Keto-enol tautomerization reactions derivation

The solvent polarity, which is defined as the overall solvation capability of a liquid derived from all possible, non-specific and specific intermolecular interactions between solute and solvent molecules [4], cannot be represented by a single value encompassing all aspects, but constants such as the refractive index, the dielectric constant, the Hildebrand solubility parameter, the permanent dipole moment, the partition coefficient logP [5] or the normalised polarity parameter TN [6] are generally employed to describe the polarity of a medium. The effect of a solvent on the equilibrium position of chemical reactions, e.g. the keto-enol tautomerism, may also be used. However, these constants reflect only on some aspects of many possible interactions of the solvent, and the assignment to specific interactions is difficult if not impossible. 

Compounds presenting the possibility of keto-enol tautomerism react easily with lead tetraacetate to afford products of a-acetoxylation. This reaction is found in the case of ketones and phenols, although it is frequently accompanied by other products derived from alternate mechanistic pathways. 

Stork reaction Alkylation or acylation of a ketone or aldehyde using its enamine derivative as the nucleophile. Acidic hydrolysis regenerates the alkylated or acylated ketone or aldehyde, (p. 1055) tautomerism An isomerism involving the migration of a proton and the corresponding movement of a double bond. An example is the keto-enol tautomerism of a ketone or aldehyde with its enol form. (p. KM2)... 

When 1-hexyne is treated with a catalytic amount of sulfuric acid in an aqueous solvent, initial reaction with the acid gives the expected secondary vinyl carbocation 103, and the most readily available nucleophile in this reaction is water (from the aqueous solvent). Nucleophilic addition of water to 103 leads to the vinyl oxonium ion 104. Loss of a proton in an acid-base reaction (the water solvent is the base) generates a product (105) where the OH unit is attached to the C=C unit, an enol. Enols are unstable and an internal proton transfer converts enols to a carbonyl derivative, an aldehyde, or a ketone. This process is called keto-enol tautomerization and, in this case, the keto form of 105 is the ketone 2-hexanone (106). (Enols are discussed in more detail in Chapter 18, Section 18.5.) Note that the oxygen of the OH resides on the secondary carbon due to preferential formation of the more stable secondary carbocation followed by reaction with water, and tautomerization places the carbonyl oxygen on that same carbon, so the product is a ketone. When a disubstituted alkyne reacts with water and an acid catalyst, the intermediate secondary vinyl cations are of equal stability and a mixture of isomeric enols is expected each will tautomerize, so a mixture of isomeric ketones will form. 

The phenoxy radical, derived from an H-abstraction reaction by NO, couples with another -NO molecule to give 4-nitrocyclohexa-2,5-dienone, which readily rearranges, after a keto-enol tautomerism to 2,6-di- -butyl-4-nitrophenol. 2,4-di-t-butylphenol and 4-r-butylphenol react in the same way, whereas the presence of three substituents on the aromatic ring of 2,4,6-tri-r-butylphenol and 2,6-di- butyl-4-methylphenol does not allow keto-enol tautomerism in these cases 2,4,6-tri- -butyl-4-nitrocyclohexa-2,5-dienone and 2,6-di-r-butyl-4-methylnitrocyclohexa-2,5-dienone were the sole nitro compounds obtained. The reaction between NO and 2,6-di-t-butylphenol or 2,4-di- -butylphenol in methanol resulted in phenoxyl radical dimerisation together with nitration. As shown in (Scheme 5.100), phenoxyl radical dimerises to give an... 

In the care of reaction (51) it is the o-complex which is undergoing deprotonation. The pKj, values of these ketone-like o-coraplexes can be derived from the constant Kt for the tautomeric keto-enol equilibrium of the phenol and the acid dissociation constant K, for the phenol. One gets pK pteto) 1-... 

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. 

The reaction that initially implicates a keto-enol tautom-erism eqniUbrium between B and C progresses with an intramolecular cycloaddition involving the CN group activated by the phenolic OH, followed by a tautomerization. The formation of the desired spiro derivatives 117 releases the catalyst 116, which begins another catalytic cycle. 

An alternate scheme for preparing these compounds starts with a prefabricated pyrimidone ring. Aldol condensation of that compound (95), which contains an eneamide function, with pyridine-3-aldehyde (80), gives the product 96. Catalytic hydrogenation gives the product of 1,4 reduction. The resulting pyrimidinedione, of course exists in the usual tautomeric keto (97a) and enol (97b) forms. Reaction with phosphorus oxyxchloride leads to the chloro derivative 98. Displacement with methoxide gives 99. Reaction of this last intermediate with the furylalkylamine derivative 92 leads to the H-2 blocker lupitidine (100) [22]. 

Unlike formamidine, acetamidine and benzamidine react with both aromatic and aliphatic a-hydroxyketones to give imidazoles exclusively. It has been suggested that aryl groups favour the enolic form (2) of the tautomeric mixture, resulting in the formation of oxazoles as major products. Aliphatic groups favour the keto form (1), from which imidazoles are derived. That amidines more complex than formamidine favour imidazole formation may be a consequence of steric hindrance to reaction of the enolic hydroxy groups with the amidine carbon in (2). The general reaction has been used to prepare such compounds as 4,5-dipropyl imidazole (25% yield from tris(formylamino)-methane and 5-hydroxyoctan-4-one), and a variety of 2-imidazolones and 2-aminoimidazoles [8]. The fact that oxazoles can be converted into imidazoles with some ease extends the applicability of this reaction. 

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