Birch reduction

The Birch reduction is the organic reduction of aromatic rings with sodium in liquid ammonia to form 1,4-cyclohexadienes. The reaction was reported by the Australian chemist Arthur John Birch (1915–1995) in 1944.^{[1] [2]} This reaction provides an alternative to catalytic hydrogenation, which usually reduces the aromatic ring all the way to a cyclohexane (after the initial reduction to a cyclohexadiene, catalytic reduction of the remaining (nonaromatic) double bonds is easier than the first reduction).

Lithium and potassium can substitute for sodium, and alcohol such as ethanol and tert-butanol can be used instead of ammonia.

Several reviews have been published.^{[3] [4] [5] [6]}

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Reaction mechanism

A solution of sodium in liquid ammonia consists of the electride salt [Na(NH₃)_x]⁺ e^-, associated with the intense blue color of these solutions. The solvated electrons add to the aromatic ring to give a radical anion followed by the dianion. These carbanions deprotonate the ammonia to form the cyclohexadiene.

Birch alkylation

In the presence of an alkyl halide the carbanion can also undergo nucleophilic substitution with carbon-carbon bond formation. In substituted aromatic compounds an electron-withdrawing substituent, such as a carboxylic acid^[7], stabilizes a carbanion and the least-substituted olefin is generated. With an electron-donating substituent the opposite effect is obtained.^[8] The reaction produces more of the less thermodynamically stable non-conjugated 1,4-addition product than the more stable conjugated 1,3-diene because the largest orbital coefficient of the HOMO of the conjugated pentadienyl anion intermediate is on the central carbon atom. Once formed, the resulting 1,4-cyclohexadiene is unable equilibrate to the thermodynamically more stable product; therefore, the observed kinetic product is produced. Experimental alkali metal alternatives that are safer to handle, such as the M-SG reducing agent, also exist.

In Birch alkylation the anion formed in the Birch reduction is trapped by a suitable electrophile such as a haloalkane.^[9] For example, in the reaction depicted below, 1,4-dibromobutane is added to t-butyl benzoate to form an alkylated 1,4-cyclohexadiene product.^[10]:

References

See also

• Benkeser reduction
• Solvated electron