Nitroarene complexes

A special example is the replacement of a nitro group on [(Tj6-nitrobenzene)FeCp]+.78,79 Reaction with O-, S-, N- and stabilized C-nucleophiles gives overall addition-elimination (equation 25). The nitroarene complexes are prepared by oxidation of the corresponding aniline complexes and are readily available.78,79... 

The advantage of a fluoride leaving group was clear in reaction of a hydroxy-carbamate anion with (fluorobenzene)FeCp complex, where the yield was 72% for F and 41% for Cl [79]. The (l,2-difluorobenzene)FeCp complex underwent smooth double substitution with phenethyl amine, whereas a dichloro complex led to monosubstitution. Replacement of a nitro group in q -nitrobenzene-FeCp" is possible with O, S, and N-nucleophiles [70,80,81]. The nitroarene complexes, which are not known in the Cr(CO)3 series, are prepared by oxidation of the corresponding aniline complexes. 

The iridium complex 35 has been also used as catalyst for the transfer hydrogenation of substituted nitroarenes [34]. Good to very good conversions were observed (2.5 mol%, in refluxing isopropanol, 12 h). A mixture of two products was obtained, the relative ratio of which depends on the concentration of added base (KOH) and catalyst. (Scheme 2.5)... 

Scheme 2.5 Products formed in the transfer hydrogenation reduction of nitroarenes by complex 35...

Arenes, on complexation with Cr, Fe, Mn, and so forth, acquire strongly electrophilic character such complexes in reactions with nucleophiles behave as electrophilic nitroarenes.71 Synthesis of aromatic nitriles via the temporary complexation of nitroarenes to the cationic cyclopentadienyliron moiety, cyanide addition, and oxidative demetalation with DDQ has been reported (Eq. 9.43).72... 

A few additional Pd-catalyzed schemes have been employed for Ilac type cyclization chemistry. Palladium-phenanthroline complexes were used by the Ragaini group to prepare indoles via the intermolecular cyclization of nitroarenes and alkynes in the presence of carbon monoxide <06JOC3748>. Jia and Zhu employed Pd-catalysis for the annulation of o-haloanilines with aldehydes <06JOC7826>. A one-pot Ugi/Heck reaction was employed in the preparation of polysubstituted indoles from a four-component reaction system of acrylic aldehydes, bromoanilines, acids, and isocyanides <06TL4683>. 

Direct analysis of complex samples have revealed neutral losses as most useful5. Thus, nitroarenes have been identified based on the losses of OH from the [M + H]+ ions109. However, the fact that some isomers will be detected with a very low sensitivity, if at all, puts a strong limitation to this strategy. 

The main consequence of EDA interactions between solvents of high donicity and nitroarenes is that amines in benzene (or in other similar solvents) compete with the solvents in complexing the nitroarenes253,254. This fact explains the ratio jfcchlorofornyjfcbenzene ] 0 or qlc molecular complex between aniline and FDNB (see Table 1). 

The major route of formation of these nitro compounds is via the reaction of VOCs with the NO arising from hot flue gases, such as automobile exhaust gases and gas streams used for drying food stuffs, etc. In these combustion systems the aliphatics can react with nitro compounds or arenes to produce nitro-PAH and nitroarenes. Some of the NO produced are thus converted into C-nitroso compounds. The interactions and reaction chemistry of these compounds is complex and difficult to interpret. 

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. 

Method A (using iron complexes) Fe,(CO)12 (0.25 g, 0.5 mmol) and the nitroarene (l.O mmol) in PhH (I0 ml) are stirred for 2 h with TEBA-CI (0.02 g, 0.I mmol) in aqueous NaOH (lM, I0 ml) at room temperature. The organic phase is separated and filtered through silica to give the aniline. 

The anodic chlorination in some cases allows one to achieve better regioselec-tivities than chemical alternatives (p/o ratio of chlorotoluene in chlorination of toluene anodic 2.2, chemical alternative 0.5-1.0) [215]. Anodic oxidation of iodine in trimethyl orthoformate afforded a positive iodine species, which led to a more selective aromatic iodination than known methods ]216]. Aryliodination is achieved in good yield, when an aryhodide is oxidized in HOAc, 25% AC2O, 5% H2SO4 in the presence of an arene ]217, 218]. Alkyl nitroaromatic compounds, nitroaromatic ketones, and nitroanihnes are prepared in good yields and regioselectivity by addition of the corresponding nucleophile to a nitroarene and subsequent anodic oxidation of the a-complex (Table 13, number 11) ]219, 220]. 

The catalytic reduction of nitro groups is usually achieved using heterogeneous catalysts, although the iridium complex 28 has been shown to be effective for the reduction of p-nitroanisole 29 to the corresponding aniline 30 using isopropanol as the hydrogen donor (Scheme 8) [30]. In the reduction of some nitroarenes, azo compounds (Ar-N=N-Ar) could be formed as by-products or as the major product by variation of the reaction conditions. 

Iridium 2-pyridinylmethyl imidazolylidene C,N-chelates were obtained by transmetallation of the silver carbene complexes and tested for catalytic activity in the TH of benzophenone and nitroarenes by isopropanol [55]. The neutral monodentate complexes [(L-KC)Ir(COD)Clj [61a,b L = l-methyl-3-(6-mesityl-2-pyridinylmethyl)-2-imidazolylidene, l-mesityl-3-(6-mesityl-2-pyridinylmethyl)-2-... 

Grignard additions, 9, 59, 9, 64 indium-mediated allylation, 9, 687 in nickel complexes, 8, 150 ruthenium carbonyl reactions, 7, 142 ruthenium half-sandwiches, 6, 478 and selenium electrophiles, 9, W11 4( > 2 in vanadocene reactions, 5, 39 Nitrites, with trinuclear Os clusters, 6, 733 Nitroalkenes, Grignard additions, 9, 59-60 Nitroarenes, and Grignard reactivity, 9, 70 Nitrobenzenes, reductive aminocarbonylation, 11, 543... 

Oxidation of aH-complexes with air oxygen is also possible, though for neutral adducts it demands hours and even days for completion. Alkylamination of 5-azacinnoline serves as a typical example (77KGS1554). In nitroarenes the N02 group itself was often found to be an acceptor of hydride ion (see Section III.D.l). Similarly, the ring C = N bond on amination of azaheterocycles can also intercept hydride ion (78RCR1042). 

Impure samples of 1 containing a green impurity were originally believed to catalyze hydrogenation, but this activity is really due to the green complex, formulated as 2. This complex is more effective than Pd/C for hydrogenation of alkenes and alkynes, and can reduce nitroarenes to anilines directly. Reduction of internal alkynes affords cis-alkenes with slight contamination with an alkane. 

The synthesis of 7-substituted indoles from ortfe-substituted nitroarenes and alkenylmagnesium halides is known as the Bartoli reaction.315 Usually, ortfe-unsubstituted nitroarenes give complex mixtures of products. In some cases... 

Reactions of 1-alkenylmagnesium halides with orr/ro-substituted nitroso-or nitroarenes can lead to indoles [14-17]. The first stage of the complex reaction pathway from nitro compounds is reduction to the nitroso compounds, so the former are usually more readily available starting materials. 

Finally, in sect. 2.9, the conversion of 2,4,6- or higher substituted pyrylium salts into the corresponding nitrobenzene derivatives through reaction with nitromethane 27) will be discussed in more detail. Since pyrylium salts of any desired complexity can be easily prepared 28), the conversion of the nitroarenes leads to a large number of phenols of the desired constitution. This method is by far the most versatile in the preparation of a desired arylated phenol. These syntheses, however, involve many steps. 

Reduction of nitroarenes,l Nitrobenzenes substituted with chloro, methyl, or meth-oxy groups can be reduced to the corresponding anilines by formic acid and this ruth-enium(ll) complex in yields generally greater than 90%. Alcoholic solvents increase the... 

The reductive carbonylation of nitroarenes with transition metal catalysts is a very important process in industry, as the development of a phosgene-free method for preparing isocyanate is required. Ruthenium, rhodium, and palladium complex catalysts have all been well studied, and ruthenium catalysts have been shown to be both highly active and attractive. The reduction of nitroarene with CO in the presence of alcohol and amine gives urethanes and ureas [95], respectively, both of which can be easily converted into isocyanates [3,96]. 

Mechanistic studies on the reaction involving mthenium-nitrene complexes [100] or ruthenium-nitroso complexes [95] have also been reported. A stoichiometric reaction of Ru(dppe)(CO)3 (18) with ArNO gives Ru(dppe)(CO)2[CON(Ar)0] (19) (Eq. 11.51). In the first step of the catalytic reaction, nitroarene is reduced to nitrosoar-ene, while in the second step the complex 19 reacts with methanol and CO to give a... 

Ru3(CO)i2 reacts with Ar-BIAN to give Ru(Ar-BIAN)(CO)3, which in turn reacts with nitroarenes to give a Ru(Ar-BIAN)(nitrosoarene)(CO)2 complex (Eq. 11.56) [104]. 

A variety of reducing agents have been used to reduce nitroarenes to azo compounds. However, a mixture of zinc and sodium hydroxide is used most frequently. - Reduction under these conditions produces hydrazo compounds, which are then oxidized to azo compounds by dissolved atmospheric oxygen alternatively, air can be drawn through the product solution to achieve the conversion. - Occasionally, activation of the zinc dust is required prior to its use, and, since metal ions form chelated complexes with azo compounds, a vigorous post-treatment with acids is recommended. The zinc-sodium hydroxide reduction conditions are sufficiently mild that p-nitrostyrene (2 equation 2), can be reduced without reduction of the vinyl groups. ... 

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