Alkynes reduction with sodium/ammonia

Alkynes are converted to trans alkenes on reduction with sodium in liquid ammonia. 

Reduction of alkynes with sodium in ammonia,147 lithium in low-molecular-weight amines,148 or sodium in hexamethylphosphoric triamide containing /-butanol as a proton source149 leads to the corresponding is-alkene. The reaction is assumed to involve successive electron-transfer and proton-transfer steps. 

Partial reduction of alkynes with sodium in liquid ammonia or bycatalytic hydrogenation. 

The alternative reverse addition procedure can give incomplete reduction of the alkyne (33). An increase in the ratio of liquid ammonia to alkyne (34), the addition of co-solvents (23), the use of lithium rather than sodium, or the use of a higher temperature in an autoclave are advisable for the reduction of high molecular weight alkynes to overcome solubility problems which can also result in incomplete reduction. The resulting olefin is usually very pure ji isomer containing no detectable Z isomer. Use of an alcohol as a co-solvent and proton donor can accelerate the reduction, but the resulting olefin then contains a minor amount of the Z isomer. Polymer-bound alkynes can not be successfully reduced with sodium in liquid ammonia (35). 

A similar synthesis of alkynes by reductive elimination of enol phosphates of /3-oxosulfones with sodium in liquid ammonia has been reported. ... 

The reduction of alkynes may be carried out with sodium dissolving in liquid ammonia. Subsequent protonation of the dianion gives the trans-alkene. On the other hand, catalytic reduction gives the r /A-alkene. An illustration of alkyne chemistry involves the preparation of intermediates for the synthesis of vitamin A shown in Scheme 3.28. Complete hydrogenation to an alkane takes place over a platinum catalyst. 

The reduction of a carbon-carbon multiple bond by the use of a dissolving metal was first accomplished by Campbell and Eby in 1941. The reduction of disubstituted alkynes to c/ s-alkenes by catalytic hydrogenation, for example by the use of Raney nickel, provided an excellent method for the preparation of isomerically pure c -alkenes. At the time, however, there were no practical synthetic methods for the preparation of pure trani-alkenes. All of the previously existing procedures for the formation of an alkene resulted in the formation of mixtures of the cis- and trans-alkenes, which were extremely difficult to separate with the techniques existing at that time (basically fractional distillation) into the pure components. Campbell and Eby discovered that dialkylacetylenes could be reduced to pure frani-alkenes with sodium in liquid ammonia in good yields and in remarkable states of isomeric purity. Since that time several metal/solvent systems have been found useful for the reduction of C=C and C C bonds in alkenes and alkynes, including lithium/alkylamine, ° calcium/alkylamine, so-dium/HMPA in the absence or presence of a proton donor,activated zinc in the presence of a proton donor (an alcohol), and ytterbium in liquid ammonia. Although most of these reductions involve the reduction of an alkyne to an alkene, several very synthetically useful reactions involve the reduction of a,3-unsaturated ketones to saturated ketones. ... 

Predominantly trans-alkcne is obtained by reduction of alkynes with sodium or lithium in liquid ammonia. Almost entirely cw-alkene (as high as 98%) is obtained by hydrogenation of alkynes with several different catalysts a specially... 

The most recent synthesis of 1 is that of Bestmann. Acetylenic alcohol 2 was converted to the dianion and alkylated with the acetal of 4-bromopentanal to give alkyne 53. Subsequent reduction of the triple bond with sodium in ammonia generated exclusively the required ( )-olefin geometry. 0-Alkylation... 

Reduction of alkynes with sodium in liquid ammonia produces -alkenes as you will be aware though the reduction of functionalised alkynes with LiAlH4 is perhaps more common nowadays. Here are simple examples of both methods the LiAIII4 reduction will be discussed in the next chapter. 

Reduction of the C11 alkyne with sodium/liquid ammonia afforded the non-natural E-isomer. 

The total synthesis started with a Birch reduction of p-methoxytoluene (382) to obtain the dihydro compound 383, which was treated with p-toluenesulfonic acid to obtain acetal 384. CyclopropanatiOTi with ethyl diazoacetate and transaceta-lization led to compound 385, which reacted to the unsaturated keto ester 386 on treatment with base. In the next step, the keto ester 386 was methylated with methylmagnesium chloride, and it reacted selectively at the 2-positon to yield 387. Lactonization with further methylation with methyl iodide afforded homo-lactone 389, which reacted with lithium salt 390 to alkyne 391 and was reduced with sodium borohydride to diol 392. Partial reductiOTi of the triple bond to the double bond was obtained with sodium in ammonia and further treatment with acid led to hydrolysis of the acetal, which subsequently cychzed to 394 (Scheme 8.1). 

Dissolving metal reductions works very well with aldehydes and ketones, but alkenes are not readily reduced under the same conditions. For example, 1-hexene is reduced to hexane in only 41% yield with Na/MeOH/liquid NHg.14 Alkynes, on the other hand, are reduced to alkenes in good yield using dissolving metal conditions, and the experimental evidence shows that the -alkene is the major product. In a typical example, 4-octyne (60) is treated with sodium in liquid ammonia, and oct-4 -ene (64) is isolated in 90% yield. None of the Z-alkene is observed in this reaction. The reaction with sodium in liquid ammonia is an electron transfer process similar to that observed with ketones and aldehydes, but how is the E geometry of the alkene product explained ... 

Benzene was introduced in Chapter 5 (Section 5.10). Chapter 21 will discuss many benzene derivatives, along with the chemical reactions that are characteristic of these compounds. In the context of dissolving metal reductions of aldehydes, ketones, and alkynes, however, one reaction of benzene must be introduced. When benzene (65) is treated with sodium metal in a mixture of liquid ammonia and ethanol, the product is 1,4-cyclohexadiene 66. Note that the nonconjugated diene is formed. The reaction follows a similar mechanism to that presented for alkynes. Initial electron transfer from sodium metal to benzene leads to radical anion 67. Resonance delocalization as shown shordd favor the resonance contributor 67B due to charge separation. 

A ketone or aldehyde is reduced to an alcohol by reaction with sodium or lithium metal in liquid ammonia, in the presence of ethanol. This is called a dissolving metal reduction and it proceeds by an alkoxy-radical known as a ketyl 21, 22, 46, 49. Alkynes are reduced to E-alkenes under dissolving metal conditions. Benzene is reduced to a cyclohexadiene under the same conditions 23, 28, 33, 37, 41, 48. 

FIGURE 10.79 Reduction of an alkyne with sodium in ammonia gives the trans alkene. 

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. 

Reduction of an alkyne with sodium metal in liquid ammonia gives an alkene formed by anti addition of two hydrogen atoms. The reaction occurs by a very different mechanism than the catalytic hydrogenation reaction. 

When hydrogenation is performed in the presence of a poisoned catalyst (such as Lindlar s catalyst), the alkyne is reduced to a cis alkene. When R is used as the catalyst, the alkyne is reduced aU the way to an alkane. Treatment of the alkyne with sodium in liquid ammonia affords a trans alkene (dissolving metal reduction), as shown here ... 

Reduction of an alkyne to an (E)-alkene can be achieved by treating the alkyne with lithium or sodium metal in ammonia at low temperatures (Following fig.). This is called dissolving metal reduction. 

To form a trans alkene, two hydrogens must be added to the alkyne with anti stereochemistry. Sodium metal in liquid ammonia reduces alkynes with anti stereochemistry, so this reduction is used to convert alkynes to trans alkenes. 

The preparation of pure isolated E olefins is readily accomplished by the reduction of an alkyne with metallic sodium or lithium in liquid ammonia (27,32). This reaction is preferably carried out by the addition of the alkyne in an ether to a mixture of sodium (or lithium) in liquid ammonia at -30°. 

Another method for the conversMin of an alkyne to an alkene u i sodium or lithium metal as the reducing agent in liquid ammonia as )1> vent. This method is coinplomentary to the C>indlar reduction becau4ie it pro duces trans rather than cis alkenca. Kor example, decyr.e gives iMns-S-decene on treatment with lithium in liquid ammonia... 

Birch reduction of aromatic compounds involves reaction with an electron-rich solution of alkali metal lithium or sodium in liquid ammonia (sometimes called metal ammonia reduction). Usually a proton donor such as tert-butanol or ethanol is used to avoid the formation of excess amount of LiNH2 or NaNH2. The major product is normally a 1,4-diene. This reaction is related to the reduction of alkynes to frans-alkenes ° (section 6.2.2). 

In the initial studies by Campbell and Eby it was noted that 3- and 4-octyne, 3-hexyne and 5-decyne could be efficiently reduced to the corresponding rram-alkenes in good yields and with remarkably high stereoselectivity. Shortly thereafter, Henne and Greenlee reported the quantitative reduction of 1-alkynes to 1-alkenes using sodium in ammonia in the presence of ammonium ion. In the absence of ammonium ion, however, extensive metallation of the 1-alkyne occurs. In the presence of ammonium ion dialkylacetylenes are inefficiently reduced (extensive hydrogen evolution occurs, in which sodium is consumed). 

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