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84 Birch Reduction (Metal-Ammonia Reduction) A. GENERAL DESCRIPTION OF THE REACTION This reaction was first reported by Wooster in 1937,1 and subsequently by H¨ uckel et al in 1939,2 for the reduction of aromatic compounds by sodium in liquid ammonia with water; however, no structural information was provided. It was Birch who extended Wooster’s protocol in 19443 and since then had extensively explored the reduction of benzene and aromatic derivatives with alkali metal (i.e., Li, Na, K) in liquid ammonia in the presence of an alcohol (as the proton donor) to produce corresponding cyclohexa-1,4-diene derivatives.4 Therefore, the reduction of aromatic compounds by alkali metal in liquid ammonia in the presence of alcohol is generally known as the Birch reduction5,6 or metal-ammonia reduction.7 In addition, this reaction is also referred to as the Birch reaction,6ddd,8 and in uckel reduction.9 one instance is cited as the Birch-H¨ Different from the transition metal (e.g., Pd, Pt, etc.) catalyzed hydrogenation, by which aromatic compounds can be converted into the fully hydrogenated molecules,5aa the Birch reduction uses the ammonia-solvated electrons arising from alkali metal as the reducing agent, which bears the extremely negative single-electron redox potential;10 thus even the electron-enriched aromatic systems (i.e., carrying electron-donating groups) can be reduced accordingly. In addition, once the solvated electron adds to aromatic system, the radical anion is quenched by alcohol, so that cyclohexa-1,4-diene derivatives are often produced from benzene derivatives without further reduced species. It should be pointed out that the existing alkali metal also reacts with alcohol to generate hydrogen, in a reaction known as the hydrogen reaction, which competes with the reduction of aromatic compounds.5aa It is reported that both the rate5aa and yield11 of the Birch reduction decreases in the order of Li > Na > K; as a result, the relatively slower reduction of sodium with aromatic compounds

Comprehensive Organic Name Reactions and Reagents, by Zerong Wang Copyright © 2010 John Wiley & Sons, Inc.

387

388

BIRCH REDUCTION

(compared to lithium) may allow a simultaneous base-catalyzed isomerization from the isolated diene system to the more stable conjugated diene system, which in turn is further reduced to tetrahydro stage by the existing metal. In comparison, lithium has higher reducing power and does not tend to promote the isomerization of the isolated diene system.6ww Such difference between lithium and sodium presumably is the reason that Wooster et al did not exploit this reaction successfully. However, the combination of lithium-methylaminealcohol can further reduce the aromatic system into tetrahydro derivatives.6ggg It should be pointed out that iron salts can strongly catalyze the sodium-based Birch reduction, but they have less effect on the lithium-based Birch reduction.6hh Although the Birch reduction and hydrogen reaction have a similar reaction order when the alkali metal concentration is around 0.01 M (or less), the hydrogen reaction decreases in an inverse order—that is, the reaction rate increases in the order of Li < Na < K.5aa Therefore, it is suggested that the reduction be carried out at the lowest possible temperature with methanol as the quenching agent to reduce the effect from the hydrogen reaction, because ethanol shows a faster rate than does butanol.5aa,11d Moreover, it is reported that both the Birch reduction and hydrogen reaction can be accelerated by the addition of an alkali cation common to the dissolved alkali metal (e.g., alkali bromide), whereas both reactions are disfavored by the addition of the complexing cryptands of the corresponding alkali cation, indicating that the formation of intermediate ion pairs or shifting pre-equilibria in which solvated electrons are involved. In addition, I propose that the added alkali cation might further stabilize the radical anion, causing a shift in the equilibrium to the end of product. Besides the commonly used alcohols, ethers and alkyl halides could be applied as anion quenchers,9 and sometimes, a co-solvent such as THF5x is also added to the reaction system, presumably to alternate the solubility of aromatic compounds. On the other hand, one of the advantages associated with the Birch reduction is its efficient steric control on the aromatic system—that is, the formed cyclohexa-1,4-diene system is the isomer with the maximum number of alkoxy and/or alkyl groups on the residual double bonds, known as the Birch rule.5ccc,5mmm,12 In other words, the electron-withdrawing groups, (e.g., NO2 , COR, and CO2 H),13 facilitate the formation of anion and subsequent reduction at the position on the aromatic ring bearing such group, leading to the formation of 1,4-dihydro derivatives via the addition of hydrogen para to the electron-withdrawing group;14 whereas electron-donating groups (such as OMe, Me, NR2 , and SiMe3 ) are deactivating substituents,13 resulting in the generation of 2,5-dihydro derivatives by meta addition of hydrogen to the strongly deactivating group.14 When both electron-withdrawing and electron-donating groups exist on the aromatic system, the former has a greater directing effect, whereas among the electron-donating groups, the ones with oxygen or nitrogen atoms outweigh the alkyl groups. Thus the empirical order for directing is as follows: carboxy > amino/alkoxyl > alkyl.14 For the fused aromatic ring system, the ring with the most fusions to another aromatic ring is normally the one to undergo the Birch reduction;14 however, the carbon skeleton for the fused aromatic system may rearrange during the Birch reduction. As an example, the reduction of biphenylene gives the expected 1,4,4a,8btetrahydrobiphenylene and the unexpected 4,5-benzobicyclo[4.2.0]octa-2,4-diene.6ii In the fused aromatic system, a ring with a hydroxyl group would be deactivated due to the formation of anion analogous to phenoxide.14 One of the most important applications of the Birch reduction is to convert aryl alkyl ethers into 1-alkoxycyclohexa-1,4-dienes which are then used as the starting materials in organic synthesis.5x,13 Compared to the benzene series, the Birch reduction of naphthalenes may afford different products, depending on the position of the alkoxyl group.

PROPOSED MECHANISMS

389

For example, along with the loss of the methoxyl group, both 1-methoxy-2-naphthoic acid and 3-methoxy-2-naphthoic acid will be converted into 1,2,3,4-tetrahydro- or 1,2,3,4,5, 8-hexahydro-2-naphthoic acid depending on the reaction conditions or proportion of reagents used, whereas 2-methoxy-1-naphthoic acid is mainly converted into 1,4,5,8tetrahydro-2-methoxy-1-naphthoic acid without loss of the methoxyl group.6s;6ii Besides the reduction of aromatic systems, the Birch reaction has been found to reduce acetylenes stereospecifically to trans-olefins15 and to hydrogenate graphite and carbon single-walled nanotubes (SWNTs).16 However, the Birch reduction cannot reduce hexamethylbenzene and highly substituted benzene rings in [2n]-cyclophanes,5oooo probably due to the steric hindrance. In addition, the Birch reaction can also cleave the C-C bond in the presence of the aromatic system. For instance, the Birch reaction with lithium-ammonia can remove the tertiary cyano group,17 whereas potassium sand in ether can cleave pentaphenyl ethane,18 and sodium-potassium alloy is able to break 1,1,2,2-tetraphenylethane.18 It is pointed out that the cleavage of 1,2-diarylethanes increases in the order of aryl = anthracene < naphthalene < benzene, with the alkali metal order of Li < Na < K, and the solvents in the order of NH3 < THF < HMPA.5ppp It should be noted that the configuration is retained during the bond cleavage under Birch-like conditions.14 Furthermore, the Birch reduction has often been used to generate hydroxyl groups by the removal of the benzyl-protecting group.5i,20 It is interesting that in the absence of the harsh conditions of the Birch reduction, an enzyme-catalyzed reduction for the two-electron reduction of benzoyl-CoA to cyclohexa1,5-diene-1-carbonyl-CoA has been observed, where a conjugated diene is formed instead of 1,4-diene in the Birch reduction.10 Nevertheless, the Birch reduction also has certain drawbacks, including the tedious experimental procedure, the use of strongly basic solutions, and sometimes the solubility problem of substrates.5llll

B. GENERAL REACTION SCHEME R

M, R′OH NH3, < –33°C

R = alkyl, OMe, OEt, NMe2, etc. R′ = Me, Et, But, M = Li, Na, K

R

R′′

M, R′OH

R′′

NH3, < –33°C R′′ = CO2Et, CONMe2, NO2, CO2H, etc. R′ = Me, Et, But, M = Li, Na, K

C. PROPOSED MECHANISMS It is generally accepted that the metal ammonia solution (MAS) contains both the solvated electron and alkali cation, in which the solvated electron has extremely high single-electron redox potential5aa so that the solvated electron can even reduce electronenriched aromatic systems. In this reduction, the solvated electron adds to the aromatic ring to generate a radical anion, which is protonated by an alcohol; then a second solvated electron adds to the ring to form a new radical anion, followed by alcohol

390

BIRCH REDUCTION

quenching to afford the unconjugated cyclohexa-1,4-diene.5ccc,5mmm The protonation of the radical anion by alcohol is the rate-limiting step.5ccc,5mmm However, there is disagreement about the electron density of the radical anion, for which Birch himself believed that the meta position of the radical anion formed from anisole had the highest electron density (Scheme 1),21 whereas Zimmerman suggested the position ortho to the maximum number of substituents to be most electron rich, from the H¨uckel calculation (Scheme 2).5ccc Displayed here are Birch’s mechanism and Zimmerman’s explanation for the regioselectivity of the Birch reduction of anisole. The Birch reduction on a aromatic system with an electron-withdrawing group should have a similar reaction process but a different position for the solvated electron to attack.

SCHEME 1. Birch’s mechanism for the reduction of anisole.

SCHEME 2. Zimmerman’s mechanism for the reduction of anisole.

D. MODIFICATION This reaction has been extensively modified. One of the important modifications is the quenching of radical anion by an alkylation agent to give an alkyl-substituted cyclohexa1,4-diene derivative,5xa,5nnnn,13,22 a condition known as the Birch reductive alkylation.22b On the other hand, a pyrrole nucleus bearing an extremely enriched electron density and

CITED EXPERIMENTAL EXAMPLES

391

an acidic hydrogen atom normally does not undergo the Birch reduction;5oo however, the pyrrole nucleus has been successfully reduced to a dihydropyrrole derivative by replacing the acidic hydrogen atom with an N-t-butylcarboxyl (Boc) group.5oo In addition, an ammonia-free methodology has been developed by using a di-tert-butylbiphenyl radical anion (generated from di-tert-butylbiphenyl and lithium in THF) to provide electrons and bis(methoxyethyl)amine as an acid, so that a number of electrophiles, such as silyl halides, chloroformates, acid chlorides, and enolizable aldehydes, can be trapped by the radical anion.5f Furthermore, this reaction has been carried out via electrochemical reduction in an aqueous media,5llll using a concentrated aqueous solution of tetrabutylammonium hydroxide.23 Moreover, calcium has been applied successfully for the ultrasound-promoted reduction of the aromatic compound;24 likewise, a photo-reduction6f system has been developed to reduce aromatic compounds by irradiation of an acetonitrile/water (9:1) solution in the presence of NaBH4 and m- or p-dicyanobenzene.5wwww

E. APPLICATIONS This reaction has been widely used to convert the aryl alkyl ethers into 1alkoxycyclohexa-1,4-diene derivatives. In addition, this reaction has been used to remove the benzyl group on oxygen, cleave the C-C bonds, and reduce acetylenes into trans-olefins stereospecifically.

F. RELATED REACTIONS This reaction is closely related to the Benkeser Reduction.

G. CITED EXPERIMENTAL EXAMPLES

Reference 5f. A Schlenk tube containing 119 mg small strips of lithium ribbon (17.0 mmol), antibumping granules, and 4.5 g di-tert-butylbiphenyl (16.9 mmol) was evacuated and purged with argon several times. The mixture was ground with a magnetic stirrer until all the lithium became a dark powder. Freshly distilled THF (50 mL) was then added, and the mixture was cooled down to −78◦ C before a mixture of 1.1 g N-Boc ethyl 2-pyrrole carboxylate (4.4 mmol) and 0.8 mL bis-methoxyethylamine (5.3 mmol) in 25 mL freshly distilled THF was added dropwise to the turquoise solution. The mixture was then stirred at −78◦ C for a further 15 min after which 1.0 g freshly prepared MgBr2 (5.4 mmol) in 20 mL THF was added, and the mixture was stirred for an additional 30 min. Distilled isobutyraldehyde

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(0.7 mL, 7.0 mmol) was then added. After 30 min the reaction mixture was quenched with 10 mL saturated NH4 Cl aqueous solution. Stirring was continued at −78◦ C for a further 30 min and then the solution was warmed to ambient temperature. The reaction mixture was poured into a 50 mL 1 M HCl solution and extracted with Et2 O (3 × 60 mL). The combined organic layers were dried over Na2 SO4 , filtered, and evaporated under reduced pressure. The residue was purified by gradient column chromatography by eluting with neat petroleum ether to recover the di-tert-butylbiphenyl and then 5% acetone in petroleum ether to afford 1.07 g product as a colorless oil, in a yield of 74%.

NH(CH2)6 Na

N +

+

3

Reference 6ddd. To a 500-mL three-necked flask equipped with a stirrer, air condenser, and nitrogen inlet tube were added 6.4 g naphthalene (0.05 mol) and 4.6 g dispersed sodium (0.2 mol), followed by 100 mL hexamethylenimine. A red color developed within 20 min. The mixture was stirred at 25◦ C for 12 h, and the unreacted sodium, which had agglomerated, was removed. The remaining solution was cooled and treated cautiously with water until the reaction mixture became colorless, then acidified with 10% aqueous HCl. After the hydrocarbons had been removed by extraction with ether, the aqueous layer was basified with dilute NaOH, and surplus hexamethylenimine was removed by steam distillation. The steam distillation residue was extracted with ether. Drying over anhydrous Na2 SO4 and distillation yielded 6.2 g 2-N-hexamethylenyl tetralin, in a yield of 55%. Other references related to the Birch reduction are cited in the literature.25

H. REFERENCES 1. 2. 3. 4.

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