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Sodium-Ammonia Reduction of an Alkyne

Sodium-Ammonia Reduction of an Alkyne THE OVERALL REACTION  [Pg.373]

Step 1 Electron transfer from sodium to the alkyne. The product is an anion radical. [Pg.373]

Step 2 The anion radical is a strong base and abstracts a proton from ammonia. [Pg.373]

Step 4 Proton transfer from ammonia converts the alkenyl anion to an alkene. [Pg.373]

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). Metal-ammonia reduction of alkynes is outlined in Mechanism 9.1. [Pg.373]

On dissolving in liquid ammonia, sodium atoms dissociate into sodium ions and electrons, both of which are solvated by ammonia. To reflect this, the solvated electrons are represented in the equation as e (am). [Pg.355]

Step 1 Electron transfer. An electron adds to one of the triply bonded carbons to [Pg.355]

Proton transfer. The anion radical formed in the first step is strongly basic and abstracts a proton from ammonia. This is believed to be the ratedetermining step. The alkenyl radical that results is a mixture of rapidly equilibrating E and Z stereoisomers. [Pg.355]

Step 3 Electron transfer. The alkenyl radical reacts with a solvated electron to [Pg.355]


FIGURE 9.4 Mechanism of the sodium-ammonia reduction of an alkyne. [Pg.376]

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. [Pg.130]

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°. [Pg.32]

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. ... [Pg.478]

Reduction of an Alkyne by Sodium in Liquid Ammonia (Section 7.sc)... [Pg.21]

Alkynes can also be reduced to alkenes by using either sodium or lithium metal in liquid ammonia or in low-molecular-weight primary or secondary amines. The alkali metal is the reducing agent and, in the process, is oxidized to M, which dissolves as a metal salt in the solvent for the reaction. Reduction of an alkyne to an alkene by lithium or sodium in liquid ammonia, NH3(Z), is stereoselective it involves mainly anti addition of two hydrogen atoms to the triple bond. [Pg.322]

FIGURE 10.79 Reduction of an alkyne with sodium in ammonia gives the trans alkene. [Pg.452]

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. [Pg.229]

An alternative method for the conversion of an alkyne to an alkene uses sodium or lithium metal as the reducing agent in liquid ammonia as solvent. This method is complementary to the Lindlar reduction because it produces... [Pg.268]

Finally, the reduction of the carbon-carbon triple bond of an alkyne can also be accomplished by using sodium or lithium in liquid ammonia. This reaction is especially useful because it produces the (ZT)-isomer of the alkene product. (Recall that the (Z)-isomer can be prepared by catalytic hydrogenation of the alkyne see Section 11.12.)... [Pg.945]

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). [Pg.32]

Dissolving-Metal Reduction of Aromatic Compounds and Alkynes. Dissolving-metal systems constitute the most general method for partial reduction of aromatic rings. The reaction is called the Birch reduction,214 and the usual reducing medium is lithium or sodium in liquid ammonia. An alcohol is usually added to serve as a proton source. The reaction occurs by two successive electron transfer/proto-nation steps. [Pg.436]

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). [Pg.232]

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. [Pg.77]

Dear and Pattison regard lithium-ammonia as superior to sodium-ammonia for the stereospecific /rans-reduction of alkynes. The reaction is carried out in tetrahydrofurane under pressure in an autoclave at room temperature. [Pg.1035]

What can we do if we want the tra s-alkene rather than the ds-isomer from alkyne reduction This can be accomplished using a dissolving metal reduction. When an alkali metal such as sodium is added to liquid ammonia, it is ionized to give solvated electrons (these are blue, but that s a story for the physical chemists...). One electron is added to the alkyne to give a radical anion (Figure 11.94). Because electrons repel each other, the orbitals containing the lone pair and the odd electron are on opposite sides of the triple bond. The lone pair is protonated by the solvent then a further electron and proton are added to complete the process. Thus, 4-octyne is cleanly reduced to fraKS-4-octene. [Pg.480]

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 ... [Pg.316]


See other pages where Sodium-Ammonia Reduction of an Alkyne is mentioned: [Pg.201]    [Pg.201]    [Pg.359]    [Pg.1318]    [Pg.201]    [Pg.1221]    [Pg.201]    [Pg.201]    [Pg.359]    [Pg.1318]    [Pg.201]    [Pg.1221]    [Pg.220]    [Pg.41]    [Pg.272]    [Pg.576]    [Pg.32]    [Pg.880]    [Pg.397]    [Pg.126]    [Pg.929]    [Pg.1061]    [Pg.61]   


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