Acetylide ions formation

McColm et al. (1977) considered that the eomposition ranges seem to be related to the amounts of R(IV) found in the respective nitrides and carbides (Lorrenzelli et al. 1970, Atoji 1962). In the cerium systems, Ce(IV) is present up to 70%, while in the praseodymium case the amount is less and in the lanthanum system the higher oxidation state is absent. This suggests that Ce(IV) and Pr(IV) do assist in preventing the catenation that leads to acetylide ion formation. However, the reviewers suggest that this factor is probably minor while the size factor of the rare earth atom is essential. 

You have already had considerable experience with caibanionic compounds and their- applications in synthetic organic chemistry. The first was acetylide ion in Chapter 9, followed in Chapter 14 by organometallic compounds—Grignaid reagents, for exanple—that act as sources of negatively polarized car bon. In Chapter 18 you learned that enolate ions—reactive intermediates generated from aldehydes and ketones—are nucleophilic, and that this property can be used to advantage as a method for carbon-carbon bond formation. 

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... 

Introduction 392 9-2 Nomenclature of Alkynes 393 9-3 Physical Properties of Alkynes 394 9-4 Commercial Importance of Alkynes 395 9-5 Electronic Structure of Alkynes 396 9-6 Acidity of Alkynes Formation of Acetylide Ions 397 9-7 Synthesis of Alkynes from Acetylides 399 9-8 Synthesis of Alkynes by Elimination Reactions 403 Summary Syntheses of Alkynes 404 9-9 Addition Reactions of Alkynes 405... 

We can see more clearly now why reaction with acetyiides to form alkynes is limited in practice to primary halides. Under the conditions of the reaction— a solvent of low polarity (liquid ammonia or ether) and a powerful nucleophilic reagent (acetylide ion)—we would expect substitution, that is, alkync formation, to take place by an Sn2 mechanism. Primary halides should therefore form alkynes fastest tertiary halides the slowest. 

The required Cg acetal was prepared from the monodiethylacetal of malonaldehyde which was first converted by deethanolation to the conjugated enol ether. This was reacted with acetylide ion in anhydrous ammonia, followed by mild acidic dehydration and then diethylacetal formation with ethyl orthoformate, as depicted in Scheme 14a. 

With this critical reaction beautifully orchestrated, all efforts could now be directed towards closing the final juncture of the 10-membered enediyne ring, a requirement whose implementation was projected to involve the addition of an acetylide ion to a suitable group at C-7. However, since this operation would ultimately lead to the excision of the alcohol function at C-7, it was decided to use this motif productively for one additional operation before attempting its displacement, namely directing the formation of the central C3-C8 endocyclic epoxide. Thus, 49 was treated with mCPBA in buffered CH2CI2 at 0 C, giving rise to 50 in 88% yield with complete facial selectivity for the drawn product. Next, treatment of 50 with TB AF in THF at 0 °C, followed by silyl reprotection of the phenolic position under standard conditions (TBSCl,... 

In this case, the equilibrium favors formation of the acetylide ion, because it is more stable (a weaker base) than the amide ion. In contrast, consider what happens when a hydroxide ion is used as the base ... 

In this case, the equilibrium does not favor formation of the acetylide ion, because the acetylide ion is less stable (a stronger base) than the hydroxide ion. Therefore, hydroxide is not sufficiently basic to produce a significant amount of the acetylide ion. That is, hydroxide cannot be used to deprotonate acetylene. 

Methylsulfinyl carbanion (dimsyl ion) is prepared from 0.10 mole of sodium hydride in 50 ml of dimethyl sulfoxide under a nitrogen atmosphere as described in Chapter 10, Section III. The solution is diluted by the addition of 50 ml of dry THF and a small amount (1-10 mg) of triphenylmethane is added to act as an indicator. (The red color produced by triphenylmethyl carbanion is discharged when the dimsylsodium is consumed.) Acetylene (purified as described in Chapter 14, Section I) is introduced into the system with stirring through a gas inlet tube until the formation of sodium acetylide is complete, as indicated by disappearance of the red color. The gas inlet tube is replaced by a dropping funnel and a solution of 0.10 mole of the substrate in 20 ml of dry THF is added with stirring at room temperature over a period of about 1 hour. In the case of ethynylation of carbonyl compounds (given below), the solution is then cautiously treated with 6 g (0.11 mole) of ammonium chloride. The reaction mixture is then diluted with 500 ml of water, and the aqueous solution is extracted three times with 150-ml portions of ether. The ether solution is dried (sodium sulfate), the ether is removed (rotary evaporator), and the residue is fractionally distilled under reduced pressure to yield the ethynyl alcohol. 

McLafferty, Fred Warren, 732 McLafferty rearrangement. 416, 732 Mechanism (reaction), 139 acetal formation, 717-718 acetylide alkylation, 272 acid chloride formal ion with SOCl2, 795... 

Fluonnated ylides have also been prepared in such a way that fluorine is incorporated at the carbon p to the carbamomc carbon Various fluoroalkyl iodides were heated with tnphenylphosphine in the absence of solvent to form the necessary phosphonium salts Direct deprotonation with butyllithium or lithium dusopropy-lamide did not lead to y hde formation, rather, deprotonation was accompanied by loss of fluonde ion However deprotonation with hydrated potassium carbonate in dioxane was successful and resulted in fluoroolefin yields of45-80% [59] (equation 54) p-Fluorinated ylides may also be prepared by the reaction of an isopropyli-denetnphenylphosphine yhde with a perfluoroalkanoyl anhydride The intermediate acyl phosphonium salt can undergo further reaction with methylene tnphenylphosphorane and phenyllithium to form a new ylide, which can then be used in a Wittig olefination procedure [60] (equation 55) or can react with a nucleophile [61] such as an acetylide to form a fluonnated enyne [62] (equation 56)... 

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