Birch reduction

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. [cite journal | author = Birch, A. J. | journal = J. Chem. Soc. | year = 1944' | pages = 430] [cite journal | author = Birch, A. J. | journal = J. Chem. Soc. | year = 1945 | pages = 809 | doi = 10.1039/jr9450000809 | title = 212. Reduction by dissolving metals. Part II] [cite journal | journal = J. Chem. Soc. | year = 1946 | pages = 593 | doi = 10.1039/jr9460000593 | title = 119. Reduction by dissolving metals. Part III | author = Birch, A. J.] [cite journal | journal = J. Chem. Soc. | year = 1947 | pages = 102 | doi = 10.1039/jr9470000102 | title = 25. Reduction by dissolving metals. Part IV | author = Birch, A. J.] [cite journal | journal = J. Chem. Soc. | year = 1947 | pages = 1642 | doi = 10.1039/jr9470001642 | title = 327. Reduction by dissolving metals. Part V | author = Birch, Arthur J. | unused_data = Birch, A. J.] [cite journal | author = Birch, A. J. | journal = J. Chem. Soc. | year = 1949 | pages = 2531 | doi = 10.1039/jr9490002531 | title = 532. Reduction by dissolving metals. Part VI. Some applications in synthesis] 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).

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

An example is the reduction of naphthalene [OrgSynth | author = Vogel, E.; Klug, W.; Breuer, A. | year = 1974 | title = 1,6-Methano [10] annulene | collvol = 6 | colvolpages = 731 | prep = cv6p0731] :

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Several reviews have been published. [cite journal | author = Birch, A. J.; Smith, H. | journal = Quart. Rev. | year = 1958 | volume = 12 | pages = 17 | format = review | doi = 10.1039/qr9581200017 | title = Reduction by metal–amine solutions: applications in synthesis and determination of structure] [cite journal | author = Caine, D. | journal = Org. React. | year = 1976 | volume = 23 | pages = 1–258 | format = review] [cite journal | author = Rabideau, P. W.; Marcinow, Z. | journal = Org. React. | year = 1992 | volume = 42 | pages = 1–334 | format = review] [cite journal | author = Mander, L. N. | journal = Comp. Org. Syn. | year = 1991 | volume = 8 | pages = 489–521 | format = review]

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 [JerryMarch] . :

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 [OrgSynth | author = Kuehne, M. E.; Lambert, B. F. | year = 1963 | title = 1,4-Dihydrobenzoic acid | collvol = 5 | collvolpages = 400 | prep = cv5p0400] , stabilizes a carbanion and the least-substituted olefin is generated. With an electron-donating substituent the opposite effect is obtained. [OrgSynth | author = Paquette, L. A.; Barrett, J. H. | year = 1969 | title = 2,7-Dimethyloxepin | collvol = 5 | collvolpages = 467 | prep = cv5p0467] 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. [OrgSynth | author = Taber, D. F.; Gunn, B. P.; Ching Chiu, I. | year = 1983 | title = Alkylation of the anion from Birch reduction of o-Anisic acid: 2-Heptyl-2-cyclohexenone | collvol = 7 | collvolpages = 249 | prep = cv7p0249] , for example:

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In the reaction depicted below, 1,4-dibromobutane is added to t-butyl benzoate to form an alkylated 1,4-cyclohexadiene product. [cite journal | title = Formation of Benzo-Fused Carbocycles by Formal Radical Cyclization onto an Aromatic Ring | author = Derrick L. J. Clive and Rajesh Sunasee | joural = Org. Lett. | year = 2007 | volume = 9 | issue = 14 | pages = 2677–2680 | doi = 10.1021/ol070849l | journal = Organic Letters] :

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References

ee also

* Benkeser Reaction
* Solvated electron