Benzenes 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). 

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... 

Krapcho and Bothner-By made additional findings that are valuable ii understanding the Birch reduction. The relative rates of reduction o benzene by lithium, sodium and potassium (ethanol as proton donor) wer found to be approximately 180 1 0.5. In addition, they found that ben zene is reduced fourteen times more rapidly when methanol is the protoi donor than when /-butyl alcohol is used. Finally, the relative rates of reduc tion of various simple aromatic compounds by lithium were deteiTnined these data are given in Table 1-2. Taken together, the above data sho that the rate of a given Birch reduction is strikingly controlled by the meta... 

Estrone methyl ether (100 g, 0.35 mole) is mixed with 100 ml of absolute ethanol, 100 ml of benzene and 200 ml of triethyl orthoformate. Concentrated sulfuric acid (1.55 ml) is added and the mixture is stirred at room temperature for 2 hr. The mixture is then made alkaline by the addition of excess tetra-methylguanidine (ca. 4 ml) and the organic solvents are removed. The residue is dissolved in heptane and the solution is filtered through Celite to prevent emulsions in the following extraction. The solution is then washed threetimes with 500 ml of 10 % sodium hydroxide solution in methanol to remove excess triethyl orthoformate, which would interfere with the Birch reduction solvent system. The heptane solution is dried over sodium sulfate and the solvent is removed. The residue is satisfactory for the Birch reduction step. Infrared analysis shows that the material contains 1.3-1.5% of estrone methyl ether. The pure ketal may be obtained by crystallization from anhydrous ethanol, mp 99-100°. Acidification of the methanolic sodium hydroxide washes affords 10-12 g of recovered estrone methyl ether. 

Many expensive reductions such as the Birch reduction of naphthalene to isotetralin, benzene to cyclohexene, with metallic sodium and liquid ammonia, or reduction with LiAlHa, can generally be carried out electrochemically at much lower cost and under safe conditions. Electrochemical processes allow fluorinations to be carried out without using fluorine gas. Conducting polymers have been made by electrochemical processes which operate under ambient conditions, and the polymer can be synthesized, doped and shaped in film form in a single step. 

Reduction of aromatic compounds to dihydro derivatives by dissolved metals in liquid ammonia (Birch reduction) is one of the fundamental reactions in organic chemistry308. When benzene derivatives are subjected to this reduction, cyclohexa-1,4-dienes are formed. The 1,4-dienes obtained from the reduction isomerize to more useful 1,3-dienes under protic conditions. A number of syntheses of natural products have been devised where the Birch reduction of a benzenoid compound to a cyclohex-1,3-diene and converting this intermediate in Diels-Alder fasion to polycyclic products is involved (equation 186)308f h. 

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. 

Problem 10.22 In the Birch reduction benzene is reduced with an active metal (Na or Li) in alcohol and liquid NHj(-33 °C) to a cyclohexadiene that gives only OCHCH CHO on ozonolysis. What is the reduction product ... 

Perhaps it should be mentioned also the orientation of the Birch reduction which is strongly dependent on the nature of the aromatic substituents. Donor-substituted benzenes furnish predominantly 1-substituted 1,4-cyclohexadienes while acceptor-substituted analogues give 3-substituted 1,4-cyclohexadienes. The regioselectivities can be explained by the destabilizing d-d pairing in the intermediates from d-substi-tuted cyclohexadienyl radical anions leading to the 3-substituted products, and the... 

However, benzene and its derivatives can be reduced to cyclohexa-dienes by solutions of metals such as Li, Na, K, Zn, and Hg in a weakly acidic solvent, such as liquid ammonia, amines, or ether-alcohol mixtures. This general type of reaction is known as the Birch reduction after the Australian chemist, A. J. Birch. With benzene, reduction with metals leads to 1,4-cyclo-hexadiene ... 

The initial step of the Birch reduction is an electron transfer to the lowest unoccupied molecular orbital of the benzene tt system (see Figure 21-5) to form a radical anion ... 

Reduction of substituted benzenes with sodium (or lithium) in liquid ammonia in the presence of a proton source (such as methanol, ethanol, etc.) leads to a substituted, non-conjugated cyclohexadiene as a result of 1,4-addition of hydrogen (the Birch reduction).16 With benzene the product is cyclohexa-1,4-diene as a result of the following mechanistic pathway. 

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. 

R. Taylor. We tried the Birch reduction and obtained amino peaks in the ir because amination occurs under the reaction conditions. We tried to get over this problem by hydrogenating over platinum at low pressure, initially in hexane and then in benzene. Benzene must be redistilled first because it contains di-octyl phthalate. Having adopted this precaution, a purple solution was obtained which became bright yellow after 2-3 days. Of course benzene is reduced as well to cyclohexane. Another problem arises because the (hydrogenated) product, although initially soluble in... 

Fig. 3. Distribution of formal charge in the anionic intermediate of the Birch reduction of benzene calculated (a) by the Huckel method, (b) by the Pople method.

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. 

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