Acetylene deprotonation

The proton of terminal acetylenes is acidic (pKa= 25), thus they can be deprotonated to give acetylide anions which can undergo substitution reactions with alkyl halides, carbonyls, epoxides, etc. to give other acetylenes. 

Deprotonation of terminal acetylenes by organolithiurn compounds in organic solvents or by alkali metal amides is an extremely fast reaction, even at very... 

The ring closure seems to involve intramolecular attack of the deprotonated methylene group of primary adduct at the second acetylene bond (75DIS). 

Acetylene and terminal alkynes are CH-acidic compounds the proton at the carbon-carbon triple bond can be abstracted by a suitable base. Such a deprotonation is the initial step of the Glaser reaction as well as the Eglinton... 

Where do hydrocarbons lie on the acidity scale As the data in Table 8.1 show, both methane (pKa 60) and ethylene (plC, = 44) are very weak acids and thus do not react with any of the common bases. Acetylene, however, has piCa = 25 and can be deprotonated by the conjugate base of any acid whose pKa is greater than 25. Amide ion (NH2-), for example, the conjugate base of ammonia (pKa - 35), is often used to aeprotonate terminal aikynes. 

The pH dependence of nitrogenase activity has been interpreted in terms of a group with a pi a = 6.3 that must he deprotonated for activity and another group with a pi a = 9 that must be protonated for activity 128). The pi a of the latter group was moved about 0.5 pH units more acid in the presence of acetylene and carbon monoxide and the group with the pi of 6.3 was moved about 0.4 pH units more acid by acetylene. The behavior of the group with the pZa of 9 is fully consistent with earlier observations (50) on the effect of acetylene on... 

While ephedrine derivatives showed some selectivity, the most promising results were obtained with cinchona alkaloids. Lithium alkoxides and lithium acetylides (n-BuLi or LiHMDS used to deprotonate both the acetylene and the alcohol) gave better results than the corresponding sodium or magnesium salts. Higher enan-tioselectivity was obtained in THF (homogeneous) than in toluene or diethyl ether (heterogeneous). 

Acetylene is sufficiently acidic to allow application of the gas-phase proton transfer equilibrium method described in equation l7. For ethylene, the equilibrium constant was determined from the kinetics of reaction in both directions with NH2-8. Since the acidity of ammonia is known accurately, that of ethylene can be determined. This method actually gives A f/ acid at the temperature of the measurement. Use of known entropies allows the calculation of A//ac d from AG = AH — TAS. The value of A//acij found for ethylene is 409.4 0.6 kcal mol 1. But hydrocarbons in general, and ethylene in particular, are so weakly acidic that such equilibria are generally not observable. From net proton transfers that are observed it is possible sometimes to put limits on the acidity range. Thus, ethylene is not deprotonated by hydroxide ion whereas allene and propene are9 consequently, ethylene is less acidic than water and allene and propene (undoubtedly the allylic proton) are more acidic. Unfortunately, the acidity of no other alkene is known as precisely as that of ethylene. 

The calculated deprotonation energies of ethane, ethylene and acetylene by SCF Hartree-Fock (FIF) and MP2 methods follow the expected order 456, 455 (basis... 

Regarding the first problem, the most elemental treatment consists of focusing on a few points on the gas-phase potential energy hypersurface, namely, the reactants, transition state structures and products. As an example, we will mention the work [35,36] that was done on the Meyer-Schuster reaction, an acid catalyzed rearrangement of a-acetylenic secondary and tertiary alcohols to a.p-unsaturatcd carbonyl compounds, in which the solvent plays an active role. This reaction comprises four steps. In the first, a rapid protonation takes place at the hydroxyl group. The second, which is the rate limiting step, is an apparent 1, 3-shift of the protonated hydroxyl group from carbon Ci to carbon C3. The third step is presumably a rapid allenol deprotonation, followed by a keto-enol equilibrium that leads to the final product. 

Cutting and Parsons described the transformation of acetylenic alcohols 314 into allenyl phenyl thioethers 316 by a two-step procedure (Scheme 8.85) [174], Deprotonation of alkynes 314 with n-butyllithium is followed by addition of phenylsulfenyl chloride, forming sulfenyloxy intermediates which subsequently rearrange to allenic sulfoxides 315. Treatment of allenes 315 with methyllithium results in loss of the sulfoxide moiety to form allenyl sulfides 316 in reasonable yields. 

Standard organolithium reagents such as butyllithium, ec-butyllithium or tert-butyllithium deprotonate rapidly, if not instantaneously, the relatively acidic hydrocarbons of the 1,4-diene, diaryhnethane, triarylmethane, fluorene, indene and cyclopentadiene families and all terminal acetylenes (1-alkynes) as well. Butyllithium alone is ineffective toward toluene but its coordination complex with A/ ,A/ ,iV, iV-tetramethylethylenediamine does produce benzyllithium in high yield when heated to 80 To introduce metal into less reactive hydrocarbons one has either to rely on neighboring group-assistance or to employ so-called superbases. 

This synthetic strategy proved to be unsuitable for [M(CO)5] (M = Cr, W) metal fragments due to the thermal instability of the corresponding non-donor-substituted allenylidenes [M(=C=C=CR R )(C0)5] (R, R = usually alkyl or aryl groups). An alternative general synthetic procedure using deprotonated functionalized acetylenes has been successfully applied (for example tris-amino or alkoxo prop-1-ynes see Equation 2.2) [4a]. 

Since the bases used for the metallation of the acetylenes are much stronger (in a thermodynamic sense) than the acetylides, the deprotonations are essentially complete, a condition that has to be met for most functionalization reactions. Some ethynyladons form an excepdon. Ethynylcyclohexanol, for example, can be obtained in yields greater than 90% by adding cyclohexanone to a suspension of potassium acetylide (or even KOH) in THF while introducing acetylene [2], The farmadon of potassium acetylide is likely to be an equilibrium ... 

However, these compounds, or, more properly, those species with the same Mg C ratios and resulting stoichiometries are not fanciful. They are two of the best known magnesium carbides and more often written in an ionic dialect, as Mg ( 2) and (Mg +) i.e. they are the magnesium salts of totally deprotonated acetylene and... 

Carboxylic acids are more acidic than alcohols and acetylene. Strong aqueous bases can completely deprotonate carboxylic acids, and salts of carboxylic acids are formed. Strong aqueous mineral acids readily convert the salt back to the carboxylic acids. Saits are soluble in water but insoluble in nonpolar solvents, e.g. hexane or dichloromethane. 

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