Internal triples

Hydrocarbons that contain a carbon-carbon triple bond are called alkynes Non cyclic alkynes have the molecular formula C H2 -2 Acetylene (HC=CH) is the simplest alkyne We call compounds that have their triple bond at the end of a carbon chain (RC=CH) monosubstituted or terminal alkynes Disubstituted alkynes (RC=CR ) have internal triple bonds You will see m this chapter that a carbon-carbon triple bond is a functional group reacting with many of the same reagents that react with the double bonds of alkenes... 

Hydrogenation of alkynes with internal triple bonds gives cis alkenes... 

Hoffmaim-La Roche has produced -carotene since the 1950s and has rehed on core knowledge of vitamin A chemistry for the synthesis of this target. In this approach, a five-carbon homologation of vitamin A aldehyde (19) is accompHshed by successive acetalizations and enol ether condensations to prepare the aldehyde (46). Metal acetyUde coupling with two molecules of aldehyde (46) completes constmction of the C q carbon framework. Selective reduction of the internal triple bond of (47) is followed by dehydration and thermal isomerization to yield -carotene (21) (Fig. 10). 

The internal triple bond of 2-butyne is stabilized relative to the tenninal triple bond ... 

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... 

The reaction has also been applied to internal triple bonds ... 

A slightly different process takes place using substrates containing an internal triple bond, such as 6/3-50. Here, the first reaction occurs at the terminal double bond to give 6/3-51 as the final product, as shown by Barrett and coworkers [243]. 

Hydrogenation of alkynes in the presence of P-2 catalyst causes syn addition of hydrogen to take place, and the alkene that is formed from an alkyne with an internal triple bond has the (Z) or cis configuration. 

This means that when we prepare Grignard reagents, we are effectively limited to alkyl halides or tro analogous organic halides containing C=C bonds, internal triple bonds, ether linkages, and -NR2 groups. 

Various diynes were thus converted into the corresponding linear monocar-boxylic acid. Diynes having both a terminal and an internal triple bond exclusively gave the product with CO2 attached to the terminal bond mostly at the 2-position. 

Diacetylenes having an internal and a terminal triple bond can be reduced selectively at the internal triple bond if they are first converted to sodium acetylides at the terminal bond by sodamide prepared in situ from sodium in... 

Concerning chemical selectivities in multiple unsaturated acetylenic compounds, conjugated enynes are reduced with modest selectivity. The most difficult case involves the selective reduction of an enyne with a terminal double bond and an internal triple bond, because the difference in the rates of their hydrogenation is minimal. The hydrogenation of nonterminal, unconjugated diynes to (Z,Z)-dienes can be achieved in good yield. 

Fluorinated alkynes are usually prepared from the corresponding alkenes, but a twofold didechlorination of the corresponding alkane has also been applied. Alkynes bearing a fluorine atom on the triple bond are extremely reactive and hence special precautions have to be taken in their preparation under ordinary conditions, this fluorine atom is lost to afford a non-halogenated triple bond. Perfluoroalkynes with an internal triple bond can even be prepared using zinc. Examples of dehalogenations of fluorinated compounds to provide fluoroalkynes are listed in Table 3. 

Although these catalytic partial hydrogenations of alkynes may well be regarded as the procedure of choice for (Z)-alkenes,25 other catalytic systems have been explored. These include a sodium hydride-sodium alkoxide-nickel(n) acetate reagent,26 and a sodium borohydride-palladium chloride-polyethylene glycol system.27 Diisobutylaluminium hydride (DIBAL) has also been used for the conversion of alkynes into (Z)-alkenes.28 ( )-Alkenes are formed when the internal triple bond is reduced with sodium in liquid ammonia.29... 

It is noteworthy that the addition of thiols to form propargylic sulfides is not catalysed by the neutral complex [Cp RuCl(p2-SR)2RuCl(Cp )], but requires the utilization of a cationic precursor such as [Cp RuCl(p2-SMe)2Ru(Cp )(H20)]0Tf [85]. With this catalytic system,propargylic alcohols bearing an internal triple bond are also transformed into propargylic sulfides, which indicates that in this special case, the reaction does not involve a ruthenium allenylidene as an active species. 

A new method for the addition of cyanoboranes to alkynes was developed (Equation (51)).264 The B-Pd-CN intermediate obtained by oxidative addition was trapped in situ by a internal triple bond with high stereo- and... 

The a,p-acetylenic ketones can be synthesized in good yields by the selective mono-hydroboration-oxidation process of conjugated diynes. The monohydroboration of conjugated diynes with disiamylborane places boron preferentially at the internal triple position of the diyne system. The resultant organoboranes on treatment with sodium hydroxide and 30% H202 afforded the a,P-acetylenic ketones (Eq. 33) 79). 

Among the haloboranes, dibromoborane-methylsulfide has proven to be an excellent hydroborating reagent. Its reaction with terminal and internal alkynes cleanly affords the corresponding vinylic dibromoboranes without the concomitant formation of 1,1-dibora derivatives. This reagent reacts only with internal triple bond in the presence of terminal double bond, and with disubstituted terminal double bond in the presence of monosubstituted one 36c). 

The experiments have shown that B-Br-9-BBN attacks only the terminal triple bond whereas internal triple bond, internal and terminal double bonds can withstand the bromoboration reaction conditions (Eqs. 119 and 120). 

A practical gold-catalyzed route to 4-substituted oxazolidin-2-ones 160 has been described. This synthesis starts from JV-Boc propargylamines 158. This route was applied to both terminal and internal triple bonds although the latter react much more slowly <07EJ03503>. 

Hydrogen adds exclusively to the terminal triple bond of monosubstituted conjugated diynes but the initially formed enynes react rapidly with hydrogen, and even from the beginning, products of over-hydrogenation appear - . After the absorption of one mole of hydrogen by 1,3-pentadiyne in the presence of Pd(CaCOs), the product contains 67% l-penten-3-yne. No products have been detected which correspond to the initial addition of hydrogen to the internal triple bond. 

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