Alkynes, addition reactions hydrogenation

Alkyne addition reactions involving [Os(NH3)5] proceed via 7r-enol and 7r-vinyl ether complexes, " while treatment of [Os(NH3)5(i7 -C6H6)] with hydrogen in methanol in the presence of Pd/C catalyst gives, selectively, [Os(NH3)5(i7 -hexene)]"-". ... 

In Summary Alkynes are very similar in reactivity to alkenes, except that they have two TT bonds, both of which may be saturated by addition reactions. Hydrogenation of the first TT bond, which gives cis alkenes, is best achieved by using the Lindlar catalyst. Alkynes are converted into trans alkenes by treatment with sodium in liquid ammonia, a process that inclndes two snccessive one-electron reductions. 

We have already discussed one important chemical property of alkynes the acidity of acetylene and terminal alkynes In the remaining sections of this chapter several other reactions of alkynes will be explored Most of them will be similar to reactions of alkenes Like alkenes alkynes undergo addition reactions We 11 begin with a reaction familiar to us from our study of alkenes namely catalytic hydrogenation... 

Hydrogen bromide (but not hydrogen chloride or hydrogen iodide) adds to alkynes by a free radical mechanism when peroxides are present m the reaction mixture As m the free radical addition of hydrogen bromide to alkenes (Section 6 8) a regioselectiv ity opposite to Markovmkov s rule is observed... 

Nitrogen-containing heterocyclic compounds, including 1,2,3,4-tetrahydroqui-noline, piperidine, pyrrolidine and indoline, are also popular hydrogen donors for the reduction of aldehydes, alkenes, and alkynes [75, 76]. With piperidine as hydrogen donor, the highly reactive 1-piperidene intermediate undergoes trimer-ization or, in the presence of amines, an addition reaction [77]. Pyridine was not observed as a reaction product. 

Disubstituted alkynes and terminal alkynes form E-dibromoalkenes [4] when the tribromide is formed in situ in an essentially basic medium, an addition reaction followed by elimination of hydrogen bromide results in the conversion of terminal alkynes into the 1-bromoalkynes [5]. When the addition reaction is conducted in methanol, l,l-dibromo-2,2-dimethoxyalkanes are produced, in addition to the 1,2-dibromoalkenes [6], The dimethoxy compounds probably result from the initial intermediate formation of bromomethoxyalkenes. Under similar conditions, alkenes yield methoxy bromo compounds [7]. 

In organic chemistry, reduction is defined as a reaction in which a carbon atom forms fewer bonds to oxygen, O, or more bonds to hydrogen, H. Often, a C=0 bond or C=C bond is reduced to a single bond by reduction. A reduction that transforms double C=C or C=0 bonds to single bonds may also be classified as an addition reaction. Aldehydes, ketones, and carboxylic acids can be reduced to become alcohols. Alkenes and alkynes can be reduced by the addition of H2 to become alkanes. 

Alkenes are much less reactive in the H-P bond addition reactions than alkynes. However, 1,3,2-dioxaphospholane 2-oxide (4a), a five-membered hydrogen phosphonate of pinacol, is exceptionally reactive with alkenes although dialkyl and diaryl phosphonates such as 4b-d are totally unreactive (Scheme 25) [26]. Very interestingly, six-membered hydrogen phosphonates 4e,f are also unreactive under identical conditions. 

Addition of hydrogen atoms in the presence of a metal catalyst to double or triple bonds is known as hydrogenation or catalytic hydrogenation. Alkenes and alkynes are reduced to alkanes by the treatment with H2 over a finely divided metal catalyst such as platinum (Pt—C), palladium (Pd—C) or Raney nickel (Ni). The platinum catalyst is also frequently used in the form of Pt02, which is known as Adams s catalyst. The catalytic hydrogenation reaction is a reduction reaction. 

Nonactivated alkynes react with hydrogen fluoride/polyvinylpyridine normally by addition of two equivalents of hydrogen fluoride, to form geminal difluoro derivatives. See also Houben-Weyl, Vol. 5/3, pp 108-113. Hydrogen fluoride/polyvinylpyridine is used in the presence of Lrichlorofluoromethane to fluorinatc hex-l-yne and hex-3-yne to give 2,2-difluoro- (56%) and 3,3-difluorohexane (59%) after a reaction time of 72 hours 35 for a modified procedure, see ref 31. 

The addition of hydrogen across multiple bonds is one of the most widely studied of catalytic reactions. Alkenes and alkynes, as well as di- and polyunsaturated systems can all be hydrogenated, provided the suitable experimental conditions are used. Studies on the ways in which these compounds react with hydrogen have revealed very complex reaction patterns. Because of their resonance stabilization, carbocyc-lic aromatic hydrocarbons are more difficult to hydrogenate than are other unsaturated compounds. 

A closely related synthesis utilizes alkynic epoxides, which add hydrogen sulfide in the presence of a base to form thiophenes in good yield. For example, when the epoxide (184) was stirred in an aqueous solution of barium hydroxide with slow addition of hydrogen sulfide gas, and the product extracted after neutralization with acetic acid, the substituted thiophene (186) was obtained in 60-70% yield (53ZOB1569). In these experiments R1 = Me or Et, R2 = H or Me and R3 = Ph. The same synthetic method was used to produce a thiophene having an optically active substituent, with relatively little racemization. In this case R1 = EtC HMe while R2 and R3 = H, but the yield was equivalent (73JOC2361). This reaction also undoubtedly proceeds via an intermediate (185), a structure related to a 1 -mercapto-1,3-butadiene. 

The principles of radical addition reactions of alkenes appear to apply equally to alkynes, although there are fewer documented examples of radical additions to triple bonds. Two molecules of hydrogen bromide can add to propyne first to give cis-1 -bromopropene (by antarafacial addition) and then 1,2-dibromopropane ... 

Further chemistry of alkenes and alkynes is described in this chapter, with emphasis on addition reactions that lead to reduction and oxidation of carbon-carbon multiple bonds. First we explain what is meant by the terms reduction and oxidation as applied to carbon compounds. Then we emphasize hydrogenation, which is reduction through addition of hydrogen, and oxidative addition reactions with reagents such as ozone, peroxides, permanganate, and osmium tetroxide. We conclude with a section on the special nature of 1-alkynes— their acidic behavior and how the conjugate bases of alkynes can be used in synthesis to form carbon-carbon bonds. 

There are several methods available for the electrophilic addition of hydrogen and nitrogen to alkenes, dienes and alkynes. While the direct electrophilic addition of amines to these substrates is not feasible, aminomercuration-demercuration affords a very useful indirect approach to such amines. The addition of amides to C—C multiple bonds can be effected directly through the Ritter reaction or by the less direct, but equally useful, amidomercuration-demercuration process using either nitriles or amides. Similarly, H—N3 addition to alkenes can be carried out directly or via mercuration to produce organic azides. 

Consideration of the oxidation level reveals diat while one carbon is reduced (the one to which hydrogen adds), die other is oxidized (die one to which the oxygen adds). There is no net change in oxidation level of the alkene functional group. Likewise die reverse processes of these addition reactions, namely, elimination of HX from alkyl halides and dehydration of alcohols to give alkenes, are not redox processes. Additions of water to alkynes is analogous. In this case, however, the product is a ketone, the oxidation level of the ketone is seen to be the same as the alkyne, and so no net change in oxidation level has occurred. 

Triple bonds are linear and the carbons are sp-hybridized (Figure 3.16). Alkynes, like alkenes, undergo addition reactions. A hydrogen connected to a triply bonded carbon is weakly acidic and can be removed by a very strong base such as sodium amide, NaNH2, to give acetylides. 

The double and triple bonds in alkenes and alkynes have extra electrons capable of forming additional bonds. Therefore, the carbon atoms attached to these bonds can add atoms without losing any atoms already bonded to them the multiple bonds are said to be unsaturated. Therefore, alkenes and alkynes both undergo addition reactions, in which pairs of atoms are added across unsaturated bonds, as shown in the reaction of ethylene with hydrogen to give ethane ... 

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