Birch reduction, mechanisms

The mechanism by which the Birch reduction of benzene takes place (Figure 118) IS analogous to the mechanism for the metal-ammonia reduction of alkynes It involves a sequence of four steps m which steps 1 and 3 are single electron transfers from the metal and steps 2 and 4 are proton transfers from the alcohol... 

Various other observations of Krapcho and Bothner-By are accommodated by the radical-anion reduction mechanism. Thus, the position of the initial equilibrium [Eq. (3g)] would be expected to be determined by the reduction potential of the metal and the oxidation potential of the aromatic compound. In spite of small differences in their reduction potentials, lithium, sodium, potassium and calcium afford sufficiently high concentrations of the radical-anion so that all four metals can effect Birch reductions. The few compounds for which comparative data are available are reduced in nearly identical yields by the four metals. However, lithium ion can coordinate strongly with the radical-anion, unlike sodium and potassium ions, and consequently equilibrium (3g) for lithium is shifted considerably... 

When double bonds are reduced by lithium in ammonia or amines, the mechanism is similar to that of the Birch reduction (15-14). ° The reduction with trifluoro-acetic acid and EtsSiH has an ionic mechanism, with H coming in from the acid and H from the silane. In accord with this mechanism, the reaction can be applied only to those alkenes that when protonated can form a tertiary carbocation or one stabilized in some other way (e.g., by a OR substitution). It has been shown, by the detection of CIDNP, that reduction of a-methylstyrene by hydridopenta-carbonylmanganese(I) HMn(CO)5 involves free-radical addition. ... 

Direct Electron Transfer. We have already met some reactions in which the reduction is a direct gain of electrons or the oxidation a direct loss of them. An example is the Birch reduction (15-14), where sodium directly transfers an electron to an aromatic ring. An example from this chapter is found in the bimolecular reduction of ketones (19-55), where again it is a metal that supplies the electrons. This kind of mechanism is found largely in three types of reaction, (a) the oxidation or reduction of a free radical (oxidation to a positive or reduction to a negative ion), (b) the oxidation of a negative ion or the reduction of a positive ion to a comparatively stable free radical, and (c) electrolytic oxidations or reductions (an example is the Kolbe reaction, 14-36). An important example of (b) is oxidation of amines and phenolate ions ... 

It was realized that the mechanism of Birch reduction involves protonation of the anion-radical formed by the addition of one electron to the reacting aromatic compound. This is followed by rapid addition of a second electron and protonation of the forming carbanion to yield nonconjugated alicyclic products. Protonation of the anion-radical by added alcohol is the rate-limiting stage. Recent calculations show that the ortho and meta positions in anisole are most enhanced in density by electron introduction. The para position is not appreciably affected (Zimmerman and Alabugin 2001 Scheme 7.9). 

Birch reduction of 3,4,5-trimethoxybenzoic acid gives in 94% yield a dihydrobenzoic acid which bears only two methoxy substituents. Suggest a plausible structure for this product based on the mechanism of the Birch reduction. 

One of the important mechanisms by which orally administered steroids are inactivated involves the formation of water-soluble derivatives at the 17 position, a process that is greatly reduced in 17a-alkyl-17(3-hydroxy derivatives. Extensive use of the resulting orally active compounds has since revealed that 17 alkylation also leads to increased liver toxicity. Preparation of the first of these compounds, nor-methandrolone (32-3), starts by addition of methylmagnesium iodide to estrone methyl ether (9-1) to give the 17a methyl derivative. Birch reduction followed by acid hydrolysis leads to normethandrolone (32-3) [16]. 

III. The Birch Reduction of Aromatic Steroids /II Mechanism of the reduction of aromatic compounds / 12 Factors influencing the rate of reduction / 14 Protonation of reduction intermediates / 17... 

Intramolecular protonation on the more hindered face of a steroid from a neighbouring hydroxyl group best explains a reversal of diastereoselectivity in the Birch reduction of styrene double bonds.266 The kinetics and product distribution of lithium metal reduction of benzaldehyde to benzyl alcohol in THF have been studied electron transfer from Li to PhCHO occurs in a slow step, but absorption of the PhCHO onto the metal surface is also crucial in determining the overall kinetics. The proposed mechanism successfully accounts for the formation of minor products, benzoin and... 

It was realized that the mechanism of the Birch reduction involves protonation of the anion radical formed by addition of one electron to the reacting aromatic compound. This... 

Birch reduction, Li can also be used, radical anion mechanism... 

Aromatic Substitutions Using Organometallic Reagents 790 17-14 Addition Reactions of Benzene Derivatives 796 Mechanism 17-9 The Birch Reduction 797 17-15 Side-Chain Reactions of Benzene Derivatives 798... 

The mechanism of the Birch reduction (shown next) is similar to the sodium/liquid ammonia reduction of alkynes to fnmy-alkencs (Section 9-9C). A solution of sodium in liquid ammonia contains solvated electrons that can add to benzene, forming a radical anion. The strongly basic radical anion abstracts a proton from the alcohol in the solvent, giving a cyclohexadienyl radical. The radical quickly adds another solvated electron to form a cyclohexadienyl anion. Protonation of this anion gives the reduced product. 

Propose mechanisms for the Birch reductions of benzoic acid and anisole just shown. Show why the observed orientation of reduction is favored in each case. 

A common illicit synthesis of methamphetamine involves an interesting variation of the Birch reduction. A solution of ephedrine in alcohol is added to liquid ammonia, followed by several pieces of lithium metal. The Birch reduction usually reduces the aromatic ring (Section 17-14C), but in this case it eliminates the hydroxyl group of ephedrine to give methamphetamine. Propose a mechanism, similar to that for the Birch reduction, to explain this unusual course of the reaction. 

In this case the Birch-reduction system (sodium in liquid ammonia) is used. Normally this system is employed to reduce aromatic rings to 1,4-dihydrobenzenes. Here it is advantageous because in contrast to catalytic hydrogenation it does not touch olefinic double bonds and the expected alcohol is generated in 95 % yield (see universal mechanism on the left). 

Equation 5 is known as the Birch reduction. This reaction is mediated by Group 1 metals (usually sodium) in a liquid ammonia solution in the presence of a certain amount of alcohol. The net result is also the addition of hydrogen, but in this case only two hydrogen atoms are added. The reason for this peculiarity becomes apparent upon examination of the mechanism of this process. When a metal like sodium is dissolved in liquid ammonia, it dissociates into a sodium cation and a solvated electron (Scheme 2.6). The first step in the Birch reduction is the attack of the solvated electron on the aromatic system of toluene to... 

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