize (by a two-step process) to carbonyl compounds. Tautomerization forms the same 1,4-addition product that results from protonation on carbon. 

In mordant dyes, phenols, naphthols, and enolizable carbonyl compounds, such as pyrazolones, are generally the couplers. As a rule, 2 1 metal complexes are formed ia the afterchroming process. A typical example of a mordant dye is Eriochrome Black T (18b) which is made from the important dyestuff iatermediate nitro-l,2,4-acid, 4-amiQO-3-hydroxy-7-nitro-l-naphthalenesulfonic acid [6259-63-8]. Eriochrome Red B [3618-63-1] (49) (Cl Mordant Red 7 Cl 18760) (1, 2,4-acid — l-phenyl-3-methyl-5-pyrazolone) is another example. The equiUbrium of the two tautomeric forms depends on the nature of the solvent. 

Tetrazole, l-(p-substituted phenyl)-antimicrobial activity, 5, 835 Tetrazole, 5-thio-rearrangements, 5, 823 Tetrazole, 2-thioacyl-reactions, 5, 109 Tetrazole, 5-(o-tolyl)-tautomerism, 5, 804 Tetrazole, 5-(p-tolyl)-dipole moments, 5, 795 tautomerism, 5, 804 Tetrazole, 5-(trimethylsilylamino)-synthesis, 5, 832 Tefrazolecarbaldehydes reactions, 5, 820 Tetrazole-5-carbaldehydes reactions, 5, 820 Tetrazolecarbonitriles reactions, 5, 820 Tetrazole carbonyl compounds reactions, 5, 820 Tetrazolecarboxylic acid, 5-aryl-acidity, 5, 816... 

The aminoketone 1, required as starting material, can be obtained by a Neber rearrangement from a A -tosylhydrazone. Another route to a-aminoketones starts with the nitrosation of an a-methylene carbonyl compound—often in situ—to give the more stable tautomeric oxime 7, which is then reduced in a subsequent step to yield 1 ... 

A carbonyl compound with a hydrogen atom on its a carbon rapidly equilibrates with its corresponding enol (Section 8.4). This rapid interconversion between two substances is a special kind of isomerism known as keto-enol tautomerism, from the Greek Canto, meaning "the same," and meros, meaning "part." The individual isomers are called tautomers. 

Carbonyl compounds are in a rapid equilibrium with called keto-enol tautomerism. Although enol tautomers to only a small extent at equilibrium and can t usually be they nevertheless contain a highly nucleophilic double electrophiles. For example, aldehydes and ketones are at the a position by reaction with Cl2, Br2, or I2 in Alpha bromination of carboxylic acids can be similarly... 

In azo couplings with carbonyl compounds, three tautomeric products are possible, compared with only two for phenols and aromatic amines (discussed in Section 12.1). The ketohydrazone 12.75 is most often dominant, but for easily enolizable 1,3-dicarbonyl compounds (X=CO-R and similar structures) the azoenol 12.76 is the major product. The azoketone 12.77 is often postulated as primary product, but has rarely been identified in an unambiguous fashion using modern methods. The CH2 group should be easily detectable in the lH NMR spectrum. 

A very common form of tautomerism is that between a carbonyl compound containing an a hydrogen and its enol form " ... 

The isomerization of allylic alcohols provides an enol (or enolate) intermediate, which tautomerizes to afford the saturated carbonyl compound (Equation (8)). The isomerization of allylic alcohols to saturated carbonyl compounds is a useful synthetic process with high atom economy, which eliminates conventional two-step sequential oxidation and reduction.25,26 A catalytic one-step transformation, which is equivalent to an internal reduction/oxidation process, is a conceptually attractive strategy due to easy access to allylic alcohols.27-29 A variety of transition metal complexes have been employed for the isomerization of allylic alcohols, as shown below. 

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]. 

Hydroxymethylenecyclopropanols (340) have been shown" to be intermediates in the photochemical rearrangement of a, -unsaturated carbonyl compounds (339) to 1,4-dicarbonyl compounds (341). The products are eventually obtained by double tautomerization of the enol and cyclopropanol portions of (340). 

Primary and secondary nitroso compounds tautomerize to isonitroso compounds - oximes of aldehydes and ketones, respectively. Their reductions are dealt with in the sections on derivatives of carbonyl compounds (pp. 106,132). 

N-Metalated azomethine ylides generated from a-(alkylideneamino) esters can exist as tautomeric forms of the chelated ester enolate (Scheme 11.8). On the basis of the reliable stereochemical and regiochemical selectivities described below, it is clear that the N-metalated tautomeric contributor of these azomethine ylides is important. Simple extension of the above irreversible lithiation method to a-(alkylideneamino) esters is not very effective, and cycloadditions of the resulting lithiated ylides to a,(3-unsaturated carbonyl compounds are not always clean reactions. When the a-(alkylideneamino) esters bear a less bulky methyl ester moiety, or when a,(3-unsaturated carbonyl compounds are sterically less hindered, these species suffer from nucleophihc attack by the organometalhcs, or the metalated cycloadducts undergo further condensation reactions (81-85). 

An example where this abstraction from the carbonyl compound competes with photocycloaddition was discussed in the previous section (p. 312). In that case, 2-methylbenzophenone underwent intramolecular hydrogen abstraction, yielding the tautomeric enol,46 a process which completely inhibited the photocycloaddition reaction.37... 

Many such activated acyl derivatives have been developed, and the field has been reviewed [7-9]. The most commonly used irreversible acyl donors are various types of vinyl esters. During the acylation of the enzyme, vinyl alcohols are liberated, which rapidly tautomerize to non-nucleophilic carbonyl compounds (Scheme 4.5). The acyl-enzyme then reacts with the racemic nucleophile (e.g., an alcohol or amine). Many vinyl esters and isopropenyl acetate are commercially available, and others can be made from vinyl and isopropenyl acetate by Lewis acid- or palladium-catalyzed reactions with acids [10-12] or from transition metal-catalyzed additions to acetylenes [13-15]. If ethoxyacetylene is used in such reactions, R1 in the resulting acyl donor will be OEt (Scheme 4.5), and hence the end product from the acyl donor leaving group will be the innocuous ethyl acetate [16]. Other frequently used acylation agents that act as more or less irreversible acyl donors are the easily prepared 2,2,2-trifluoro- and 2,2,2-trichloro-ethyl esters [17-23]. Less frequently used are oxime esters and cyanomethyl ester [7]. S-ethyl thioesters such as the thiooctanoate has also been used, and here the ethanethiol formed is allowed to evaporate to displace the equilibrium [24, 25]. Some anhydrides can also serve as irreversible acyl donors. 

Oxetanes can be formed by intramolecular reaction between a carbonyl group and an alkene, and this has been used (4.74) in making analogues of thromboxane A, (one of the compounds responsible for the control of blood clotting), albeit usually as the minor product. A special case of intramolecular reaction is seen for a,p-unsaturated carboxylic acids 14.75), where the product is an oxete that is tautomeric with a p-lactone. Oxetes may also be formed by photocycloaddition of ketones or aldehydes with alkynes the oxete normally ring-opens at room temperature to give an a,p-unsaturated carbonyl compound (4.76), but at lower temperatures its spectral... 

The Tautomerism of Heterocycles J. Elguero etal., Adv. Heterocycl. Chetn., Suppl. 1,1976. Ring-Chain Isomeric Transformations of Hydroxy-, Amino-, and Mercapto-Derivatives of Carbonyl Compounds and Their Heteroanalogues R. Valters, Russ. Chem. Rev. Engl. Transl.), 1974, 43, 665-678. 

Cyanuric acid exists in two tautomeric forms corresponding to keto-enol tautomerism in carbonyl compounds. The keto form predominates, and most of the reactions of cyanuric acid have their counterparts in the chemistry of the cyclic imides. Many of the reactions involve the replacement of all three imido hydrogens (Scheme 31). Usually, the reaction cannot be controlled to produce the mono- or di-substituted isocyanurates specificially, but there are exceptions, e.g. the reaction between cyanuric acid and aziridine (Scheme 31) (B-79MI22001, 63JOC85, 63AHC(2)245). 

Hydrolytic replacement of a fluorine attached to a C = C bond by a hydroxy group leads to the hydroxy compound or its tautomeric carbonyl compound. For example, 2,3,3,4,4,4-hexa-fluoro-l-(phenyldiazenyl)but-l-ene (7) is hydrolyzed in acidic media to give the phenyl-hydrazone in a mixture with its tautomer.13... 

Water is solvent for all compounds except diphenylmethane, in which case ethyl ether is used. Ionization constants for carbonyl compounds and nitriles are gross acid constants, uncorrected for tautomerism. 

The treatment of 1,2-diols with Dess-Martin periodinane may lead either to a 1,2-dicarbonyl compound,14 or to an oxidative breakage of a C-C bond14,72 depending on stereoelectronic factors. When a 1,2-dicarbonyl compound is obtained, very often, one of the carbonyl groups tautomerizes to the enol form. Under controlled conditions, very often, it is possible to selectively oxidize one of the alcohols in a 1,2-diol, particularly when this alcohol is an allylic one.73... 

Isomer stabilities and activation energies have been calculated for keto-enol tautomerization of simple carbonyl compounds, MeC(R)=X (X = O R = H, Me) 129 both specific and bulk solvent effects have been analysed. Related isomerizations of acid derivatives (R = F, CN) and other related structures (R = H X = CF12, NH, S) are compared. 

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