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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

A Practical and Chemoselective Ammonia-Free Birch Reduction
P. Lei, Y. Ding, X. Zhang, A. Adijiang, H. Li, Y. Ling, J. An, Org. Lett., 2018, 20, 3439-3442.

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.