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.
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.
Several reviews have been published.
[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.
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, stabilizes a carbanion and the least-substituted olefin is generated. With an electron donating substituent the opposite effect is obtained. The non-conjugated 1,4-addition product is preferred over the conjugated 1,3-diene which is explained in terms of the principle of least motion. 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.
[edit] See also
- Benkeser reduction
- Solvated electron
