Carbanions acetylide anion

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... 

Potassium or lithium derivatives of ethyl acetate, dimethyl acetamide, acetonitrile, acetophenone, pinacolone and (trimethylsilyl)acetylene are known to undergo conjugate addition to 3-(t-butyldimethylsiloxy)-1 -cyclohexenyl t-butyl sulfone 328. The resulting a-sulfonyl carbanions 329 can be trapped stereospecifically by electrophiles such as water and methyl iodide417. When the nucleophile was an sp3-hybridized primary anion (Nu = CH2Y), the resulting product was mainly 330, while in the reaction with (trimethylsilyl)acetylide anion the main product was 331. 

Here too, a second alkylation can be made to take place yielding RC=CR or R C=CR. It should, however, be remembered that the above carbanions—particularly the acetylide anion (57)—are the anions of very weak acids, and are thus themselves strong bases, as well as powerful nucleophiles. They can thus induce elimination (p. 260) as well as displacement, and reaction with tertiary halides is often found to result in alkene formation to the exclusion of alkylation. 

We can consider decarboxylation reactions in terms that are analogous to those in proton transfer reactions the reactivity of the carbanion in carboxylation reactions is analogous to internal return observed in proton transfer reactions from Bronsted acids. Kresge61 estimated that the rate constant for protonation of the acetylide anion, a localized carbanion (P A 21), is the same as the diffusional limit (1010 M s1). However, achieving this rate is highly dependent on the extent of localization of the carbanion. Jordan62 has shown that intermediates in thiazolium derivatives are also likely to be localized carbanions, which implies that protonation of these intermediates could occur at rates approaching those of other localized carbanions. 

Finally, acetylide anions have been alkylated with propargyl halides to give excellent yields of dialkynes643,644. Similar reactions have been used in the synthesis of a wide variety of natural products including lactones and macrolides645,646 and leukotrienes647-651. With many halides, reaction with acetylide anions is not useful however, due to elimination side-reactions caused by the significant basicity of the carbanion. 

This reaction follows the general mechanism for nucleophilic addition (Section 20.2A)—that is, nucleophilic attack by a carbanion followed by protonation. Mechanism 20.6 is shown using R"MgX, but the same steps occur with organolithium reagents and acetylide anions. 

A possible explanation for these sequences can be found in the electronic configuration of the carbanions. In the alkyl, phenyl, and acetylide anions, the unshared pair of electrons occupies respectively an an sp, and an sp orbital. The availability of this pair for sharing with acids determines the basicity of the particular anion. As we proceed along the series sp, sp sp, the p character of the orbital decreases and the s character increases. Now, an electron in a orbital is at some distance from the nucleus and is held relatively loosely an electron in an s orbital, on the other hand, is close to the nucleus and is held more tightly. Of the three anions, the alkyl ion is the strongest base since its pair of electrons is held most loosely, in an sp orbital. The acetylide ion is the weakest base since its pair of electrons is held most tightly, in an sp orbital. 

A systematic investigation of alkynic carbanion structures has been reported by Weiss and cowoikers. These structures all incorporate either r-butylacetylide or phenylacetylide anion. They differ by the ligand that is incorporated. Perhaps this sequence of structures most tq)tly demonstrates the rich variety of aggregate structural types that an acetylide anion can choose. 

Alkylation of (bromomethyl)chlorodimethylsilane by the acetylide anion followed by addition of the thioacetate anion opened a new access to a silyl tethered yne-vinylsulflde precursor that found use in new radical cascades. Unsymmetrical ansa-fluorenyl containing ligands incorporating a CFl2-SiMe2 bridge have been described and result from a dialkylation with fluorenyl carbanions. ... 

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... 

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

The most convenient method for the preparation of sodium acetylide appears to be by reaction of acetylene with sodium methylsulfinyl carbanion (dimsylsodium). The anion is readily generated by treatment of DMSO with sodium hydride, and the direct introduction of acetylene leads to the reagent. As above, the acetylide may then be employed in the ethynylation reaction. 

Solvent for Displacement Reactions. As the most polar of the common aprotic solvents, DMSO is a favored solvent for displacement reactions because of its high dielectric constant and because anions are less solvated in it (87). Rates for these reactions are sometimes a thousand times faster in DMSO than in alcohols. Suitable nucleophiles include acetylide ion, alkoxide ion, hydroxide ion, azide ion, carbanions, carboxylate ions, cyanide ion, halide ions, mercaptide ions, phenoxide ions, nitrite ions, and thiocyanate ions (31). Rates of displacement by amides or amines are also greater in DMSO than in alcohol or aqueous solutions. Dimethyl sulfoxide is used as the reaction solvent in the manufacture of high performance, polyaryl ether polymers by reaction of bis(4,4,-chlorophenyl) sulfone with the disodium salts of dihydroxyphenols, eg, bisphenol A or 4,4,-sulfonylbisphenol (88). These and related reactions are made more economical by efficient recycling of DMSO (89). Nucleophilic displacement of activated aromatic nitro groups with aryloxy anion in DMSO is a versatile and useful reaction for the synthesis of aromatic ethers and polyethers (90). 

A mixed Li /Mg+ aggregate corresponding to (213) is formed with either phenyl or methyl carbanions. An unusual lithium/magnesium acetylide is formed with stoichiometry Li2[(PhCsC)3Mg(TMEDA)]2 and is depicted as (214). The same authors also report the ion pair characterized as the mixed benzyllithium/magnesium TMEDA complex (215). ° A different mixed lithium/magnesium aggregate depicted as (216) is found for the THF-solvated anion of tris(trimethylsi-lyl)methyl carbanion. ... 

The conjugate base of an alkyne is an alkyne anion (older literature refers to them as acetylides), and it is generated by reaction with a strong base and is a carbanion. It funetions as a nucleophile (a source of nucleophilic carbon) in Sn2 reactions with halides and sulfonate esters. Acetylides react with ketones, with aldehydes via nucleophilic acyl addition and with acid derivatives via nucleophilic acyl substitution. Acetylides are, therefore, important carbanion synthons for the creation of new carbon-carbon bonds. Some of the chemistry presented in this section will deal with the synthesis of alkynes and properly belongs in Chapter 2. It is presented here, however, to give some continuity to the discussion of acetylides. 

Diketone anions of 1-aryl-4,4,4-trifluorobutane-l,3-dione (85) undergo nucleophilic addition of sodium acetylide to give tertiary 1,4-alkynediols (86). Double cyclization gives a 2,2 -bifuran. While the mechanism of acetylide addition has not been proved, a likely first step is the formal reaction of two carbanions to give a C-C bond. °... 

Carbon nucleophiles play a central role in organic chemistry, as they form the basis of carbon-carbon bond formation. A few are shown in Figure 1.2, including such carbanionic species as organolithiums (RLi), Grignard reagents (typically written as RMgBr), and the cyanide (CN ) and acetylide (R-C=C ) anions. Other examples such as enolates, enols, and enamines will be briefly discussed in Section 1.15. 

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