Catalytic hydrogenations of benzene and benzene derivatives usually yield completely hydrogenated products, since alkenes that are intermediately formed are more easily reduced than aromatic rings. However, a selective reduction that yields cyclohexadienes is also possible when employing a reaction with a one-electron transfer mechanism, the Birch reduction.
The Birch reduction is carried out in liquid ammonia with dissolved sodium (or lithium or potassium). By solving sodium in liquid ammonia, a sodium cation and a solvated electron are formed. The latter is a particularly powerful reducing agent. In addition, the reaction mixture contains stoichiometric amounts of an alcohol, such as ethanol, which is required as proton donor that protonates anionic intermediates. Under such reaction conditions, only a small amount of the side product hydrogen is then formed by the direct reaction between the reducing agent and the alcohol.
The first step of the mechanism of the Birch reduction is a one-electron transfer into an antibonding π orbital of the aromatic system. The resulting product is a radical anion, which is then protonated by ethanol, yielding a cyclohexadienyl radical. This resonance-stabilized allyl radical is converted into a cyclohexadienyl anion by an additional one-electron transfer. Subsequently, the cyclohexadienyl anion is also protonated by ethanol. Surprisingly, the final protonation exclusively yields the 1,4-cyclohexadiene and not the thermodynamically more stable, conjugated 1,3-cyclohexadiene. In substituted aromatic compounds, the substituents control the position of the 1,4-double bonds:
- Electron-donating substituents, such as alkoxy or alkyl groups, are located at one of the double bonds of the product.
- Electron-withdrawing substituents, like carboxyl or amide groups, for instance, are located at one of the sp3-hybridized carbons in the ring of the product.
Alkynes are selectively converted into trans alkenes when they are reduced by a solution of sodium (or lithium) in liquid ammonia that contains stoichiometric amounts of an alcohol, such as ethanol. As in the case of Birch reduction of aromatic systems, the first step of this reaction is a one-electron transfer into an antibonding π orbital of the alkyne, which yields a radical anion. Subsequently, protonation of the radical anion, an additional one-electron transfer, and a concluding protonation yield a trans alkene.
|Zimmerman, H. E. (1961): Orientation in Metal Ammonia Reductions. In: Tetrahedron. 16 , 169-176|
|Zimmerman, H. E.; Wang, P. A. (1993): The Regioselectivity of the Birch Reduction. In: J. Am. Chem. Soc.. 115 , 2205-2216|