Partial reduction, of aromatic compounds

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). 

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. 

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. ... 

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. 

Formic acid is a good reducing agent in the presence of Pd on carbon as a catalyst. Aromatic nitro compounds are reduced to aniline with formic acid[100]. Selective reduction of one nitro group in 2,4-dinitrotoluene (112) with triethylammonium formate is possible[101]. o-Nitroacetophenone (113) is first reduced to o-aminoacetophenone, then to o-ethylaniline when an excess of formate is used[102]. Ammonium and potassium formate are also used for the reduction of aliphatic and aromatic nitro compounds. Pd on carbon is a good catalyst[103,104]. NaBH4 is also used for the Pd-catalyzed reduction of nitro compounds 105]. However, the ,/)-unsaturated nitroalkene 114 is partially reduced to the oxime 115 with ammonium formate[106]... 

A special use of sodium arsenite (applied in aqueous alkaline solutions) is partial reduction of trigeminal halides to geminal halides [220] and reduction of aromatic nitro compounds to azoxy compounds [221]. 

More abundant are reductions with sodium sulfite which is applied in aqueous solutions (solubility 24%). Its specialties are reduction of peroxides to alcohols [257], of sulfonyl chlorides to sulfinic acids [252], of aromatic diazonium compounds to hydrazines [253], and partial reduction of geminal polyhalides [254] Procedure 43, p. 216). 

The enantioselective hydrogenation of olefins, ketones and imines still represents an important topic and various highly enantioselective processes based on chiral Rh, Ru or Ir complexes have been reported. However, most of these catalysts failed to give satisfactory results in the asymmetric hydrogenation of aromatic and heteroaromatic compounds and examples of efficient catalysts are rare. This is especially the case for the partial reduction of quinoline derivatives which provide 1,2,3,4-tetrahydroquinolines, important synthetic intermediates in the preparation of pharmaceutical and agrochemical products. Additionally, many alkaloid natural products consist of this stmctural key element. 

The reduction of aromatic nitro compounds to the corresponding amines was catalyzed by [Ru3(CO)i2] in combination with aliphatic amine cocatalysts (95). A mixture of diglyme and water was used as a solvent, turnover frequencies were about 5000 h-1, and a CO partial pressure of 20-50 atm was necessary. The reaction is highly selective for aromatic amines. It was speculated that the reaction proceeds via an intramolecular hydrogen transfer in a hydrido-metal-nitrene intermediate without prior formation of H2 in the water gas shift reaction. 

Although pseudomonads are well known for aerobic decomposition of aromatic compounds, some strains of Pseudomonas, as well as many other bacteria, are able to degrade aromatic compounds under completely anerobic conditions.166 167 Benzoate can be converted to benzoyl-CoA and the ring can be partially reduced in two ATP- and NADH-dependent reactions (Eq. 25-9). The first of these reduction steps is unusual because ATP is apparently needed to drive the reaction.166,166a,b This is analogous to the need for ATP in nitrogen fixation (Eq. 24-6, step b). 

Selective hydrogenation of one or two nitro groups in an aromatic dinitro compound is the basis for the synthesis of otherwise unattainable molecules. Carefully chosen catalysts, under totally different reaction conditions, have been met with success. Partial reduction of 2,6-dinitroanilines to nitrophenylenediamines occurs in 60-90% yield over 10% palladium-on-carbon at RT [equation (a)], although most heterogeneous catalysts do not afford such selectivity. 

The reduction of nitroaromatic compounds is a very exothermic reaction (560 kJ mol ). When reaction control is lost, decomposition of the aromatic nitro compound or of partially hydrogenated intermediates could be triggered easily. 

Other complex metal hydrides can only rarely be applied to reduction of C=C bonds. Sodium borohydride, which can be used in aqueous-alcoholic solution, seems not normally to attack ethylenic bonds. A few cases only of partial reduction of cyclic iminum salts96 and of selective reduction of unusually activated ethylenic bonds97 have been reported. However, some polynitro aromatic compounds, e.g., 1,3,5-trinitrobenzene, can be converted in high yield with saturation of the aromatic system into nitrocyclohexanes or nitro-cyclohexenes.98 Sodium hydridotrimethoxyborate has proved valuable as a mild reducing agent for preparation of a series of nitroparaffins from nitro-alkenes."... 

It should, however, be noted that aromatic amino-nitro compounds are often more conveniently prepared by heating a halo-nitro compound with ammonia or an amine (e.g., N,N-dimethy -o- and -p-nitroaniline194) or by partial reduction of a polynitro compound (cf. pp. 463 et seq. and 575). 

Sulfide. This is used mainly for the partial reduction of pol3mitro aromatic compounds to nitroamines and for the reduction of nitroanthra-quinones to aminoanthraquinones. 

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