Birch Reduction
The Birch reaction is the 1,4-reduction of aromatic rings to the corresponding unconjugated cyclohexadienes and heterocycles by alkali metals (Na, Li, K) dissolved in liquid ammonia in the presence of alcohol.
General features:
1. In addition to aryl rings, pyridines, pyrroles, and furans are also reduced. 2. The regioselectivity of the reduction depends on the nature of the substituent. 3. Non-conjugated double bonds are not reduced in these conditions. 4. The rate of the reduction is lower in the presence of electron-donating substituents.
Birch Reduction
Reaction Mechanism
1. The solution of the metal in ammonia provides electrons. The electrons are taken up by the aromatic ring to form the corresponding radical anion. 2. Protonation by the alcohol to form a cyclohexadienyl radical. 3. Formation of a cyclohexadienyl carbanion by electron transfer. 4. Formation of the unconjugated cyclohexadiene product.
Experimental Procedure
To a solution of p–methoxyphenethyl alcohol (65.7 mmol, 1.0 eq) and tert-butanol (20 mL) in liquid NH3 at -78 °C was added Li (6.0 eq) portionwise. After 5 h at this temperature, the reaction mixture was slowly quenched with NH4Cl (8.0 eq) and the cooling bath removed. The mixture was stirred overnight at room temperature with a flow of N2 to evaporate the residual NH3 and water was added. The product was then extracted with MTBE. The combined organic phases were dried over MgSO4 and concentrated under reduced pressure to obtain the desired product.
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