TheBirch reduction orMetal-Ammonia reduction^{[citation needed]} is an organic reaction that is used to convertarenes to1,4-cyclohexadienes. The reaction is named after the Australian chemistArthur Birch and involves theorganic reduction ofaromatic rings in anaminesolvent (traditionally liquidammonia) with analkali metal (traditionally sodium) and aproton source (traditionally analcohol). Unlikecatalytichydrogenation, Birch reduction does not reduce the aromatic ring all the way to acyclohexane.

Another example is the reduction ofnaphthalene in ammonia anddiethyl ether:

A solution of sodium in liquid ammonia consists of the intensely blueelectride salt [Na(NH₃)_x]⁺ e⁻. Thesolvated electrons add to the aromatic ring to give aradical anion, which then abstracts a proton from the alcohol. The process then repeats at either theortho orpara position (depending on substituents) to give the final diene.^[1] The residual double bonds do not stabilize further radical additions.^[2][3]

The reaction is known to bethird order – first order in the aromatic, first order in the alkali metal, and first order in the alcohol.^[4] This requires that therate-limiting step be the conversion of radical anion B to the cyclohexadienyl radical C.

That step also determines the structure of the product. AlthoughArthur Birch originally argued that the protonation occurred at themeta position,^[5] subsequent investigation has revealed that protonation occurs at either theortho orpara position. Electron donors tend to induceortho protonation, as shown in the reduction ofanisole (1). Electron-withdrawing substituents tend to inducepara protonation, as shown in the reduction ofbenzoic acid (2).^[6]

Solvated electrons will preferentially reduce sufficiently electronegative functional groups, such asketones ornitro groups, but do not attackalcohols,carboxylic acids, orethers.^[6]

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 theexchange integrals to account for varying interatomic distances, produces maximum electron density at the central atom 1,^[7][8][9] a result confirmed by more modernRHF computations.^[10]

The result is analogous to conjugated enolates. When those anions (but not the enoltautomer) kinetically protonate, they do so at the center to afford the β,γ-unsaturated carbonyl.^[7][11]

Traditional Birch reduction requirescryogenic temperatures to liquify ammonia andpyrophoric alkali-metal electron donors. Variants have developed to reduce either inconvenience.

Many amines serve as alternative solvents: for example, bis(methoxymethyl)amine inTHF^[12][13] or mixedn-propylamine andethylenediamine.^[14] Pure secondary and tertiary amines, however, fail to dissolve alkali metals.^[15]

To avoid direct alkali, there are chemical alternatives, such asM-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.^[16]

InBirch alkylation theanion formed in the Birch reduction is trapped by a suitableelectrophile such as ahaloalkane, for example:^[17]

In substituted aromatics, anelectron-withdrawing substituent, such as acarboxylic acid, will stabilize thecarbanion to generate the least-substitutedolefin;^[18] anelectron-donating substituent has the opposite effect.^[19]

TheBenkeser reduction is thehydrogenation ofpolycyclic aromatic hydrocarbons, especiallynaphthalenes usinglithium orcalciummetal in low molecular weight alkylamines solvents. Unlike traditional Birch reduction, the reaction can be conducted at temperatures higher than the boiling point of ammonia (−33 °C).^[21][22]

For the reduction of naphthalene with lithium in a mixedethylamine-dimethylamine solution, the principal products are bicyclo[3.3.0]dec-(1,9)-ene, bicyclo[3.3.0]dec-(1,2)-ene and bicyclo[3.3.0]decane.^[23][24]

The directing effects of naphthalene substituents remain relatively unstudied theoretically. Substituents adjacent to the bridge appear to direct reduction to the unsubstituted ring; β substituents (one bond further) tend to direct reduction to the substituted ring.^[6]

Arthur Birch, building on earlier (1937) work by Wooster and Godfrey who used water,^[25] developed the reaction in the 1940s while working in theDyson Perrins Laboratory at theUniversity of Oxford.^[26] Birch's original procedure usedsodium andethanol,^[5][27][28]Alfred L. Wilds later discovered that lithium gives better yields.^[29][30]

The reaction was difficult to understand mechanistically, with controversy lasting into the 1990s.

The case with electron-withdrawing groups is obvious, because the Birch alkylation serves as a trap for the penultimate dianion D. This dianion appears even in alcohol-free reactions. Thus the initial protonation ispara rather thanipso, as seen in the B-C transformation.^[31][32][33]

For electron-donating substituents, Birch initially proposedmeta attack, corresponding to the location of greatest electron density in a neutralbenzene ring, a position endorsed by Krapcho and Bothner-By.^[4][34] These conclusions were challenged by Zimmerman in 1961, who computed electron densities of the radical and diene anions, revealing that theortho site which was most negative and thus most likely to protonate.^[7][9] But the situation remained uncertain, because computations remained highly sensitive to transition geometry. Worse, Hückel orbital and unrestricted Hartree-Fock computations gave conflicting answers. Burnham, in 1969, concluded that the trustworthiest computations supportedmeta attack;^[35] Birch and Radom, in 1980, concluded that bothortho andmeta substitutions would occur with a slight preference forortho.^[36]

In the earlier 1990s, Zimmerman and Wang developed an experiment technique to distinguish betweenortho andmeta protonation. The method began with the premise that carbanions are much more basic than the corresponding radical anions and thus protonate less selectively. Correspondingly, the two protonations in Birch reduction should exhibit anisotope effect: in a protium–deuterium medium, the radical anion should preferentially protonate and the carbanion deuterate. Indeed, a variety ofmethoxylated aromatics exhibited lessortho deuterium thanmeta (a 1:7 ratio). Moreover, modern electron density computations now firmly indicatedortho protonation; frontier orbital densities, most analogous to the traditional computations used in past studies, did not.^[10]

Although Birch remained reluctant to concede thatortho protonation was preferred as late as 1992,^[37] Zimmerman and Wang had won the day: modern textbooks unequivocally agree that electron-donating substituents promoteortho attack.^[6]

• Caine, D. (1976). "Reduction and Related Reactions of α,β-Unsaturated Carbonyl Compounds with Metals in Liquid Ammonia".Org. React. (review).23:1–258.doi:10.1002/0471264180.or023.01.ISBN 0471264180.

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