Birch Reduction

The Birch Reduction Offers Access To Substituted 1,4-cyclohexadienes.

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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.