Iron, reduction of nitroarenes

Ionization potential. See Ionization energy a- and p-lonone, 1049 lonophore, 624,1023 Iron, reduction of nitroarenes by, 878 Iron(III) salts as catalysts in halogenation of arenes, 446, 448—450 Isoamyl acetate, in bananas, 85, 788 Isobutane, 57. See also 2-Methylpropane Isobutene. See 2-Methylpropene... 

In many respects the apparently analogous reduction of nitroarenes with triruthenium dodecacarbonyl under basic phase-transfer conditions is superior to that of the iron carbonyl-mediated reductions. However, the difference in the dependence of the two processes on the concentration of the aqueous sodium hydroxide and the pressure of the carbon monoxide suggests that they may proceed by different mechanisms. Although the iron-based system is most effective under dilute alkaline conditions in the absence of carbon monoxide, the use of 5M sodium hydroxide is critical for the ruthenium-based system, which also requires an atmosphere of carbon monoxide [11]. The ruthenium-based reduction has been extended to the... 

Reduction of nitroarenes (cf., 1, 440). Nitroarenes are reduced to amines in generally good yields by hydrazine hydrate with hydrated /8-iron(III) oxide, /3-FciO, H2O as catalyst. 

Selective reduction to hydroxylamine can be achieved in a variety of ways the most widely applicable systems utilize zinc and ammonium chloride in an aqueous or alcoholic medium. The overreduction to amines can be prevented by using a two-phase solvent system. Hydroxylamines have also been obtained from nitro compounds using molecular hydrogen and iridium catalysts. A rapid metal-catalyzed transfer reduction of aromatic nitroarenes to N-substituted hydroxylamines has also been developed the method employs palladium and rhodium on charcoal as catalyst and a variety of hydrogen donors such as cyclohexene, hydrazine, formic acid and phosphinic acid. The reduction of nitroarenes to arylhydroxyl-amines can also be achieved using hydrazine in the presence of Raney nickel or iron(III) oxide. ... 

The iron cluster [Fe4S4(SPh)4] catalyzes the reduction of nitroarenes to arylamines. " A less hydridic nucleophile [HFe(CO)4] has also found application as a selective reducing agent for nitroarenes." Although [HFe(CO)4]"] is known to reduce aldehydes, ketones and acid halides," in THF solvent with tri-fluoroacetic acid, it selectively reduces nitrobenzenes to anilines in the presence of aldehyde and acid halide groups. 

For the reduction of nitroarenes to aminoarenes by the catalytic hydrazine H-transfer reduction method, the classical hydrogenation catalysts Ni, Pd and Pt are most commonly used [1] [2]. In a more extended study [3] we were able to confirm previously reported observations [4] that these reductions can also be catalysed by modified iron oxides hydroxides. This method for the production of many aromatic amines offers several advantages compared to the conventional processes still employed in industry, such as the environmentally imfavourable Bechamp [5] and Zinin reductions [6]. It is an outstanding feature of the novel reduction method presented here that further reducible substituents in nitroazo compounds, such as... 

The cluster Ru3(CO)i2 is known to react in the presence of bases to afford, under certain experimental conditions, [HRu3(CO)u] [42]. Since the corresponding iron cluster is known to react with nitrobenzene to yield an hydrido imido cluster [HFe3(CO)9(NPh)] , which can be protonated and afford small amounts of aniline [43], a catalytic cycle was proposed, and supported by some model reactions, for the ruthenium-catalysed reduction of nitroarenes by CO/H2O, which includes the intermediate formation of the trinuclear hydrido cluster [44-46]. However, one of us has recently shown that [HFe3(CO)n] and the corresponding imido complex play no role in the Fe3(CO)i2-promoted reduction of nitrobenzene [6, 47] and the proposal of an active role of [HRu3(CO)ii] appears to be questionable. 

Miyake and co-workers (40) have published a synthesis of ellipticine that features a novel reductive phenylation of nitroarenes (41) (Scheme 4). Nitration of 5,8-dimethyl-l, 2,3,4-tetrahydroisoquinoline (22) gave an inseparable mixture of nitro compounds 23. Treatment of this mixture with iron pentacarbonyl and triflic acid in the presence of benzene gave a 2 1 mixture of amines 24 and 25. Separation of these isomers and diazotization of each with nitrous acid, conversion to the azide, and thermolysis yielded ellipticine (1) and isoellipticine (27) (5,11-dimethyl-10f/-pyrido[3,4- )]carbazole), respectively, following Pd/C dehydrogenation of the initially formed nitrene insertion product (e.g., 26). The overall yield of ellipticine is 9%. 

Reductions of mtroarenes. Nitroarenes were first reduced with iron and an acid by Bechamp in 1854. The use of iron in glacial acetic acid was reported in 1952 by Spring et al Owsley and Bloomfield have now made a systematic study of this method and conclude that it is a useful alternative to catalytic hydrogenation and is simpler to carry out than reductions with tin or tin(II) chloride with... 

Iron-substituted mesoporous aluminophosphate was used as a novel, efficient and ecofriendly catalyst for reductive cleavage of azo dyes and reduction in nitroarenes and catalytic transfer hydrogenation of nitro and carbonyl compounds. Unlike most of the iron-containing molecular sieves, dislodgement of tetrahedral Fe(III) was not observed upon various process treatments such... 

Following the isolation of these complexes, all of the mechanistic studies on the carbonylation and reduction reactions of nitroarenes catalysed by Ru3(CO)i2, even in the presence of several promoters, have focused on the reactivity of these or related clusters [157-164]. Moreover, many studies have been also conducted on analogous osmium [165-172] and iron (see paragraph 6.6.) clusters, including insertion reactions of isocyanates, which yield potential intermediates in the carbonylation reaction (Insertion reaction of other cumulenes into the Ru-N bond will not be discussed here. However, see the paragraph of the synthesis of heterocycles later in this chapter). Although not all of the previously mentioned studies were intended to be a basis for a mechanistic understanding of the reactions here discussed, they still contain a lot of information on the possible transformations of amido or imido moieties on a trinuclear cluster. 

In this paragraph, we will discuss catalytic systems in which an iron compound is the only catalyst, that is we will not consider those systems in which an iron compound is used as a cocatalyst in what is considered to be a palladium- or rhodium-based eatalytic system. These last systems have already been discussed in paragraphs 6.3.1. and 6.5.2. Synthetic applications of iron-based systems for the carbonylation (Chapter 3) and reduction (Chapter 4) of nitroarenes have already been described. 

Primary Amines—Several new and improved methods for the preparation of aromatic primary amines have been described. " The catalytic reduction of aromatic nitro-compounds to amines with hydrazine hydrate in the presence of iron(iii) oxide hydroxide proceeds smoothly and in high yields. Nitroarenes (1) react with Grignard reagents in the presence of copper(i) iodide, and are alkylated selectively on the aromatic moiety, accompanied by reduction of the nitro-group to the amine (Scheme 1). Both aryl and alkyl azides are rapidly... 

Nitroarenes are reduced to anilines (>85%) under the influence of metal carbonyl complexes. In a two-phase system, the complex hydridoiron complex [HFe,(CO)u]2-is produced from tri-iron dodecacarbonyl at the interface between the organic phase and the basic aqueous phase [7], The generation of the active hydridoiron complex is catalysed by a range of quaternary ammonium salts and an analogous hydrido-manganese complex is obtained from dimanganese decacarbonyl under similar conditions [8], Virtually no reduction occurs in the absence of the quaternary ammonium salt, and the reduction is also suppressed by the presence of carbon monoxide [9], In contrast, dicobalt octacarbonyl reacts with quaternary ammonium fluorides to form complexes which do not reduce nitroarenes. 

Reduction with iron in hydrochloric acid is one of the most common methods for converting nitroarenes to arylamines. 

For M = Ru, formamides and amines were the principal products. Substitution of Fe3(CO)i2 for Ru3(CO)i2 results in the formation of carbamate esters (ArNHCOOMe) as the major products, with ureas as the main by-products. Without the metal carbonyl, nitroarenes are recovered imchanged from the reaction mixture. An imido-alcoxycarbonyl complex was suggested as an intermediate in the reaction and the difference between iron and ruthenium was proposed to be due to the different facility of this intermediate to undergo protonation and reductive elimination to give carbamate, or insertion of CO into the M-N bond, followed by hydrogenation, affording formamide (Scheme 13) ... 

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