Birch reduction

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The Birch reduction is the organic reduction of aromatic rings with sodium in liquid ammonia to form 1,4-cyclohexadienes. The reaction was invented by Arthur Birch in 1944.^[1]

The metal can also be lithium or potassium and the hydrogen atoms are supplied by an alcohol such as ethanol or tert-butanol. Sodium in liquid ammonia gives an intense blue color due to a solvated electron.

Several reviews have been published.^[2]^[3]^[4]^[5]

[edit] Reaction mechanism

The first step of a Birch reduction is a one-electron reduction of the aromatic ring to a radical anion. Sodium is oxidized to the sodium ion Na⁺. This intermediate is able to dimerize to the dianion. In the presence of an alcohol the second intermediate is a free radical which takes up another electron to form the carbanion. This carbanion abstracts another proton from the alcohol to form the cyclohexadiene.

Reaction mechanism of the Birch reduction

In the presence of an alkyl halide the carbanion can also engage in nucleophilic substitution with carbon-carbon bond formation. In substituted aromatics an electron withdrawing substituent, such as a carboxylic acid^[6], stabilizes a carbanion and the least-substituted olefin is generated. With an electron donating substituent the opposite effect is obtained.^[7] 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.

[edit] References

^ (a) Birch, A. J. J. Chem. Soc. 1944, 430. (b) Birch, A. J. J. Chem. Soc. 1945, 809. (c) Birch, A. J. J. Chem. Soc. 1946, 593. (d) Birch, A. J. J. Chem. Soc. 1947, 102 & 1642. (e) Birch, A. J. J. Chem. Soc. 1949, 2531.
^ Birch, A. J.; Smith, H. Quart. Rev. 1958, 12, 17. (Review)
^ Caine, D. Org. React. 1976, 23, 1-258. (Review)
^ Rabideau, P. W.; Marcinow, Z. Org. React. 1992, 42, 1-334. (Review)
^ Mander, L. N. Comp. Org. Syn. 1991, 8, 489-521. (Review)
^ Kuehne, M. E.; Lambert, B. F. (1963). "1,4-Dihydrobenzoic acid". Organic Syntheses 43: 22.
^ Paquette, L. A.; Barrett, J. H. (1969). "2,7-Dimethyloxepin". Organic Syntheses 49: 62.
Taber, D. F.; Gunn, B. P.; Ching Chiu, I. (1983). "Alkylation of the anion from Birch reduction of o-Anisic acid: 2-Heptyl-2-cyclohexenone". Organic Syntheses 61: 59.
Vogel, E.; Klug, W.; Breuer, A. (1974). "1,6-Methano-10-annulene". Organic Syntheses 54: 11.