Acetylide anion stability

Acetylides are formed by treating terminal acetylenes with a strong base, sodium amide in liquid ammonia being the one most commonly employed. Acetylenes with a hydrogen atom attached to the triple bond are weakly acidic (pATa about 25) due to the stability of the acetylide anion (see Section 4.3.4),... 

Acetylide anions are stabilized by the large amount of "s character" of the orbital that holds the electron. 

No matter where the equilibria in the earlier steps of this reaction settle out, the thermodynamic stability of the acetylide anion will drag the overall equilibrium far to the right. When water is added to neutralize the solution and end the reaction, the acetylide will be protonated and 1-butyne can be isolated. The overall reaction is summarized in Figure 12.13. 

The stability of the various cumulenic anions depends to a large extent upon the nature of the groups linked to the cumulenic system. Whereas solutions of lithiated allenic ethers and sulfides in diethyl ether or THF can be kept for a limited period at about O C, the lithiated hydrocarbons LiCH=C=CH-R are transformed into the isomeric lithium acetylides at temperatures above about -20 C, probably via HC C-C(Li )R R Lithiated 1,2,4-trienes, LiCH=C=C-C=C-, are... 

Cumulenic anions, C=C=C and C=C=C=C, without strongly electron-withdrawing substituents are much stronger bases than acetylides, "CsC- and are therefore also stronger nucleophiles. In view of the poor stability of the cumulenic anions at normal temperatures this is a fortunate circumstance the usual functionalization reactions such as alkylation, trimethylsilylation and carboxylation in most cases proceed at a sufficient rate at low temperatures, provided that the... 

Other li acetylides Li-C=C-R with R = hexyl [21] or benzylether dendrons [22, 23] (up to the fourth generation) have also been attached to (Figure 3.3), and various different electrophiles have been used to complete the reaction with the intermediate li-fuUeride (Scheme 3.2 and Figure 3.3). Besides the protonation, alkyl-, benzyl-, cycloheptatrienyl-, benzoyl- or vinylether-derivatives or formaldehyde and dichloro-acetylene were used as electrophiles [12,20]. Most of these electrophiles are attached to the anion in the expected C-2 position. The 1,4-adducts are available by quenching the anion with the tropylium cation or benzoyl chloride [12]. The fuUerene anion can be stabilized by introduction of benzylether dendrons. The lifetimes of the anions change with the size of the dendrons [22]. 

In contrasi to many acetylenes RCsCH, chloroacetylene can be successfully coupled with ketones in liquid ammonia via the lithium compound [80,85], The excellent yield in the reaction with acetone indicates that practically no formation of enolate occurs. Similar good results have been obtained with lithiated ethynyl thioethers, (LiCsCSR), lithiated enyne thioethers, (LiCsCCH=CHSR), lithiated 1,3-diynes (RC=CC=CLi), and lithiated aiylacetylenes (LiCsCAryl)[2], A possible explanation for the small extent of enolization of the ketone is that all these acetylides are less basic due to some stabilization of the anion. 

The unique feature of the alkyllithium compounds that makes them useful as diene initiators is their character as exceedingly powerful bases yet they are soluble in organic solvents and quite thermally stable. Alkyllithium compounds are sufficiently basic to add to hydrocarbon monomers. However, lithium salts of stabilized anions, such as acetylide and fluorenyl anions, are too weakly basic to add to such double bonds. Similarly, alkoxides and mercaptides fail to react with hydrocarbon monomers, but lithium alkyl amides react analogously to alkyllithium compounds. 

An alternative strategy for the preparation of mononuclear ylide complexes is to start from gold(I) precursors which already contain an ylide ligand. Displacement in such complexes of tetrahydrothiophene, SC4H8, by neutral or anionic ligands (including polyfunctional phosphines, acetylides, carbonyl metalates and methanide complexes) leads to a variety of mono-, di- and tetranuclear compounds of remarkable stability (Scheme 9)55,114,211... 

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