The Birch reduction is an organic reaction that is used to convert arenes to 1,4-cyclohexadienes. The reaction is named after the Australian chemist Arthur Birch and involves the organic reduction of aromatic rings in an amine solvent (traditionally liquid ammonia) with an alkali metal (traditionally sodium) and a proton source (traditionally an alcohol). Unlike catalytic hydrogenation, Birch reduction does not reduce the aromatic ring all the way to a cyclohexane.
An example is the reduction of naphthalene in ammonia and ethanol:
A solution of sodium in liquid ammonia consists of the intensely blue electride salt [Na(NH<sub>3</sub>)<sub>x</sub>]+ e−. The solvated electrons add to the aromatic ring to give a radical anion, which then abstracts a proton from the alcohol. The process then repeats at either the ortho or para position (depending on substituents) to give the final diene. The residual double bonds do not stabilize further radical additions.[1] [2]
The reaction is known to be third order – first order in the aromatic, first order in the alkali metal, and first order in the alcohol.[3] This requires that the rate-limiting step be the conversion of radical anion B to the cyclohexadienyl radical C.
That step also determines the structure of the product. Although Arthur Birch originally argued that the protonation occurred at the meta position, subsequent investigation has revealed that protonation occurs at either the ortho or para position. Electron donors tend to induce ortho protonation, as shown in the reduction of anisole (1). Electron-withdrawing substituents tend to induce para protonation, as shown in the reduction of benzoic acid (2).[4]
Solvated electrons will preferentially reduce sufficiently electronegative functional groups, such as ketones or nitro groups, but do not attack alcohols, carboxylic acids, or ethers.
The second reduction and protonation also poses mechanistic questions. Thus there are three resonance structures for the carbanion (labeled B, C and D in the picture). Simple Hückel computations lead to equal electron densities at the three atoms 1, 3 and 5, but asymmetric bond orders. Modifying the exchange integrals to account for varying interatomic distances, produces maximum electron density at the central atom 1,[5] [6] [7] a result confirmed by more modern RHF computations.[8]
| Approximation | Density Atom 3 | Density Atom 2 | Density Atom 1 | Bond Order 2–3 | Bond Order 1–2 | |
|---|---|---|---|---|---|---|
| Hückel (1st approx) | 0.333 | 0.00 | 0.333 | 0.788 | 0.578 | |
| 2nd approx | 0.317 | 0.00 | 0.365 | 0.802 | 0.564 | |
| 3rd approx | 0.316 | 0.00 | 0.368 | 0.802 | 0.562 |
Traditional Birch reduction requires cryogenic temperatures to liquify ammonia and pyrophoric alkali-metal electron donors. Variants have developed to reduce either inconvenience.
Many amines serve as alternative solvents: for example, THF[10] [11] or mixed n-propylamine and ethylenediamine.[12]
To avoid direct alkali, there are chemical alternatives, such as M-SG reducing agent. The reduction can also be powered by an external potential or sacrificial anode (magnesium or aluminum), but then alkali metal salts are necessary to colocate the reactants via complexation.[13]
In Birch alkylation the anion formed in the Birch reduction is trapped by a suitable electrophile such as a haloalkane, for example:
In substituted aromatics, an electron-withdrawing substituent, such as a carboxylic acid, will stabilize the carbanion to generate the least-substituted olefin; an electron-donating substituent has the opposite effect.
The Benkeser reduction is the hydrogenation of polycyclic aromatic hydrocarbons, especially naphthalenes using lithium or calcium metal in low molecular weight alkyl amines solvents. Unlike traditional Birch reduction, the reaction can be conducted at temperatures higher than the boiling point of ammonia (−33 °C).[14]