Substituted benzenes Birch reduction

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... 

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. 

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. 

One of the solvated electrons is transferred into an antibonding 7t -orbital of the aromatic compound, and a radical anion of type C is formed (Figure 17.82). The alcohol protonates this radical anion in the rate-determining step with high regioselectivity. In the case under scrutiny, and starting from other donor-substituted benzenes as well, the protonation occurs in the ortho position relative to the donor substituent. On the other hand, the protonation of the radical anion intermediate of the Birch reduction of acceptor-substituted benzenes occurs in the para-position relative to the acceptor substituent. 

Several examples of the Birch reduction of substituted benzene derivatives are shown in the following equations. Note that substituents such as alkyl and alkoxy groups prefer to be attached to one of the carbons of the double bonds of the product, while a carboxyl group prefers to be attached to one of the singly bonded carbons. Benzene derivatives with other types of substituents are usually not employed as reactants in the Birch reduction because the substituents are not stable to the reaction conditions. 

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... 

Closely related to the Birch reduction, benzene, and other aromatic compounds afforded reduction-silylation products (equation 10). Electron-transfer reactions of bis(phenylethynyl)dimethylsilane gives 2,5-dianion of 3,4-diphenylsilole that is a useful intermediate to variously substituted siloles (equation 11). ... 

The example depicted below indicates preferential cleavage of the electron-rich enol ether double bond over the trisubstituted one by the electrophilic ozone. Thus, Birch reduction of methoxy-substituted benzenes followed by ozonolysis of the resultant enol ethers provides a powerful route to functionally substituted (Z)... 

TTHE REACTION OF benzene or one of its derivatives with an alkali metal in liquid ammonia containing an alcohol cosolvent is known as the Birch reduction (1-6). As expected with anionic intermediates, 1-substituted-1,4-dihydro products result from electron-withdrawing groups, and 2,5-dihydro products result from electron-donating groups, as shown in Scheme I. Application of these conditions to polynuclear aromatic compounds produces complications because the products formed initially are subject to reduction under the reaction conditions (3). Although methods such as the use of iron salts minimize side reactions, more recently developed procedures avoid... 

Claisen-Schmidt condensation, 720, 728 with methylamine, 673, 873 nitration, 467, 873 reductive amination, 881 with vinyllithium, 556 Benzenamine, 859. See also Aniline Benzene, 54, 399-406, 433 34 acidity of, 552, 577 Birch reduction of, 413-414 derivatives, nomenclature of, 406-408 electrophilic aromatic substitution in,... 

Fig. 9. Spin distribution in the iso(valence)electronic radical anions of l,4-bis(trimethylsilyl)- and 1,4-dimethyl-substituted (R = H, CH3) benzenes and the protonation quenching products of their Birch reduction (R = CH3). Coupling constants of phenyl hydrogen atoms are a and of substituents [1].

Fig. 6 Reaction schemes for the Birch reduction. The regioselectivity depends on the nature of the substituting groups bonded to the benzene ring.

FIGURE 13.69 In the Birch reduction of substituted benzenes, electronreleasing groups appear on one of the double bonds, but electron-withdrawing groups are located at the methylene position. 

The regioselectivity of Birch reduction of aikoxy-substituted benzenes is the same as for a Iky I benzenes. What did Arthur Birch isoiate when he carried out the foiiowing reaction over... 

---