Metal-ammonia reduction alkynes

A useful alternative to catalytic partial hydrogenation for converting alkynes to alkenes IS reduction by a Group I metal (lithium sodium or potassium) m liquid ammonia The unique feature of metal-ammonia reduction is that it converts alkynes to trans alkenes whereas catalytic hydrogenation yields cis alkenes Thus from the same alkyne one can prepare either a cis or a trans alkene by choosing the appropriate reaction conditions... 

The stereochemistry of metal-ammonia reduction of alkynes differs from that of catalytic hydrogenation because the mechanisms of the two reactions are different The mechanism of hydrogenation of alkynes is similar to that of catalytic hydrogenation of alkenes (Sections 6 1-6 3) A mechanism for metal-ammonia reduction of alkynes is outlined m Figure 9 4... 

The mechanism by which the Birch reduction of benzene takes place (Figure 118) IS analogous to the mechanism for the metal-ammonia reduction of alkynes It involves a sequence of four steps m which steps 1 and 3 are single electron transfers from the metal and steps 2 and 4 are proton transfers from the alcohol... 

Mechanism 9-1 Metal-Ammonia Reduction of an Alkyne 407 Mechanism 9-2 Acid-Catalyzed Keto-EnolTautomerism 411 Mechanism 9-3 Base-Catalyzed Keto-EnolTautomerism 413 9-10 Oxidation of Alkynes 414... 

The metal-ammonia reduction proceeds by addition of an electron to the alkyne to form a radical anion, followed by protonation to give a neutral radical. Protons are provided by the ammonia solvent or by an alcohol added as a cosolvent. Addition of another electron, followed by another proton, gives the product. 

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). 

The selective hydrogenation of a triple bond to give an alkene without concomitant positional or geometric isomerization is particularly important in synthetic procedures and many industrial processes. In the absence of any isomerization, selective partial hydrogenation of a disubstituted alkyne produces the cis alkene. Small amounts of the trans alkene are sometimes formed in these reactions, but catalytic processes do not lead to the production of the irans olefin as the primary product. The Irons alkenes can be produced as a primary product by metal-ammonia reduction of disubstituted alkynes.2... 

Metal ammonia reduction of alkynes (trans product)... 

Metal-ammonia reduction (Section 9.10) Group I metals—sodium Is the one usually employed—in liquid ammonia as the solvent convert alkynes to trans alkenes. The reaction proceeds by a four-step sequence in which electron-transfer and proton-transfer steps alternate. 

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