Enolization capability

E is the so-called enol constant and measures the enolization capability of the diketo form ( = 1 for ethyl acetoacetate by definition). Thus, the so-called desmotropic constant L is a measure of the enoHzation power of the solvent. By definition, the values of L are equal to the equilibrium constants of ethyl acetoacetate E = 1), determined in different solvents [24]. This desmotropic constant seems to be the first empirical solvent parameter. It describes the relative solvation power of a solvent for diketo and enol forms of 1,3-dicarbonyl compounds. It was measured only for a few solvents and was soon forgotten. 

Methods that involve C—C bond formation with the establishment of two new stereogenic centers are of considerable interest in this context. The aldol reaction has proven very useful in this regard, particularly in view of die development of powerful chiral enolates capable of controlling the stereochemical course of reactions with chiral aldehydes via the principle of double asymmetric synthesis. ... 

It IS important to recognize that an enol is a real substance capable of mdepen dent existence An enol is not a resonance form of a carbonyl compound the two are constitutional isomers of each other... 

Enolates can also serve as carbon nucleophiles in carbonyl addition reactions. The addition reaction of enolates with carbonyl compounds is of very broad scope and is of great synthetic importance. Essentially all of the enolates considered in Chapter 7 are capable of adding to carbonyl groups. The reaction is known as the generalized aldol addition. 

Isolated double bonds do not interfere with this reaction sequence, but other ketones (saturated or a, -unsaturated) will also form enol acetates, which in turn are capable of further reaction with 7V-iodosuccinimide. 

C-20 enol acetates react with peracids in the same manner as their C-17(20) counterparts, giving a 20,21-epoxide capable of cleavage to the 21-hydroxy-20-ketone ... 

Because the pK s of the aldehyde and water are similar, the solution contains significant quantities of both the aldehyde and its enolate. Moreover, their reactivities are complementary. The aldehyde is capable of undergoing nucleophilic addition to its carbonyl group, and the enolate is a nucleophile capable of adding to a carbonyl group. And as shown in Figure 18.4, this is exactly what happens. The product of this step is an alkoxide, which abstracts a proton from the solvent (usually water or ethanol) to yield a (3-hydroxy aldehyde. A compound of this type is known as an aldol because it contains both an aldehyde function and a hydroxyl group (aid + ol = aldol). The reaction is called aldol addition. 

Which tautomer is lower in energy, acetone or propen-2-oll Use equation (1) to calculate the equilibrium distribution of the two at room temperature. If an experiment is capable of detecting concentrations as low as 1 % of the total, would you expect to observe both keto and enol forms of acetone at room temperature ... 

When a solution of magnesium methoxide (prepared by the reaction of magnesium with methanol) is saturated with carbon dioxide, an active carboxylating agent, MMC, is produced. The reagent carboxylates substrates capable of enolization apparently by promoting formation of the magnesium chelate of the a-adduct. The reaction has been... 

Ferrocen-l,l -diylbismetallacycles are conceptually attractive for the development of bimetal-catalyzed processes for one particular reason the distance between the reactive centers in a coordinated electrophile and a coordinated nucleophile is self-adjustable for specific tasks, because the activation energy for Cp ligand rotation is very low. In 2008, Peters and Jautze reported the application of the bis-palladacycle complex 56a to the enantioselective conjugate addition of a-cyanoacetates to enones (Fig. 31) [74—76] based on the idea that a soft bimetallic complex capable of simultaneously activating both Michael donor and acceptor would not only lead to superior catalytic activity, but also to an enhanced level of stereocontrol due to a highly organized transition state [77]. An a-cyanoacetate should be activated by enolization promoted by coordination of the nitrile moiety to one Pd(II)-center, while the enone should be activated as an electrophile by coordination of the olefinic double bond to the carbophilic Lewis acid [78],... 

Among the compounds capable of forming enolates, the alkylation of ketones has been most widely studied and applied synthetically. Similar reactions of esters, amides, and nitriles have also been developed. Alkylation of aldehyde enolates is not very common. One reason is that aldehydes are rapidly converted to aldol addition products by base. (See Chapter 2 for a discussion of this reaction.) Only when the enolate can be rapidly and quantitatively formed is aldol formation avoided. Success has been reported using potassium amide in liquid ammonia67 and potassium hydride in tetrahydrofuran.68 Alkylation via enamines or enamine anions provides a more general method for alkylation of aldehydes. These reactions are discussed in Section 1.3. 

Among Michael acceptors that have been shown to react with ketone and ester enolates under kinetic conditions are methyl a-trimethylsilylvinyl ketone,295 methyl a-methylthioacrylate,296 methyl methylthiovinyl sulfoxide,297 and ethyl a-cyanoacrylate.298 Each of these acceptors benefits from a second anion-stabilizing substituent. The latter class of acceptors has been found to be capable of generating contiguous quaternary carbon centers. 

The dienone intermediate (53a), as well as enolising to the phenol (52a), is itself capable of undergoing a Cope rearrangement to yield a second dienone (cf. 56a), whose enol is the p-substituted phenol (c/ 57a). Enolisation normally predominates, but where (51) has o-substituents, i.e. (54a), o-enolisation cannot take place, and only the p-phenol (57a) is then obtained. That this product is indeed formed not by direct migration of the allyl group, but by two successive shifts, is suggested by the double inversion of the position of the, 4C label in the allyl group that is found to occur ... 

With dimethyl(trimethylsilyl)amine, the nickel enolate 162 is capable of reacting with benzaldehyde to produce the condensation product 163 (Scheme 60). 

The ring-opening of the cyclopropane nitrosourea 233 with silver trifiate followed by stereospecific [4 + 2] cycloaddition yields 234 [129]. (Scheme 93) Oxovanadium(V) compounds, VO(OR)X2, are revealed to be Lewis acids with one-electron oxidation capability. These properties permit versatile oxidative transformations of carbonyl and organosilicon compounds as exemplified by ring-opening oxygenation of cyclic ketones [130], dehydrogenative aroma-tization of 2-eyclohexen-l-ones [131], allylic oxidation of oc,/ -unsaturated carbonyl compounds [132], decarboxylative oxidation of a-amino acids [133], oxidative desilylation of silyl enol ethers [134], allylic silanes, and benzylic silanes [135]. 

From the reactions shown in Scheme 5, it is obvious that only those uronic acid derivatives whose elimination proceeds with the formation of enolic or aldehydic groups, or both, afford products capable of reducing the Cu(II) ion. Although such structures can be expected from hexo- and hepto-furanuronic, as well as from hep-topyranuronic, acid derivatives, glycosides of pentofuranuronic and of hexopyranuronic acid derivatives do not exhibit reducing properties. However, in view of this generalization, the zero reducing power observed for compound 26 requires a different explanation. 

Reipig (39,40), Pfaltz (41), and Andersson and their co-workers (42) independently showed that these catalysts are capable of effecting the selective cyclopropanation of enol ethers and enolsilanes. Methyl vinyl ketone and acetophenone enolsilanes provide high selectivities in the cyclopropane products, but both isomers are formed equally. The trisubstituted dihydropyran 65 leads to cyclopropane adducts in high diastereoselectivities and enantioselectivities using 55c CuOTf as catalyst. 

In the free state and in their reactions the simple aldehydes and ketones are in general known only in the aldo- and keto-forms. Erlen-meyer suggested the rule that the isomeric enol-structure, which might be formed at first in the production of acetaldehyde from glycol, for example, should in no case be capable of existence. 

Now such enolates have an exceptionally reactive double bond which makes them capable of taking part in all kinds of addition reactions. The ethyl acetoacetate synthesis is the most important process of this type. The enolate is able to combine with a second molecule of the ester the latter is added as the two fragments R—jC=0 and OR... 

Whether or no both forms of a tautomeric substance are capable of isolation in the free state depends chiefly on the velocity of rearrangement of the more labile form. The isolation of keto- and enol-forms in a permanent crystalline state was first carried out with unsymmetrical dibenzoylacetone (Claisen, 1896) ... 

The proportion of the /rans-O-alkylated product [101] increases in the order no ligand < 18-crown-6 < [2.2.2]-cryptand. This difference was attributed to the fact that the enolate anion in a crown-ether complex is still capable of interacting with the cation, which stabilizes conformation [96]. For the cryptate, however, cation-anion interactions are less likely and electrostatic repulsion will force the anion to adopt conformation [99], which is the same as that of the free anion in DMSO. This explanation was substantiated by the fact that the anion was found to have structure [96] in the solid state of the potassium acetoacetate complex of 18-crown-6 (Cambillau et al., 1978). Using 23Na NMR, Cornelis et al. (1978) have recently concluded that the active nucleophilic species is the ion pair formed between 18-crown-6 and sodium ethyl acetoacetate, in which Na+ is co-ordinated to both the anion and the ligand. 

An interesting approach to form a divinylcyclopropane structure capable of rearranging into seven-membered functionalized derivatives consists of the silyloxylation of cyclic ketones 541 followed by a spontaneous Cope rearrangement to produce the cyclic enol esters 542 which then hydrolyzed to ketones 543 (equation 2 1 3)265. 

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