Acetylide anion formation

Retrosynthetic cleavage of the indicated bond in 9 provides acetylenic aldehyde 23 as a potential precursor. It was anticipated that the action of a suitable base on 23 would result in the formation of an acetylide anion, a competent carbon nucleophile that could... 

On the other hand, following the same sequences from the differently protected serine-derived nitrone 168, through the formation of hydroxylamines 169, C2 epimers of carboxylic acid and aldehydes are obtained, i.e., (2S,3R)-170 and (2S,3R)-171. Moreover, the syn adducts 164 were exclusively obtained in the addition of Grignard reagents to the nitrone 163, whereas the same reactions on nitrone 168 occurred with a partial loss of diastereoselectivity [80]. Q, j6-Diamino acids (2R,3S)- and (2R,3R)-167 can also be prepared from the a-amino hydroxylamines 164 and 169 by reduction, deprotection and oxidation steps. The diastereoselective addition of acetylide anion to N,N-dibenzyl L-serine phenyhmine has been also described [81]. 

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. 

Finally, the peculiar formation of iodoalkynes from iodine and acetylenes with relatively low (< 25) pK values in liquid ammonia should be mentioned [121]. The most likely intermediates occurring are acetylide "anions" formed in very low concentrations from the acetylene and the base ammonia. The conversions proceed very slowly and iodinadon of the lithiated alkynes in the same solvent is undoubtedly a far superior method. 

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 formation nor survive in a solution containing carbanions. Acetylide ions add to carbonyl... 

Carbon nucleophiles are very useful species because their reactions with carbon electrophiles result in the formation of carbon—carbon bonds. Section 10.8 introduced acetylide anions as nucleophiles that could be used in Sm2 reactions. These nucleophiles are prepared by reacting 1-alkynes with a strong base such as sodium amide. The relatively acidic hydrogen on the. vp-hybridized carbon is removed in this acid-base reaction ... 

Formation of acetylide anions from terminal alkynes (11 6B)... 

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

In this equilibrium, acetylene is the stronger acid and sodium amide is the stronger base, and the position of equilibrium lies considerably toward the right and favors formation of the acetylide anion and ammonia (Section 2.4). Table 4.1 gives pA values for an alkane, alkene, and an alkyne hydrogen. Also given for comparison is the value for water. 

To see how the use of an acetylide anion can lead to the formation of a new carbon-carbon bond, consider chloromethane, CH3CI. The C — Cl bond of chloromethane is polar covalent, with carbon bearing a partial positive charge because of the difference in electronegativity between carbon and chlorine. 

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