Alkylation of a terminal alkyne

Terminal alkynes by virtue of the presence of an acidic hydrogen atom can be converted into the corresponding alkynylsodium or alkynylmagnesium halide (see Section 5.4.2, p. 531) which may then be alkylated to give a homologous alkyne. 

Acetylene itself can form both the mono- or di-sodium salt the former is the main product when a large excess of acetylene is used in the reaction with soda-mide in liquid ammonia. 

In these alkylation reactions primary alkyl halides (the bromide for preference) should be used as the alkylating agents, since secondary and tertiary halides undergo extensive olefin-forming elimination reactions in the presence of the strongly basic acetylide ion. A typical synthesis is that of hex-l-yne (Expt 5.26). 

A modified procedure which is suitable for the introduction of a secondary or tertiary alkyl group uses the trialkynylalane as the intermediate. This is formed from aluminium chloride and an alkynyllithium, and then subsequently allowed to react with the tertiary or secondary halide.45 

(1) Occasionally the reaction mixture does not become completely black nor free from suspended solid here the acetylide is an insoluble (or sparingly soluble) form, but it gives satisfactory results in the preparation of hex-l-yne. The saturated solution of the soluble forms of sodium acetylide in liquid ammonia at — 34 °C is about 4.1 m. 

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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. 

The trans double bond in the target molecule is a product of reduction of a triple bond with Li in NH3.The alkyne was formed by an alkylation of a terminal alkyne with bromomethane. The terminal alkyne was synthesized from the starting alkene by bromination, followed by dehydrohalogenation. 

Next ask, "What is an immediate precursor of 2-hexyne " We ve seen that an internal alkyme can be prepared by alkylation of a terminal alkyne anion. In the present... 

As we have seen many times in this chapter, the only method we have learned for achieving this transformation is the alkylation of a terminal alkyne. Our synthesis must therefore involve an alkylation step. This should be taken into account when performing a retrosynthetic analysis. 

The housefly (Musca domestica) uses the hydrocarbon (Z)-9-tricosene (318 in Scheme 58) as the major component of its sex pheromone. Simple syntheses involve Wittig coupling 153, 154) or alkylation of a terminal alkyne and subsequent reduction 155, 156) to afford the desired Z-isomer in ca. 95% purity. Erucic acid 157, 158) or oleic acid 159, 160) have served as starting material of known Z stereochemistry. The transition metal-catalyzed olefin cross-metathesis reaction has been applied by Rossi 161) to synthesize (318) as a mixture of /Z-isomers together with the other possible Cis and C28 olefins (Scheme 58). 

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

Since Bruce s pioneering work in the area of ruthenium vinylidene chemistry (1), it has been well known that isomerization of a terminal alkyne to a vinylidene on a metal center is not only favorable but also effects a reversal in the reactivity of the carbon atoms. However, hydration catalysis was not possible, because alkyl migration from a proposed acyl intermediate led to an... 

The cis double bond can be formed by hydrogenation of an alkyne, which can be synthesized by an alkylation reaction of a terminal alkyne. 

In all of these problems, an acetylide ion (or an anion of a terminal alkyne) is alkylated by haloalkane. 

Synthesis of secootkiry eokois from l-aOtynes. Dihydroboration at room temperature of a terminal alkyne with either (I) or 9-BBN gives a 1,1-diborylalkane (3) this is treated at 0-5° with I eq. of methyllithium in ether. The product (4) rearranges to (S). An alkyl halide (100 excess) is then added, and the resultant secondary organo-borane (6) is oxidized with alkaline hydrogen peroxide. Secondary alcohols (7) are obtained in 70-85% yield. 

When an alkyne undergoes the acid-catalyzed addition of water, the product of the reaction is an enol. The enol immediately rearranges to a ketone. A ketone is a compound that has two alkyl groups bonded to a carbonyl (C=0) group. An aldehyde is a compound that has at least one hydrogen bonded to a carbonyl group. The ketone and enol are called keto-enol tautomers they differ in the location of a double bond and a hydrogen. Interconversion of the tautomers is called tautomerization. The keto tautomer predominates at equilibrium. Terminal alkynes add water if mercuric ion is added to the acidic mixture. In hydroboration-oxidation, H is not the electrophile, H is the nucleophile. Consequently, mercuric-ion-catalyzed addition of water to a terminal alkyne produces a ketone, whereas hydroboration-oxidation of a terminal alkyne produces an aldehyde. 

Vinylboranes are versatile intermediates, and there are modifications that convert them to alkenes. Hydro-boration of a terminal alkyne such as cyclohexyl acetylene (78) with dicyclohexylborane gave ( )-alkenyl-borane (79). Subsequent treatment with iodine and sodium hydroxide resulted in transfer of an alkyl group from boron to the alkenyl carbon, generating cis-1,2-dicyclohexylethene (80) in 77% yield (92% isomeric... 

Formal carboboration of a terminal alkyne could be achieved by bromoboration and subsequent palladium-catalysed substitution of alkyl for bromine with alkylzinc chloride. 

Fig. I. Reaction of a terminal alkyne with a strong base. Alkylation...

Each alkyne can be synthesized by alkylation of an appropriate alkyne anion. First decide which new carbon-carbon bond or bonds must be formed by alkylation and which alkyne anion nucleophile and haloalkane pair is required to give the desired product. Synthesis of a terminal alkyne from acetylene requires only one nucleophilic substitution, and synthesis of an internal alkyne from acetylene requires two nucleophilic substitutions. 

A multi-component reaction of a terminal alkyne, sulfur, electrophile (E-X) and carbodiimides, R R CH-N=C=N-R, produces 1,2-dihydrothiopyrimidines and 2,3-dihydropyrimidinthiones (43, R" derived from alkyne, E = H, alkyl). The expected N=C cleavage of the diimide is accompanied by an unexpected C(5p )-H cleavage, such that the carbodiimide acts as sources of H -I- R R -C-N -I- C=N-R , with subsequent reorganization to give products. 

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