Terminal alkyne acetylide

Metal derivatives of terminal alkynes, RC2H. Transition metals form complex acetylides (e.g. (M(C = CR) ]- ) often containing the metal in low oxidation states. 

Terminal alkynes are only reduced in the presence of proton donors, e.g. ammonium sulfate, because the acetylide anion does not take up further electrons. If, however, an internal C—C triple bond is to be hydrogenated without any reduction of terminal, it is advisable to add sodium amide to the alkyne solution Hrst. On catalytic hydrogenation the less hindered triple bonds are reduced first (N.A. Dobson, 1955, 1961). 

Both chlorines of 1,1-dichloroethylene (340) react stepwise with different terminal alkynes to form the unsymmetrical enediyne 341 [250]. The coupling of the dichloroimine 342 with tin acetylide followed by hydrolysis affords the dialkynyl ketone 343[2511. The phenylthioimidoyl chloride 344 undergoes stepwise reactions with two different tin acetylides to give the dialkynylimine 345[252],... 

Terminal alkynes react with propargylic carbonates at room temperature to afford the alka-l, 2-dien-4-yne 14 (allenylalkyne) in good yield with catalysis by Pd(0) and Cul[5], The reaction can be explained by the transmetallation of the (7-allenylpailadium methoxide 4 with copper acetylides to form the allenyKalk-ynyl)palladium 13, which undergoes reductive elimination to form the allenyl alkyne 14. In addition to propargylic carbonates, propargylic chlorides and acetates (in the presence of ZnCb) also react with terminal alkynes to afford allenylalkynes[6], Allenylalkynes are prepared by the reaction of the alkynyl-oxiranes 15 with zinc acetylides[7]. 

Solutions of sodium acetylide (HC=CNa) may be prepared by adding sodium amide (NaNH2) to acetylene m liquid ammonia as the solvent Terminal alkynes react similarly to give species of the type RC=CNa... 

The most striking difference between alkenes and alkynes is that terminal alkynes are weakly acidic. When a terminal alkyne is treated with a strong base, such as sodium amide, Na+ -NH2, the terminal hydrogen is removed and an acetylide anion is formed. 

Because of its generality, acetylide alkylation is an excellent method for preparing substituted alkynes from simpler precursors. A terminal alkyne can be prepared by alkylation of acet dene itself, and an internal alkyne can be prepared by further alkylation of a terminal alkyne. 

Strategy Compare the product with the starting material, and catalog the differences. In this case, we need to add three carbons to the chain and reduce the triple bond. Since the starling material is a terminal alkyne that can be alkylated, we might first prepare the acetylide anion ol 1-pentyne, let it react with 1-bromopropane, and then reduce the product using catalytic hydrogenation. 

What is an immediate precursor of a terminal alkyne " Perhaps sodium acetylide and an alkyl halide. 

Terminal alkynes are weakly acidic. The alkyne hydrogen can be removed by a strong base such as Na+ NH2 to yield an acetylide anion. An acetylide... 

Acetylide anion (Section 8.7) The anion formed by removal of a proton from a terminal alkyne. 

SN displacement reactions, 27-9 a-Selenocyclohexanones, 77 Senecioyl chloride, 33 Silmagnesiation, platinum-catalysed, 8 Silametallation of terminal alkynes, 7-9 Silver acetylide, 49 Silver trifluoroacctate, 42,127 Silyl cuprates, 7... 

Organic compounds can be metalated at suitably acidic positions by active metals and by strong bases.The reaction has been used to study the acidities of very weak acids (see p. 228). The conversion of terminal alkynes to acetylid ions is one... 

Triple bonds can also be selectively reduced to double bonds with DIBAL-H, " with activated zinc (see 12-36), with hydrogen and Bi2B-borohydride exchange resin, ° or (internal triple bonds only) with alkali metals (Na, Li) in liquid ammonia or a low-molecular-weight amine.Terminal alkynes are not reduced by the Na—NH3 procedure because they are converted to acetylide ions under these conditions. However, terminal triple bonds can be reduced to double bonds by the... 

Normally, copper-catalysed Huisgen cycloadditions work with terminal alkynes only. The formation of a Cu-acetylide complex is considered to be the starting point of the catalyst cycle. However, the NHC-Cu complex 18 was able to catalyse the [3-1-2] cycloaddition of azides 17 and 3-hexyne 23 (Scheme 5.6). 

The coupling of terminal alkynes with aryl or alkenyl halides catalysed by palladium and a copper co-catalyst in a basic medium is known as the Sonogashira reaction. A Cu(I)-acetylide complex is formed in situ and transmetallates to the Pd(II) complex obtained after oxidative addition of the halide. Through a reductive elimination pathway the reaction delivers substituted alkynes as products. 

In the case of 3-alkynylamines, IH proceeds exclusively in a S-Endo-Dig process to give substituted 1-pyrrolines. The best catalysts are palladium complexes (Eq. 4.69) the reaction fails for terminal alkyne owing to the formation of a stable palladium acetylide [278]. 

There are a number of procedures for coupling of terminal alkynes with halides and sulfonates, a reaction that is known as the Sonogashira reaction.161 A combination of Pd(PPh3)4 and Cu(I) effects coupling of terminal alkynes with vinyl or aryl halides.162 The reaction can be carried out directly with the alkyne, using amines for deprotonation. The alkyne is presumably converted to the copper acetylide, and the halide reacts with Pd(0) by oxidative addition. Transfer of the acetylide group to Pd results in reductive elimination and formation of the observed product. 

Because of the slightly acidic nature of the sp C-H bonds, the reaction of metal acetylides with various electrophiles is one of the most general strategies in organic transformations.1 Traditionally, such reactions are carried out by using alkali metal acetylides which are air and water sensitive. On the other hand, there is much interest in developing transition-metal catalyzed terminal alkyne reactions involving soft and more stable C-M bonds as reaction intermediates, because many such reactions can tolerate water. 

A related cyclization of 2-(alkynyl)phenylisocyanates with terminal alkynes to oxindoles was also reported by the same group (Equation (115)).472 (E)-exo-olefinic oxoindoles are selectively obtained. It was proposed that a palladium acetylide generated by the C-H activation of terminal alkynes regioselectively inserts to the alkyne moiety and the resulting vinylpalladium intermediate adds to the C=0 part of the isocyanate to give a (Z)-oxindole. This (Z)-isomer is isomerized to the ( )-isomer under the reaction conditions through catalysis of the phosphine. 

A multi-step reaction sequence was then realized to prepare the precursor (178) for the pivotal macrocyclization reaction. Alternate stepwise chain elongations were achieved according to Schemes 28 and 29. Reaction of the tosylate prepared from the alcohol 162 with lithium acetylide afforded the alkyne 174 (Scheme 28). Following the introduction of a tosylate at the upper branch, a one-carbon chain elongation of the terminal alkyne afforded the methyl alkynoate 175. A methyl cuprate 1,4-addition was used to construct the tri-substituted C double bond stereoselectively. For this purpose, the alkynoate 175 was initially transformed into the Z-configured a,/ -unsat-... 

Stereoselective Alkene Synthesis. Terminal alkynes can also be alkylated by organoboranes. Adducts are formed between a lithium acetylide and a trialkylborane. Reaction with iodine induces migration and results in the formation of the alkylated alkyne.24 25... 

The metal vinylidene intermediates discussed elsewhere in this chapter are limited to a single carbon-substituent on account of the 1,2-migration process by which they form from terminal alkynes. Alkenylidenes—vinylidenes bearing two carbon-substituents—are formed by nucleophilic addition of the (i-carbon of a metal acetylide to an electrophile (Scheme 9.16) [30]. 

Terminal alkynes are acidic, and the end hydrogen can be removed as a proton by strong bases (e.g. organolithiums, Grignard reagents and NaNH2) to form metal acetylides and alkynides. They are strong nucleophiles and bases, and are protonated in the presence of water and acids. Therefore, metal acetylides and alkynides must be protected from water and acids. 

Heavy metal acetylides test for terminal alkynes... 

Besides electrophilic addition, terminal alkynes also perform acid-base type reaction due to acidic nature of the terminal hydrogen. The formation of acetylides and alkynides (alkynyl Grignard reagent and aUcylnyllithium) are important reactions of terminal alkynes (see Section 4.5.3). Acetylides and alkynides undergo nucleophilic addition with aldehydes and ketones to produce alcohols (see Section 5.3.2). 

They react with alkyl halides to give internal alkynes (see Section 5.5.2) via nucleophilic substitution reactions. This type of reaction also is known as alkylation. Any terminal alkyne can be converted to acetylide and alkynide, and then alkylated by the reaction with alkyl halide to produce an internal alkyne. In these reactions, the triple bonds are available for electrophilic additions to a number of other functional groups. 

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