Reduction of aromatic

The elimination of lead, the reduction of aromatics in gasoline, and the desulfurization of diesel fuels are oing to require significant reformulations of these products that will irripiy development of specific additives that allow the refiner to optimize costs while meeting the required specifications. 

Unsaturated hydrocarbons are present in nearly all products of the Clemmensen reduction of aromatic ketones and must be removed, if the hydrocarbon is requiral pure, by the above process. Secondary alcohols, often produced m small amount are not appreciably steam-volatile. 

Compounds containing two primary amino groups attached to a benzene ring can be prepared by the reduction of dinitro compounds and of nitroanilines, usually with tin or stannous chloride and hydrochloric acid or with iron and very dilute hydrochloric acid. / ara-diamines may also be obtained by the reduction of aromatic amino-azo compounds (e.g., p-aminodimethylanihne from methyl orange, see Section IV,78). p-Phenylenediamine may also be prepared from p-nitroacetanilide reduction with iron and acid yields p-amino-acetaniUde,.which may be hydrolysed to the diamine. 

By the reduction of aromatic esters with sodium and absolute ethyl alcohol, for example ... 

Birch Reductions reduction of aromatic rings Organic Reactions 1976, 23, 1. Tetrahedron 1986, 42, 6354. Cornprehensice Organic Synthesis 1991, voJ. 8, 107. 

Akhrem, A. A. Reshetova, I. G. Titov, Yu. A. 1972, Birch Reduction of Aromatic Compound, Plenum New York... 

Metal-ammonia-alcohol reductions of aromatic rings are known as Birch reductions, after the Australian chemist Arthur J Birch who demonstrated their usefulness begin nmg m the 1940s... 

Aromatic rings in lignin may be converted to cyclohexanol derivatives by catalytic hydrogenation at high temperatures (250°C) and pressures (20—35 MPa (200—350 atm)) using copper—chromium oxide as the catalyst (11). Similar reduction of aromatic to saturated rings has been achieved using sodium in hquid ammonia as reductants (12). 

The Zinin reduction is also usehil for the reduction of aromatic nitro compounds to amines in the laboratory. It requires no special equipment, as is the case with catalytic hydrogenations, and is milder than reductions with iron and acid. Usually ammonium or alkah sulfides, hydrosulftdes or polysulftdes are used as the reactant with methanol or ethanol as the solvent. 

Electrolytic reductions generally caimot compete economically with chemical reductions of nitro compounds to amines, but they have been appHed in some specific reactions, such as the preparation of aminophenols (qv) from aromatic nitro compounds. For example, in the presence of sulfuric acid, cathodic reduction of aromatic nitro compounds with a free para-position leads to -aminophenol [123-30-8] hy rearrangement of the intermediate N-phenyl-hydroxylamine [100-65-2] (61). 

Silver sulfate has been described as a catalyst for the reduction of aromatic hydrocarbons to cyclohexane derivatives (69). It is also a catalyst for oxidation reactions, and as such has long been recommended for the oxidation of organic materials during the deterrnination of the COD of wastewater samples (70,71) (see WASTES, INDUSTRIAL WATER, INDUSTRIAL WATERTTEATI NT). 

This reaction is favored by moderate temperatures (100—150°C), low pressures, and acidic solvents. High activity catalysts such as 5—10 wt % palladium on activated carbon or barium sulfate, high activity Raney nickel, or copper chromite (nonpromoted or promoted with barium) can be used. Palladium catalysts are recommended for the reduction of aromatic aldehydes, such as that of benzaldehyde to toluene. 

Reduction of Aromatic Mitro Compounds to Aromatic Amines. Mild conditions and a large variety of catalysts effect reduction of aromatic nitro compounds to aromatic amines (see Amines byreduction). 

One-electron reduction of a-dicarbonyl compounds gives radical anions known as setnidiones. Closely related are the products of one-electron reduction of aromatic quinones, the semiquinones. Both semidiones and semiquinones can be protonated to give neutral radicals which are relatively stable. 

The term Birch reduction was originally applied to the reduction of aromatic compounds by alkali metals and an alcohol in ammonia. In recent years many chemists have used the term to include all metal-ammonia reductions, whether an alcoholic proton source is present or not. The author prefers to use the term Birch reduction to designate any reduction carried out in ammonia with a metal and a proton donor as or more acidic than an alcohol, since Birch customarily used such a proton donor in his extensive pioneering work. The term metal-ammonia reduction is best reserved for reductions in which ammonia is the only proton donor present. This distinction in terminology emphasizes the importance of the acidity of the proton donor in the reduction process. 

Reduction of a conjugated enone to a saturated ketone requires the addition of two electrons and two protons. As in the case of the Birch reduction of aromatic compounds, the exact order of these additions has been the subject of study and speculation. Barton proposed that two electrons add initially giving a dicarbanion of the structure (49) which then is protonated rapidly at the / -position by ammonia, forming the enolate salt (50) of the saturated ketone. Stork later suggested that the radical-anion (51), a one electron... 

The preparation of enamines by reduction of aromatic heterocyclic bases and their quaternary salts or of lactams is not the most useful approach (97). The lithium aluminum hydride reduction of N-acyl enamines has been used with both fruitful and unsuccessful results. A series of 3-N-acetyl -d -cholestenes (104) has been prepared by desulfurization of the appropriate thiazolidine (105) (98,99). Lithium aluminum hydride reduction of the... 

In most reviews of enamine chemistry the reactions of iminium salts are scattered throughout the review and are consequently not covered in a comprehensive manner. This chapter will be an attempt to look at reactions that, at one stage or another, proceed by nucleophilic addition to the iminium intermediate. The subject of enamines has been reviewed 1-4) and certain aspects of iminium salt chemistry such as reduction of aromatic quaternary salts have been treated in detail (5). Consequently, the reduction of aromatic quaternary salts with complex hydrides will be presented here only briefly. Although the literature (especially 1950-1967) has been checked with care, the author can make no claim to completeness. The... 

The reduction of aromatic compounds 1 by alkali metals in liquid ammonia in the presence of an alcohol is called the Birch reduction, and yields selectively the 1,4-hydrogenated product " 2. 

The elaboration of a method for the reduction of aromatic rings to the corresponding dihydrobenzenes under controlled conditions by A. J. Birch opened a convenient route to compounds related to the putative norprogesterone. This reaction, now known as the Birch reduction,is typified by the treatment of... 

An unusual by-product was obtained in small yield in palladium-catalyzed reduction of 2-amino-4,5-dimethoxyindanone hydrochloride, The reduction was done in two stages first, a rapid absorption of 1 mol of hydrogen at 38 C to give the amino alcohol, and then a much slower reduction in the presence of HCIO4 at 70 "C. The rearranged by-product was shown to arise from attack of acid on the amino alcohol (50), Resistance to hydrogenolysis is characteristic of / -amino aromatic alcohols (56), a fact that makes reduction of aromatic oximino ketones to amino benzyl alcohols a useful synthetic reaction. 

This result stands in contrast to hydrogenation of 2-oximino-]-indanone (R = H), which stopped spontaneously at the 2-amino-1-indanol stage under similar conditions (43). This latter result accords with the general exp>erience in reduction of aromatic -oximino ketones (35,37 38,39,40). The amino function usually severely inhibits hydrogenolysis of the alcohol. 

Reduction of aromatic nitro compounds may give products sensitive to air. Moder and Leonard (d5) stressed the importance of rigid exclusion of air during reduction of a nitro compound to a complex air-sensitive diamine in order to maximize the yield. It is quite likely that in these cases the oxidation products will have an adverse influence on catalyst life. 

Hydroxylamines ordinarily do not accumulate in the reduction of aromatic nitro compounds for, with some exceptions, most systems in competition will reduce the hydroxylamine function preferentially. Nonetheless, systems have been found that afford the intermediate aromatic hydroxylamine in excellent yield. With hydrogen gas as a reductant and platinum-on-carbon or -on-alumina and about I wt % of DMSO based on nitro compound as a modifier, aromatic hydroxylamines can be formed in 90% yield under mild conditions. The reduction slows markedly after absorption of the second mole of hydrogen and should be stopped at this stage (80). 

Rhodium (2J) and ruthenium are excellent catalysts for the reduction of aromatic rings. It is with these catalysts that the best chance resides for preservation of other reducible functions (2,10,13,18,41,42,52). Rhodium (41) and ruthenium (45) each reduced methylphenylcarbinol to methylcyclohexyl-carbinol in high yield. Palladium, on the other hand, gives ethylbenzene quantitatively. Water has a powerful promoting effect, which is unique in ruthenium catalysis (36). 

Formation of azo-type products might be troublesome. These by-products, arising from reduction of aromatic nitro compounds, usually are assumed to be derived from the coupling of intermediate nitroso and hydroxylamine compounds. The coupling problem is accentuated in reduction of nitroso compounds because of much higher concentrations. It can be alleviated by dropwise addition of the substrate to the hydrogenation and use of acidic media. 

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