Benzene rings, Birch reduction

Another way to make cyclohexenes is by the partial reduction of benzene rings ( Birch reduction, described in Norman, p.553-557) such as ... 

Selective reduction of a benzene ring (W. Grimme, 1970) or a C C double bond (J.E. Cole, 1962) in the presence of protected carbonyl groups (acetals or enol ethers) has been achieved by Birch reduction. Selective reduction of the C—C double bond of an a,ft-unsaturated ketone in the presence of a benzene ring is also possible in aprotic solution, because the benzene ring is redueed only very slowly in the absence of a proton donor (D. Caine, 1976). 

Jenny and Reiner 107> obtained the di- and tetrahydro compounds 153 and 154 by Birch reduction of [2.2]paracyclophane. On the basis of spectroscopic findings, the authors postulate the configuration 153 a A, for the dihydro compound, where the benzene rings are slightly distorted in a tublike fashion. 

Reduction of benzenoid hydrocarbons with solvated electrons generated by the solution of an alkali metal in liquid ammonia, the Birch reaction [34], involves homogeneous electron addition to the lowest unoccupied 7t-molecular orbital. Protonation of the radical-anion leads to a radical intermediate, which accepts a further electron. Protonation of the delocalised carbanion then occurs at the point of highest charge density and a non-conjugated cyclohexadiene 6 is formed by reduction of the benzene ring. An alcohol is usually added to the reaction mixture and acts as a proton source. The non-conjugated cyclohexadiene is stable in the presence of... 

The fully delocalized n electron system of the benzene ring remains intact during electrophilic aromatic substitution reactions. However, in the Birch reduction, this is not the case. In the Birch reduction, benzene, in the presence of sodium metal in liquid ammonia and methyl alcohol, produces a nonconjugated diene system. This reaction provides a convenient method for making a wide variety of useful cyclic dienes. 

The third is partial or total reduction of an aromatic ring. Any catalogue lists a vast number of available substituted benzene rings. Saturated compound 8 can obviously be made by total reduction of 9 but it may not be obvious that partial reduction (Birch) allows the enone 11 also to be made from 9. Birch reduction is the only new method here so we shall revise the Robinson and the Diels-Alder and concentrate on Birch. 

Birch reduction11 is the partial reduction of aromatic rings by solvated electrons produced when alkali metals dissolve (and react) in liquid amines. Typical conditions are sodium in liquid ammonia or lithium in methylamine. These electrons add to benzene rings to produce, probably, a dianion 57 that is immediately protonated by a weak acid (usually a tertiary alcohol) present in solution. The anions in the supposed intermediate 57 keep as far from each other as they can so the final product is the non-conjugated diene 58. It is important to use the blue solution of solvated electrons before it reacts to give hydrogen and NaNH2. 

PAHs and soils contaminated with PAHs are readily remediated by solvated electrons in NH3. Oligomeric reduced products are obtained. These reactions are slower than dehalogenation, as was demonstrated by the rapid formation of benzene, toluene, and naphthalene in Na/NH3 from their corresponding monochloro derivatives [24,28], Table 12 summarizes data on the destruction of pure PAHs. Soils contaminated with PAHs have been remediated to below detection levels. Mononuclear aromatics (benzene, toluene, anisole, and nitrobenzene) undergo ring reduction according to the well-known Birch reduction [11-18]. 

It appears established (Bothner-By, 1959) that Birch reduction of the benzene ring proceeds by the addition of one electron to give a radical ion (2), which must be protonated (3) before addition of a second electron and proton can occur (4). 

Harsh reaction conditions are required to reduce the aromatic benzene ring. This can be achieved by catalytic hydrogenation (using high temperatures or pressures and very active catalysts) or alkali metals in liquid ammonia/ethanol (in a Birch reduction). 

Birch reduction. Sodium or lithium metal (in liquid ammonia) can donate an electron to the benzene ring to form a radical anion. On protonation (by ethanol) and further reduction/protonation, this produces 1,4-cyclohexadiene. 

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