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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 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).

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.^{[3] [4] [5] [6]}

Contents

Reaction mechanism

Birch alkylation

References

See also

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^[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.

Birch alkylation

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 ^[10]:
:

References

1. (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.
2. 1,6-Methano-10-annulene, Vogel, E.; Klug, W.; Breuer, A., , , Organic Syntheses, 1974
3. Birch, A. J.; Smith, H. ''Quart. Rev.'' '1958', ''12'', 17. (Review)
4. Caine, D. ''Org. React.'' '1976', ''23'', 1-258. (Review)
5. Rabideau, P. W.; Marcinow, Z. ''Org. React.'' '1992', ''42'', 1-334. (Review)
6. Mander, L. N. ''Comp. Org. Syn.'' '1991', ''8'', 489-521. (Review)
7. 1,4-Dihydrobenzoic acid, Kuehne, M. E.; Lambert, B. F., , , Organic Syntheses, 1963
8. 2,7-Dimethyloxepin, Paquette, L. A.; Barrett, J. H., , , Organic Syntheses, 1969
9. Alkylation of the anion from Birch reduction of o-Anisic acid: 2-Heptyl-2-cyclohexenone, Taber, D. F.; Gunn, B. P.; Ching Chiu, I., , , Organic Syntheses, 1983
10. ''Formation of Benzo-Fused Carbocycles by Formal Radical Cyclization onto an Aromatic Ring'' Derrick L. J. Clive and Rajesh Sunasee Org. Lett.; '2007'; 9(14) pp 2677 - 2680; (Letter)

See also

★ Benkeser reduction

★ Solvated electron