Alkynyl anions

Carbanions are negatively charged organic species with an even number of electrons and the charge mainly concentrated on a carbon atom. In alkyl, alkenyl, and alkynyl anions all of the... 

Alkynyl anions are more stable = 22) than the more saturated alkyl or alkenyl anions (p/Tj = 40-45). They may be obtained directly from terminal acetylenes by treatment with strong base, e.g. sodium amide (pA, of NH 35). Frequently magnesium acetylides are made in proton-metal exchange reactions with more reactive Grignard reagents. Copper and mercury acetylides are formed directly from the corresponding metal acetates and acetylenes under neutral conditions (G.E. Coates, 1977 R.P. Houghton, 1979). 

Finally (d + aV dditions of 1-alkenyl and 1-alkynyl anions to carbonyl groups should be mentioned. Examples are the addition of vinylmagnesium bromide to ketones e.g. in the first step of Torgov s steroid synthesis (I.N. Nazarov, 1957), and the famous alkynylation of... 

Phenyl trimethylsilylethyne (Chapter 20) undergoes a fluoride-ion-catalysed addition to aldehydes and ketones. This provides a remarkably mild, relatively non-basic method for the generation of an alkynyl anion or its equivalent. 

Wada, M., Sakurai, Y., Akiba, K.-y. (1984) Addition of Alkynyl Anions to Aldimines Containing A-Hydrogens A Novel Synthesis offi-Aminoacetylenes. Tetrahedron Letters, 25, 1083-1084. 

An alternative disconnection of homopropargylic alcohols substrates for intramolecular hydrosilylation is the opening of an epoxide with an alkynyl anion. This strategy was employed in a total synthesis of the macrolide RK-397 (Scheme 20). Epoxide ring opening serves to establish homopropargylic alcohol C with the appropriate stereochemistry. A hydrosilylation/oxidation protocol affords the diol E after liberation of the terminal alkyne. The... 

LDA is a strong base. Two E2 eliminations give an alkyne, which is deprotonated by the excess LDA to give an alkynyl anion. This species then reacts with Mel by an Sjsj-2 process. 

R = H, Ph) have been obtained from the anionic [Ru(N4Meg)] and ethyne or HC CPh, respectively [51], In this case, formation of an intermediate r -alkyne complex is unlikely, the probable mechanism being deprotonation of the alkyne and coordination of the alkynyl anion followed by proton transfer. 

In contrast to the alkynyl anion, coordination to a metal center results in Co, being electron-poor and subject to frontier-orbital controlled nucleophilic attack, while the... 

Special attention is warranted to the role of copper iodide and the unusual solvent trie thy lamine. Triethylamine is not a strong base, but it is nevertheless able to support a small equilibrium concentration of the alkynyl anion. Copper iodide assists in this dcprolona-tion because the resulting at complex 51 is stable. The reaction occurs catalytically, so catalytic amounts of 51 are sufficient. 

The alkyne insertion reaction is terminated by anion capture. As examples of the termination by the anion capture, the alkenylpalladium intermediate 189, formed by the intramolecular insertion of 188, is terminated by hydrogenolysis with formic acid to give the terminal alkene 192. Palladium formate 190 is formed, and decarboxylated to give the hydridopalladium 191, reductive elimination of which gives the alkene 192 [81]. Similarly the intramolecular insertion of 193 is terminated by transmetallation of 194 with the tin acetylide 195 (or alkynyl anion capture) to give the dienyne 196 [82], Various heterocyclic compounds are prepared by heteroannulation using aryl iodides 68 and 69, and internal alkynes. Although the mechanism is not clear, alkenylpalladiums, formed by insertion of alkynes, are trapped by nucleophiles... 

To prepare ethynylestradiol, we can treat estrone with 2 equivalents of an alkynyl anion ... 

We will discuss the preferred geometries and the MO descriptions of carbon radicals and the corresponding carbenium ions or carbanions in two parts. In the first part, we will examine carbon radicals, carbenium ions, and carbanions with three substituents on the carbon atom. The second part treats the analogous species with a divalent central C atom. Things like alkynyl radicals and cations are not really important players in organic chemistry and won t be discussed. Alkynyl anions, however, are extremely important, but will be covered later. 

Alkynyl anions can be reacted with elemental selenium to form alkynylselenolates 203. These alkynylselenolates 203 have been used as key intermediates for the synthesis of a variety of selenocarbonyl compounds. First reactions have already been carried out earlier,384 and some later improvements widened the scope of this reaction including the synthesis of selenoesters, selenothioesters, and selenoamides of type 204 (Scheme 60).385,386 Even a direct... 

The converse of this idea is central to a useful bit of chemoselectivity that can be obtained in the reactions of dianions. 1-Propynol can be deprotonated twice by strong bases—first, at the hydroxyl group to make an alkoxide anion (the pIQ of the OH group is about 16) and, secondly, at the alkyne (pKa of the order of 25) to make a reacts with electrophiles it always reacts at the alkynyl anion and not at the alkoxide. 

A synthesis could start with the same bromoketone as the one above reduction gives an alcohol, but alkylation of an alkynyl anion with this compound is not possible, because the anion will just deprotonate the hydroxyl group. 

Making alkenes by addition to alkynes offers two distinct advantages. Firstly, although the reaction is not connective in the sense that the Wittig and Julia reactions are, the starting materials can often be made straightforwardly by alkylation of alkynyl anions. Secondly, the same alkyne can be used to make either E- or Z-alkene—an advantage shared with the Peterson reaction but here the... 

A filled sp orbital is lower in energy than filled sp or sp orbitals since it is closer to the positively charged nucleus. This imparts sufficiently greater acidity to acetylene and 1-alkynes (pA a 24-26) so that bases such as alkyllithiums, lithium dialkylamides, sodium amide in liquid ammonia, and ethylmagnesium bromide may be used to generate the alkynyl anions (see Section 8.2). 

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