Acetylenic anion

Svndiesis (Crombie, J. Chem. Soc. (C), 1969, 1016). The acetylenic bromide corresponding to allyl bromide is called propargyl bromide and is reactive and readily available. We shall need to protect the ketone before we make the acetylene anion. It turns out tliat protection and decarboxylation can be done in one step. 

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 m the starting alkyne will neither tolerate the strongly basic conditions required for anion formation nor survive m a solution containing carbanions Acetyhde ions add to carbonyl... 

Answer Dianion (11) is formed from (9) with BuLi the acetylene anion (pKa 25) is very much more reactive than the carboxylatc ion (pKa - 5) so it reacts preferentially. 

Syntheses (a) and (c) use the same disconnect ion but with opposite polarity so that the acetylene anion In (c) is a reagent for the synthon (34),... 

Acetylene anion serves for synthon (4), the hydration with mercury ton catalysis being unambiguous. 

Disconnection of the ether la now a good idea as it leaves an ct-hydroxy ketone to be made by acyl anion equivalent addition. We prefer (10) as the acetylene anion can then serve as synthon (4). 

One of the most useful coupling reactions is the Eglintion reaction in which the oxidation of acetylene anions with ferric ion proceeds very satisfactorily. 

The use of a basic solvent (in this case diethylamine) is important to stabilize acetylenic anions.(9) The third catalyst system component, triphenyl phosphine, is presumably added to help replace lost triphenyl phosphine ligands on the palladium complex and thus prevent metal agglomeration. 

The potassium salt of the 2,2 -dipyridyl acetylene anion-radical represents another important example. In this case, the spin and charge are localized in the framework of N-C-C=C-C-N fragment. The atomic charge on each nitrogen atom is -0.447, that is, close to unity in total. The energy of this ion pair is minimal when the potassium counterion is located midway between the two rather close nitrogen lone pairs. Such a structure is consistent with the fact that the ESR spectrum of this species is almost insensitive to temperature. It means that the counterion does not hop between two remote sites of the anion-radical (Choua et al. 1999). 

The reduction of acetylenic tellurides to the vinylic ones is achieved by treatment with NaBH4 in EtOH. The formal unusual reduction of the carbon triple bond by NaBH4 can be rationalized involving the attack of a hydride ion to the tellurium atom producing a tellurol and an acetylenic anion followed by the addition of the tellurolate anion to the acetylene, DIBAL-H has later been employed as a reducing agent. ... 

With Functionalized Carbanions. Other nucleophilic carbanions, such as lithio-dithiane or sodiomalonate or acetylene anion react easily with epoxides. 

Nonempirical quantum-chemical calculations of acetylide molecules support the ready displacement of alkali metal cations to the bridge position (87IZV2777 88IZV1335, 88IZV1339). This naturally leads to the conclusion that the polarization and deformation of the ir-electronic shell of acetylene must depend on the atomic number of the cation attached to the acetylene anion. However, the acetylene activation in the reaction with ketoximes via acetylides suggests nucleophile attack at a carbanionlike complex, which is of course a week point of the hypothesis. Nevertheless, the electrophilic assistance from the alkali metal cation (Na+) to the... 

The acetylene anion radical was successfully generated in a glassy MeTHF matrix via ionizing irradiation (60Co) at 77 K. Upon illumination with light 430 nm, the following isomerization takes place (Itagaki Shiotani 1999) ... 

In order to side-step potential problems with hydroxyl-directed hydro-metallation of the alkyne, ester 14 was reduced with sodium borohydride in methanol. This also led to a small amount of alkyne desilylation (5%-10%). Fortunately, however, this was the next step of the synthesis. After selective protection of diol 15 to obtain 8, the stabilised acetylenic anion... 

Scheme 8.5 Acetylenic anion ring-closure in Danishefsky s calicheamicinone synthesis.

As shown in Appendix 1, the pKa value for hydrocyanic acid is approximately 9.3, and the pKa value for acetylene is approximately 25. Thus, the acetylene anion is more reactive than the cyanide anion and is therefore the better nucleophile. 

Trifluoromethylsulfonyl)alkynes such as 4 are easily obtained by sulfonylation of acetylenic anions with trifluoromethanesulfonic anhydride (3). ... 

Weak nucleophiles such as the acetylene anion have been introduced cleanly to form R3Sb(C=CR )2 (equation 195 ). 

The majority of the known heterocyclic dienes are pentadienes, largely as a result of the discovery that alkynes react with alkali metals and form dianions via dimer addition of the acetylene anion radical. A method that has become useful for the generation of larger cyclic dienes is the addition of an organometallic dihydride to a suitable dialkyne. Synthetic approaches to six-membered dienes tend to be unique for each system, and no general method has been developed. Some of the six-membered heterocyclic dienes have been converted to trienes for the study of systems that may be aromatic. 

Alkylation of the acetylenic anion with isobutyraldehyde gave the propargylic alcohol which was then oxidized using Collin s reagent to yield the requisite ketone. 

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