Acetylide ions

The vinyl anion synthon is best represented by an acetylide ion (frame 33). Synthesis ... 

Because acetylene is a far weaker acid than water and alcohols these substances are not suitable solvents for reactions involving acetylide ions Acetylide is instantly converted to acetylene by proton transfer from compounds that contain —OH groups... 

Amide ion is a much stronger base than acetylide ion and converts acetylene to its conjugate base quantitatively... 

Next an alkyl halide (the alkylating agent) is added to the solution of sodium acetylide Acetylide ion acts as a nucleophile displacing halide from carbon and forming a new carbon-carbon bond Substitution occurs by an 8 2 mechanism... 

The properties of organometallic compounds are much different from those of the other classes we have studied to this point Most important many organometallic com pounds are powerful sources of nucleophilic carbon something that makes them espe cially valuable to the synthetic organic chemist For example the preparation of alkynes by the reaction of sodium acetylide with alkyl halides (Section 9 6) depends on the presence of a negatively charged nucleophilic carbon m acetylide ion... 

Conjugate addition is most often observed when the nucleophile (Y ) is weakly basic The nucleophiles m the two examples that follow are C=N and C6H5CH2S respectively Both are much weaker bases than acetylide ion which was the nucleophile used m the example illustrating direct addition... 

Both CH3CH2CH2C=CH and CH3CH2C=CCH3 can be prepared by alkylation of acety lene The alkyne (CH3)2CHC=CH cannot be prepared by alkylation of acetylene because the required alkyl halide (CH3)2CHBr is secondary and will react with the strongly basic acetylide ion by elimination... 

Certain functional groups may be protected from reduction by conversion to anions that resist reduction. Such anions include the alkoxides of allylic and benzylic alcohols, phenoxide ions, mercaptide ions, acetylide ions, ketone carbanions, and carboxylate ions. Except for the carboxylate, phenoxide, and mercaptide ions, these anions are sufficiently basic to be proton-ated by an alcohol, so they are useful for protective purposes only in the... 

It looks as though all that is needed is to prepare the acetylenic anion, then alkylate it with methyl iodide (Section 9.6). There is a complication, however. The carbonyl group in the starting alkyne will neither tolerate the strongly basic conditions required for anion fonnation nor survive in a solution containing carbanions. Acetylide ions add to carbonyl... 

You have already had considerable experience with caibanionic compounds and their- applications in synthetic organic chemistry. The first was acetylide ion in Chapter 9, followed in Chapter 14 by organometallic compounds—Grignaid reagents, for exanple—that act as sources of negatively polarized car bon. In Chapter 18 you learned that enolate ions—reactive intermediates generated from aldehydes and ketones—are nucleophilic, and that this property can be used to advantage as a method for carbon-carbon bond formation. 

Carbanion (Section 9.5) Anion in which the negative charge is home by carbon. An example is acetylide ion. 

We won t study the details of this substitution reaction until Chapter 11 but for now can picture it as happening by the pathway shown in Figure 8.6. The nucleophilic acetylide ion uses an electron pair to form a bond to the positively polarized, electrophilic carbon atom of bromomethane. As the new C-C bond forms, Br- departs, taking with it the electron pair from the former C-Br bond and yielding propyne as product. We call such a reaction an alkylation because a new alkyl group has become attached to the starting alkyne. 

The alkylation reaction is limited to the use of primary alkyl bromides and alkyl iodides because acetylide ions are sufficiently strong bases to cause dehydrohalogenation instead of substitution when they react with secondary and tertiary alkyl halides. For example, reaction of bromocyclohexane with propyne anion yields the elimination product cyclohexene rather than the substitution product 1-propynylcyclohexane. 

Ketones react with acetylide ion (Section 8.7) to give alcohols. For example, the reaction of sodium acetylide with 2-butauone yields 3-methy -l-pentyn-3-ol ... 

C bond with two oxygen atoms attached to each carbon atom) (b) BrO4 (c) the acetylide ion, C22. Assign formal charges to each atom. 

This reaction shows that the methide ion is a very strong Bmnsted base. The species C22 is the acetylide ion, and the carbides that contain it are called acetylides. The acetylide ion is also a strong Bronsted base, and acctylides react with water to produce ethyne (acetylene) and the corresponding hydroxide. Calcium carbide, CaC2, is the most common saline carbide. 

Other carbanionic groups, such as acetylide ions, and ions derived from a-methylpyridines have also been used as nucleophiles. A particularly useful nucleophile is the methylsulfinyl carbanion (CH3SOCHJ), the conjugate base of DMSO, since the P-keto sulfoxide produced can easily be reduced to a methyl ketone (p. 549). The methylsulfonyl carbanion (CH3SO2CH2 ), the conjugate base of dimethyl sulfone, behaves similarly, and the product can be similarly reduced. Certain carboxylic esters, acyl halides, and DMF acylate 1,3-dithianes (see 10-10. )2008 Qxj(jatjye hydrolysis with NBS or NCS, a-keto aldehydes or a-... 

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... 

Triple bonds can also be selectively reduced to double bonds with DIBAL-H, " with activated zinc (see 12-36), with hydrogen and Bi2B-borohydride exchange resin, ° or (internal triple bonds only) with alkali metals (Na, Li) in liquid ammonia or a low-molecular-weight amine.Terminal alkynes are not reduced by the Na—NH3 procedure because they are converted to acetylide ions under these conditions. However, terminal triple bonds can be reduced to double bonds by the... 

Route (b), using acetylIde ion, has been used for the synthesis of (26) which did Indeed give the right dlastereoisomer of (23) with maleic anhydride. Synthesis... 

For example, the difference between the monoclinic CaC2 phases is the presence of only one type of acetylide ion in phase II and two distinct C2 species in phase III, as concluded from their crystal structures and from C-NMR studies (Fig. 8.3) [8]. The transformation between phases II and III is induced by heating phase II above 150°C until the metastable phase. III, is formed. Phase III remains stable even when being cooled down to room temperature. However, when the metastable phase III is ground in a mortar at room temperature, it transforms back into phase II. 

---