The Acidity of Terminal Alkynes

A different j3 hydrogen can be removed from the carbocation, so as to form a more highly substituted alkene than the initial alkene. This deprotonation step is the same as the usual completion of an El elimination. (This carbocation could experience other fates, such as further rearrangement before elimination or substitution by an S 1 process.) 

The hydrogen bonded to the carbon of a terminal alkyne, called an acetylenic hydrogen atom, is considerably more acidic than those bonded to carbons of an alkene or alkane (see Section 3.8A). The pK values for ethyne, ethene, and ethane illustrate this point  

The order of basicity of their anions is opposite that of their relative acidity  

If we include in our comparison hydrogen compounds of other first-row elements of the periodic table, we can write the following orders of relative acidities and basicities  

We see from the order just given that while terminal alkynes are more acidic than ammonia, they are less acidic than alcohols and are less acidic than water. 

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The acidity of terminal alkynes makes available routes to alkynylstannanes that do not apply to alkenylstannanes. Thus, alkynylstannanes can be made by the acidolysis of aminostannanes with alkynes, and the iV-stannylpyr-roles are recommended for this purpose as they can be readily prepared by azeotropic dehydration of the trialkyltin oxide and pyrrole, and the amine that is liberated is non-basic.242... 

Several of the trialkylaluminum and alkylaluminum halides and hydrides mentioned above are commercially available. Alkynyl, alkenyl, cyclopentadienyl, and aryl derivatives are, in general, not commercially available and must be synthesized for laboratory use. Alkynyl derivatives can be prepared by salt metathesis, as in the reaction of Et2AlCl with NaC=CEt to give Et2AlC=CEt. The acidity of terminal alkynes is sufficient for preparation of alkynyl aluminum compounds by alkane or hydrogen elimination upon reaction with a trialkylaluminum or an aluminum hydride (equation 17), respectively. TriaUcynyl aluminum compounds are typically isolated as Lewis base adducts to stabilize them against otherwise facile polymerization. Alkenyl compounds of aluminnm have similarly been prepared. 

There also exists an acidregioselective condensation of the aldol type, namely the Mannich reaction (B. Reichert, 1959 H. Hellmann, 1960 see also p. 291f.). The condensation of secondary amines with aldehydes yields Immonium salts, which react with ketones to give 3-amino ketones (=Mannich bases). Ketones with two enolizable CHj-groupings may form 1,5-diamino-3-pentanones, but monosubstitution products can always be obtained in high yield. Unsymmetrical ketones react preferentially at the most highly substituted carbon atom. Sterical hindrance can reverse this regioselectivity. Thermal elimination of amines leads to the a,)3-unsaturated ketone. Another efficient pathway to vinyl ketones starts with the addition of terminal alkynes to immonium salts. On mercury(ll) catalyzed hydration the product is converted to the Mannich base (H. Smith, 1964). 

Organoboranes undergo transmetallation. 1-Hexenylboronic acid (438) reacts with methyl acrylate via the transmetallation with Pd(OAc)2, giving methyl 2,4-nonadienoate (439)[399], The ( )-alkenylboranes 440, prepared by the hydroboration of terminal alkynes, are converted into the alkylated ( )-alkenes 441 by treatment with an equivalent amount of Pd(OAc)2 and triethylamine[400]. The ( )-octenylborane 442 reacts with CO in MeOH in the... 

The most frequent applications of these procedures he in the preparation of terminal alkynes Because the terminal alkyne product is acidic enough to transfer a proton to amide anion one equivalent of base m addition to the two equivalents required for dou ble dehydrohalogenation is needed Adding water or acid after the reaction is complete converts the sodium salt to the corresponding alkyne... 

Organic compounds can be metalated at suitably acidic positions by active metals and by strong bases.The reaction has been used to study the acidities of very weak acids (see p. 228). The conversion of terminal alkynes to acetylid ions is one... 

The addition of terminal alkynes to carbon-carbon double bonds has not been explored until recently, possibly because C=C double bonds are not as good electrophiles as C=N or C=0. In 2003, Carreira et al. reported the first conjugate addition reaction of terminal alkynes to C=C catalyzed by copper in water. The reaction proceeded with derivatives of Meldrum s acid in water in the presence of Cu(OAc)2 and sodium ascorbate (Eq. 4.35).59 However, this method was limited to C=C double bonds with two electron withdrawing groups. 

Pt-catalyzed hydration of various aliphatic and aromatic alkynes under phase transfer conditions in (CH2C1)2/H20 in the presence of Aliquat 336 led to either a Markovnikov product, mixtures of two ketones, or ketones with the carbonyl group positioned away from the bulky side.72 In the absence of the phase transfer reagent, Aliquat 336, hardly any reaction took place. Recently, a hydrophobic, low-loading and alkylated polystyrene-supported sulfonic acid (LL-ALPS-SO3H) has also been developed for the hydration of terminal alkynes in pure water, leading to ketones as the product.73 Under microwave irradiation, the hydration of terminal arylalkynes was reported to proceed in superheated water (200°C) without any catalysts.74... 

Cleavage of Zr—C a bonds occurs readily on treatment with H20 or dilute acids, while the Zr—Cp bond usually survives mild protonolysis conditions. The use of D20 or DC1/D20 permits the replacement of Zr with D. Deuterolysis provides a generally reliable method for establishing the presence of Zr—C bonds. Protonolysis or deuterolysis of Zr—Csp bonds proceeds with retention of configuration [97]. In the hydrozirconation of terminal alkynes, deuterium can be introduced at any of the three positions in the vinyl group in a completely regio- and stereoselective manner, as shown in Scheme 1.18. Although relatively little is known about the mechanistic details, the experimental results appear to be consistent with concerted c-bond metathesis (Pattern 13) between C—Zr and H— X bonds. 

Other cations (Cu2+, Pd2+, Ru3+, Ni2+, Rh3+) incorporated into Nafion-H have been found to promote hydration.36 Other metals that catalyze hydration of alkynes include gold(III),37 ruthenium(in),38 and platinum(II) (Zeise s salt39 40 and halides40), p-Methoxybenzenetellurinic acid is very effective in the hydration of terminal alkynes 41 Similar to the hydration of alkenes, photochemical acid-catalyzed hydration of alkynes is possible ... 

It has been demonstrated625 that ytterbium-aromatic imine dianion complexes can act as effective catalysts for the isomerization of terminal alkynes to internal alk-2-ynes. Isomerization of acetylenic pentafluorophenyl esters in the presence of phosphines has been found to give rise to activated dienoic acids, which have been coupled directly with amines (and alcohols) in a simple one-pot procedure626 (see Scheme 124). 

B-2. Which of the following statements best explains the greater acidity of terminal alkynes (RC=CH) compared with monosubstituted alkenes (RCH=CH2) ... 

When iodosylbenzene and one equivalent of triflic anhydride are mixed in dichloromethane, a (/ -phenylene)bisiodine(III) species, similar to that obtained from iodosylbenzene with two equivalents of triflic acid (structure 11, equation 23)38,63, is generated134. The addition of terminal alkynes to such mixtures affords (p-phenylene)bisiodonium ditriflates with one phenyl and one TfOC(R)=CH— ligand (stereochemistry unspecified)134 see equation 173. 

Figure 3.10 Synthesis of ynones through the coupling of terminal alkynes and acid chlorides in water.

All reactions of alkynes occur because they contain easily broken ft bonds or, in the case of terminal alkynes, an acidic, sp hybridized C—H bond. 

Hydroboration is especially valuable for the synthesis of stereodefined 1- alkenyl-boronic acids. A general method is the hydroboration of terminal alkynes with cate-cholborane 7 [16, 17] (eq (14)). The reaction is generally carried out at 70 without solvent, but it is very slow in THF solvent. More recent results demonstrate that the hydroboration of alkenes or alkynes with catecholborane is strongly accelerated in the presence of palladium [18]. rhodium [19]. or nickel catalysts [20], thus allowing the reaction to proceed below room temperature. 

Because of the acidity of the protons of terminal alkynes, they are easily converted into alkynyl... 

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