Other Tautomeric Reactions

Besides typical ESIPT compounds, other tautomeric systems have also been investigated by ultrafast spectroscopy. However, the number of studies is less. Cytosine was investigated by femtosecond ionization techniques to learn about the relevance of tautomerism for the photochemistry and photostabiHty of DNA [57, 58]. It was found that the isolated cytosine molecule exhibits a rather fast IC. It 

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Hydroxyisoquinolines. Hydroxy groups in the 5-, 6-, 7-, and 8-position show phenoHc reactions for example, the Bucherer reaction leads to the corresponding anainoisoquinolines. Other typical reactions include the Mannich condensation, azo-coupling reactions, and nitrosation. Both 0-methyl and /V-methyl derivatives are obtained from the methylation of 1-hydroxyisoquinoline, indicating that both tautomeric forms are present. Distillation of various hydroxy compounds, eg, 1- and 4-hydroxyisoquinoline, with zinc dust removes the oxygen. Treatment of 1-isoquinolinol with phosphoms tribromide yields 1-bromoisoquinoline [1532-71 -4] (178). 

Hydrolysis of vinyl acetate is catalyzed by acidic and basic catalysts to form acetic acid and vinyl alcohol which rapidly tautomerizes to acetaldehyde. This rate of hydrolysis of vinyl acetate is 1000 times that of its saturated analogue, ethyl acetate, ia alkaline media (15). The rate of hydrolysis is minimal at pH 4.44 (16). Other chemical reactions which vinyl acetate may undergo are addition across the double bond, transesterification to other vinyl esters, and oxidation (15—21). 

Hydroxypyridine 1-oxide is insoluble in chloroform and other suitable solvents, and, although the solid-state infrared spectrum indicates that strong intermolecular hydrogen bonding occurs, no additional structural conclusions could be reached. Jaffe has attempted to deduce the structure of 4-hydroxypyridine 1-oxide using the Hammett equation and molecular orbital calculations. This tautomeric compound reacts with diazomethane to give both the 1- and 4-methoxy derivatives, " and the relation of its structure to other chemical reactions has been discussed by Hayashi. ... 

The most important reaction of tautomeric 1,7-naphthyridinones, halogenolysis, has been covered in Section 17.1. Only a few of the other possible reactions have been reported, as illustrated by the following examples. 

It was first necessary to define the species of the heterocycle entering into reaction under any particular set of conditions. For example, basic molecules such as pyridine could react as free base or conjugate acid, whereas a potentially tautomeric compound such as 4-pyridone could react as such, or in the other tautomeric form (4-hydroxypyridine), or as the conjugate acid or base. 

While wanning the catalysis mixture to 55 C (Step D, Scheme 1) leads to no other observable reaction intermediates, the generation of intermediate 8 would allow the series of steps shown in Scheme 1. Insertion of the coordinated CO into the palladium-carbon bond would lead to the overall coupling of acid chloride, imine and carbon monoxide in conq>lex 10. The subsequent loss of HCl from 10, either via direct deprotonation or P-H elimination, would form the a-amide substituted ketene 11. The latter is known to be in rapid equilibrium with its cyclic mesoionic l,3-oxazolium-5-oxide tautomeric 12 (14). These steps would lead to the liberation of the Pd(0) catalyst, which can return to the catalytic cycle. 

Several other important reactions of tautomeric cinnolinones are represented in the more recent literature, as illustrated in the following classified examples. 

Several other useful reactions of tautomeric phthalazinones that have been employed in the more recent literature are illustrated by the following classified examples. 

Kinetics and spectroscopy of substituted phenylnitrenes, 36, 255 Kinetics, of organic reactions in water and aqueous mixtures, 14, 203 Kinetics, reaction, polarography and, 5, 1 Kinetic studies of keto—enol and other tautomeric equilibria by flash photolysis, 44, 325... 

One of the topics much under discussion in the early years of kinetics research was the nature of the two tautomeric forms, or where the proton actually resided. Laar [35] had proposed a so-caUed oscillatory model, where a hydrogen atom vibrates continuously between the two possible positions. Other early observations include dielectric effects the polarity of the solvent could help release a proton from one position, thus making a transfer possible [1]. Although a tautomerization reaction is not an ionization (Section 1.1), an ionization step does play a crucial role, and may in many cases be the rate-determining step. 

The ketone is added to a large excess of a strong base at low temperature, usually LDA in THF at -78 °C. The more acidic and less sterically hindered proton is removed in a kineti-cally controlled reaction. The equilibrium with a thermodynamically more stable enolate (generally the one which is more stabilized by substituents) is only reached very slowly (H.O. House, 1977), and the kinetic enolates may be trapped and isolated as silyl enol ethers (J.K. Rasmussen, 1977 H.O. House, 1969). If, on the other hand, a weak acid is added to the solution, e.g. an excess of the non-ionized ketone or a non-nucleophilic alcohol such as cert-butanol, then the tautomeric enolate is preferentially formed (stabilized mostly by hyperconjugation effects). The rate of approach to equilibrium is particularly slow with lithium as the counterion and much faster with potassium or sodium. 

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... 

Hydroxyquinolines (Quinolinols). A number of methods have been employed for their preparation. A modified Chichibabia reaction of quinoline ia fused KOH—NaOH at 240°C produces 70% of 2-hydroxyquiQoline [59-31-4] (121). Alternative names based on the facile keto—enol tautomerism of two of these compounds are 2(1H) and 4(lJd)-quiQolinone none of the other quinolinols show this property. The treatment of... 

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