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Further Information

Birch Reduction

The Birch Reduction offers access to substituted 1,4-cyclohexadienes.

Mechanism of the Birch Reduction

The question of why the 1,3-diene is not formed, even though it would be more stable through conjugation, can be rationalized with a simple mnemonic. When viewed in valence bond terms, electron-electron repulsions in the radical anion will preferentially have the nonbonding electrons separated as much as possible, in a 1,4-relationship.

This question can also be answered by considering the mesomeric structures of the dienyl carbanion:

The numbers, which stand for the number of bonds, can be averaged and compared with the 1,3- and the 1,4-diene. The structure on the left is the average of all mesomers depicted above followed by 1,3 and 1,4-diene:

The difference between the dienyl carbanion and 1,3-diene in absolute numbers is 2, and between the dienyl carbanion and 1,4-diene is 4/3. The comparison with the least change in electron distribution will be preferred.

Reactions of arenes with +I- and +M-substituents lead to the products with the most highly substituted double bonds:

The effect of electron-withdrawing substituents on the Birch Reduction varies. For example, the reaction of benzoic acid leads to 2,5-cyclohexadienecarboxylic acid, which can be rationalized on the basis of the carboxylic acid stabilizing an adjacent anion:

Alkene double bonds are only reduced if they are conjugated with the arene, and occasionally isolated terminal alkenes will be reduced.

Recent Literature

Ammonia Free Partial Reduction of Aromatic Compounds Using Lithium Di-tert-butylbiphenyl (LiDBB)
T. J. Donohoe, D. House, J. Org. Chem., 2002, 67, 5015-5018.