Partial reduction, of aromatic

Dissolving-Metal Reduction of Aromatic Compounds and Alkynes. Dissolving-metal systems constitute the most general method for partial reduction of aromatic rings. The reaction is called the Birch reduction,214 and the usual reducing medium is lithium or sodium in liquid ammonia. An alcohol is usually added to serve as a proton source. The reaction occurs by two successive electron transfer/proto-nation steps. 

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. 

L. N. Mander, Partial Reduction of Aromatic Rings by Dissolving Metals and by Other Methods, in Comprehensive Organic Synthesis (B. M. Trost, I. Fleming, Eds.), Vol. 8,489, Pergamon Press, Oxford, U. K., 1991. 

SCHEME 30. Ammonia-free partial reduction of aromatic esters using LiDBB135... 

Partial reduction of aromatic polynitro compounds leads to nitro amines. The most successful reagents are the alkali metal or ammonium sulfides in aqueous alcoholIn some instances, sodium bicarbonate combined with sodium sulfide gives better results because of the formation of sodium hydrosulfide, which is believed to be the main reducing agent. Also, aqueous methanol is preferred to aqueous ethanol. Nitro compounds that are sparingly soluble in alcohol solutions may be reduced by hydrogen sulfide in pyridine solution. ... 

Partial Reduction of Aromatic Rings by Dissolving Metals and by Other Methods... 

For examples of Birch reductions, see Mander LN (1991) Partial reduction of aromatic rings by dissolving metals and by other methods. In Trost BM, Fleming I (eds) Comprehensive organic synthesis, vol 8. Pergamon, Oxford, p489 and references cited therein... 

Birch reduction is a partial reduction of aromatic compounds by electron transfer from dissolving metals—usually Na in liquid ammonia or Li in ethylamine—in the presence of a weak proton donor—usually an alcohol. The reaction behaves as if dianion (30) were an intermediate, giving non-conjugated dienes (31). Electron-donating substituents repel the anions (32) to give products like (33). whilst electron-withdrawing substituents attract the anions (34), to give products like (35). 

There are other reagents for partial reduction of aromatic systems the reduction of naphthalene can be controlled to give any of the five products (42)-(46). There is no point in trying to learn these conditions but you should be aware that these compounds are available. 

Acid derivatives are also reduced by L1A1H4. Reduction of the highly reactive acid chlorides 2 and acid anhydrides27 is rapid. Partial reduction of aromatic anhydrides to lactones has been reported by controlling the stoichiometry of L1A1H4 and the anhydride.28 In most cases, however, complete reduction to the diol... 

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