Acidity, alkynes

Dichloropyrazolinones with alkali give alkynic acids (Scheme 40) (58JA599). 

Magriotis. P.A. Vourloumis, D. Scott, M.E. Tarli, A. Tetrahedron Lett, 1993, 34, 2071 SECTION 301 ALKYNE - ACID DERIVATIVES... 

The position of the triple bond can alter the reactivity of the alkynes. Acidic alkynes react with certain heavy metal ions, e.g. Ag+ and Cu+, to form precipitation. Addition of an alkyne to a solution of AgNOs in alcohol forms a precipitate, which is an indication of hydrogen attached to the triple bonded carbon. Thus, this reaction can be used to differentiate terminal alkynes from internal alkynes. 

Dissolution in sulfuric acid is sufficient to convert 5-(4-chlorophenyl)penta-2,4-diynoic acid into the 6-aryl-4-hydroxypyran-2-one (73BSF1293) other p-substituents are compatible with the synthesis. The intermediacy of a 2,4-diketo acid is assumed. Preparation of the alkynic acids is achieved through the coupling of 2-aryl-1-bromoacetylenes with propynoic acid. 

Water is too weak a base to accept protons from alkynes consequently no measurable concentration of H30 is expected from the ionization of alkynes in dilute aqueous solutions. Therefore we have no quantitative measure of 1-alkyne acidity in aqueous solution other than that it probably is about 1010 times less acidic than water, as judged from measurements in other solvents to be discussed shortly. In the gas phase, however, the situation is reversed, and ethyne is a stronger acid than water ... 

Except for very reactive alkynes, acid-catalyzed hydrations are usually sluggish. This slow hydration can be overcome by the addition of catalytic amounts of mercury(II) salts. Such hydrations are generally mild and will tolerate the presence of other functional groups. Specific examples of mercury-catalyzed hydrations are discussed in the next section. 

Table 22 Lactonization of Acyclic Alkynic Acids (Equation 76)...

Utimoto et al. [145] reported a procedure for the synthesis of macrocyclic ynones by intramolecular acylation of )-(trimethylsilyl)ethynylalkanoyl chlorides in the presence of Lewis acid. For example, in the synthesis of f — )-muscone 216), cyclization of alkyne acid chloride 238 gave the macrocycle 239 in 52% yield, which was hydrogenated to f — )-muscone (Scheme 80). 

In a similar context Amdtsen developed a new pyrrole synthesis from alkynes, acid chlorides either imines or isoquinolines, based on the reactivity of isocyanides (Scheme 35a) [197]. Although all atoms from the isocyanide are excluded from the final structure, its role in the reaction mechanism is crucial. The process takes place through the activation of the imine (isoquinoline) by the acid chloride to generate the reactive M-acyliminium salt, which is then attacked by the isocyanide to furnish a nitrilium ion. This cationic intermediate coordinates with the neighboring carbonyl group to form a miinchnone derivative, which undergoes a [3+2] cycloaddition followed by subsequent cycloelimination of the isocyanate unit, to afford the pentasubstituted pyrrole adducts 243 and 244 (Scheme 35a, b). 

Both acrylic and alkynic acids of imidazoles decarboxylate normally on heating, and bromoacrylic acids in the series are dehydrobrominated under the influence of ethanolic alkali (71 CHE 132). 

Alkyne Acidity Formation of Acetylide Anions Alkylation of Acetylide Anions 289... 

The more elaborate diene 105, prepared by enantioselective reduction of the corresponding ketone, (chapter 26) reacts with the P-tosyl alkyne acid 106 to give a good yield of one regio- and... 

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