Birch reduction

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The Birch reduction is the organic reduction of aromatic rings with sodium and an alcohol in liquid ammonia to form 1,4-cyclohexadienes. The reaction was reported by the Australian chemist Arthur John Birch (1915–1995) in 1944.[1] 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 Birch reduction

Lithium and potassium can substitute for sodium, and alcohols are ethanol and tert-butanol.


An example is the reduction of naphtalene[2]:

Naphtalene Birch Reduction

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

Contents

[edit] 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. The added alcohol supplies a proton to the carbanion, For most substrates ammonia is not acidic enough [7].

Reaction mechanism of the Birch reduction

[edit] 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[8], stabilizes a carbanion and the least-substituted olefin is generated. With an electron-donating substituent the opposite effect is obtained.[9] 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 to 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.[10], for example:

Birch Alkylation Org Synth 1990


In the reaction depicted below, 1,4-dibromobutane is added to t-butyl benzoate to form an alkylated 1,4-cyclohexadiene product.[11]:

Birch alkylation

[edit] 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. ^ Vogel, E.; Klug, W.; Breuer, A. (1974). "1,6-Methano-10-annulene". Organic Syntheses 54: 11. 
  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. ^ March, Jerry (1985). Advanced Organic Chemistry, Reactions, Mechanisms and Structure, third Edition, John Wiley & Sons. ISBN 0-471-85472-7. 
  8. ^ Kuehne, M. E.; Lambert, B. F. (1963). "1,4-Dihydrobenzoic acid". Organic Syntheses 43: 22. 
  9. ^ Paquette, L. A.; Barrett, J. H. (1969). "2,7-Dimethyloxepin". Organic Syntheses 49: 62. 
  10. ^ 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. 
  11. ^ 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) doi:10.1021/ol070849l

[edit] See also