Metal-Ammonia Reduction of Alkynes

In subsequent equations, we will not specify the components of the Lindlar palladium catalyst in detail but will simply write Lindlar Pd over the reaction arrow. 

Hydrogenation of alkynes to alkenes yields the cis (or Z) alkene by syn addition to the triple bond. 

PROBLEM 9.10 Oleic acid and stearic acid are naturally occurring compounds, which can be isolated from various fats and oils. In the laboratory, each can be prepared by hydrogenation of a compound known as stearolic acid, which has the formula CH3(CH2)7C=C(CH2)7C02H. Oleic acid is obtained by hydrogenation of stearolic acid over Lindlar palladium stearic acid is obtained by hydrogenation over platinum. What are the structures of oleic acid and stearic acid  

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

PROBLEM 9.11 Sodium-ammonia reduction of stearolic acid (see Problem 9.10) I yields a compound known as elaidic acid. What is the structure of elaidic acid I 

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

Suggest an efficient synthesis of fraA s-2-heptene fronn propyne and any necessary organic or inorganic reagents. 

The stereochemistry of metal-ammonia reduction of alkynes differs from that of catal5dic hydrogenation because the mechanisms of the two reactions are different The mechanism of hydrogenation of alkynes is similar to that of catal5dic hydrogenation of alkenes (Sections 6.1-6.3). Metal-ammonia reduction of alkynes is outlined in Mechanism 9.1. 

The mechanism includes two single-electron transfers (steps 1 and 3) and two proton transfers (steps 2 and 4). Experimental evidence indicates that step 2 is rate-determining, and that the ( )-and (Z)-alkenyl radicals formed in this step interconvert rapidly. 

Reduction of these alkenyl radicals (step 3) gives a mixture of the ( )- and (Z)-aIkenyl anions in which the more stable E stereoisomer predominates. Unlike the corresponding alkenyl radicals, the ( )- and (Z)-alkenyl anions are configurationally stable under Ae reaction conditions and yield an E/Z ratio of alkenes in step 4 that reflects the E/Z ratio of the alkenyl anions formed in step 3. 

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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... 

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

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