Reduction of Benzene Rings

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

Hydrogenation over rhodium (5% on alumina) in acetic acid at room temperature and 3-4 atm of hydrogen was successfully used for reduction of benzene rings in compounds containing functions which would be hydrogen-olyzed over platinum or palladium catalyst [390], Palladium at low temperature usually does not reduce benzene rings and is therefore suited for hydro-genolysis of benzyl derivatives (pp. 150, 151). 

It has been of interest to know if the electrochemical method can provide functional group selectivity. As test cases the inter- and intramolecular competition between reduction of benzene rings and terminal alkynes were studied 45). It was found that the amount of water in the solvent has a large effect on the ratio alkyne/benzene that reacted and that it could be used to control the ratio of alkene/dihydrobenzene prod-... 

Rhodium on carbon, Rh/C Acts as a hydrogenation catalyst in the reduction of benzene rings to yield cyclohexanes (Section 16.10). 

Electrophilic addition to alkene ch20 Reduction of benzene rings Cycloadditions ch35... 

Other reactions may also occur, such as hydrogenolysis of certain groups or reduction of benzene rings, but only the above three will be covered in this section. 

Selective reduction of a benzene ring in the presence of another reducible group is possible if the other group is first protected in some way. Ketones, for example, may be converted to acetals or enol ethers to protect them from reduction. Conversely, reduction of benzene rings takes place only slowly in the absence of a proton donor, and selective reduction of an a,(3-unsaturated carbonyl system can be effected. 

Electrophilic addition to alkenes ch19 Reduction of benzene rings Reagents for oxidation of alcohols Reagents for oxidation of alkenes Protection of aldehydes, ketones, alcohols, and amines Synthesis of peptides Cycloadditions ch34... 

Reduction of aromatic rings with lithium or calcium " in amines (instead of ammonia—called Benkeser reduction) proceeds further and cyclohexenes are obtained. It is thus possible to reduce a benzene ring, by proper choice of reagent, so that one, two, or all three double bonds are reduced. Lithium triethylborohy-dride (LiBEtsH) has also been used, to reduce pyridine derivatives to piperidine derivatives." ... 

Free radical attack at the pyridine ring is noted for its low selectivity and substituents have little effect. Arylation takes place at all three positions, but halogen atoms preferentially attack the a-, and alkyl radicals the a- and y-positions. Metals such as sodium and zinc transfer a single electron to pyridine to form anion radicals. These can dimerize by reaction at the a- or y-position to yield dipyridyls by loss of hydride ion. Thus, reduction of pyridine by chemical and catalytic means is easier than reduction of benzene. 

D. A. Hrovat, J. H. Hammons, C. D. Stevenson, and W. T. Borden, Calculations of the Equilibrium Isotope Effects on the Reductions of Benzene-dg and Cyclooctatetraene-dg, J. Am. Chem. Soc. 1997,119, 9523. B3LYP/6-31+G calculations on the title compounds and on the radical anions formed from them show that the very large difference between the equilibrium isotope effects, found by Stevenson, is due to an inverse isotope effect on the planarization of the COT ring. This explanation was subsequently confirmed by KIE measurements, carried out by C. D. Stevenson, E. C. Brown, D. A. Hrovat, and W. T. Borden, Isotope Effects on the Ring Inversion of Cyclooctatetraene, J. Am. Chem. Soc. 1998, 120, 8864. 

The use of acyl chlorides in Friedel-Craft acylations of benzene rings, as well as their reactions with organometallics and reductions to aldehydes, has been discussed in Section 15.2. 

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. 

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