Dissolving metals conjugated dienes

The reduction of conjugated dienes by dissolving metals is not extensively reported. This method appears to be nonselective, giving rise to a mixture of the expected olefins and polyolefins as by-products". 

Birch reduction11 is the partial reduction of aromatic rings by solvated electrons produced when alkali metals dissolve (and react) in liquid amines. Typical conditions are sodium in liquid ammonia or lithium in methylamine. These electrons add to benzene rings to produce, probably, a dianion 57 that is immediately protonated by a weak acid (usually a tertiary alcohol) present in solution. The anions in the supposed intermediate 57 keep as far from each other as they can so the final product is the non-conjugated diene 58. It is important to use the blue solution of solvated electrons before it reacts to give hydrogen and NaNH2. 

Catalytic hydrogenation of an enone would not be chemoselective if an isolated double bond were also present in the molecule. However, isolated double bonds are inert to dissolving metal reduction. On the other hand, a variety of functional groups are reduced with alkali metals in liquid ammonia. These include alkynes, conjugated dienes, allylic, or benzylic halides and ethers. 

Birch reduction is a partial reduction of aromatic compounds by electron transfer from dissolving metals—usually Na in liquid ammonia or Li in ethylamine—in the presence of a weak proton donor—usually an alcohol. The reaction behaves as if dianion (30) were an intermediate, giving non-conjugated dienes (31). Electron-donating substituents repel the anions (32) to give products like (33). whilst electron-withdrawing substituents attract the anions (34), to give products like (35). 

Dissolving Metal Reduction (Section 10.5) MO Description of Conjugated Dienes (Section 17.3)... 

Very little work has been done on reactions involving nucleophiles formed from hydrocarbons.124-142 The limitation on basicity of the carbanion, so that it does not react with solvent, has led to use of conjugated hydrocaibons, such as dienes or alkenes conjugated with aromatic rings. When initiated by dissolving alkali metal in liquid ammonia, complex mixtures are often produced on account of reduction processes,124 and regiochemistry and multiplicity of arylation in conjugated systems also create prob-... 

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