Nitro groups, aromatic selectivity

Reduction of Nitro- to Amino-groups. - Sulphide catalysts continue to find application in selective hydrogenation of nitro-groups. Aromatic nitro-groups in the presence of acetylenes were selectively hydrogenated (373 K, 25-70 atm, in isopropanol) to amino-groups over cobalt polysulphide and RUS2 catalysts, for example (3-nitrophenyl)acetylene to (3-aminophenyl)acetylene in 75-85% yield. ... 

Aromatic nitro groups are selectively reduced to amino groups with the catalyst in the presence of halogen, ester, and nitrile groups. Nitrobenzene is reduced to aniline quantitatively. More interestingly, dinitroarenes can be selectively reduced to nitroaniUnes in good yield. Such selectivity is not possible with most heterogeneous catalysts. ... 

A number of selected aromatic nitro compounds are collected in Table IV,16A, It will be noted that a few nitro aromatic esters have been included in the Table. These are given here because the nitro group may be the first functional group to be identified aromatic nitro esters should be treated as other esters and hydrolysed for final identification. 

It is convenient to include under Aromatic Amines the preparation of m-nitroaniline as an example of the selective reduction of one group in a polynitro compound. When wt-dinitrobenzene is allowed to react with sodium polysulphide (or ammonium sulphide) solution, only one of the nitro groups is reduced and m-nitroanUine results. Some sulphur separates, but the main reaction is represented by ... 

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]... 

The inertness of ordinary double bonds toward metallie hydrides is quite useful, since it permits reduction of, say, a carbonyl or nitro group, without disturbing a double bond in the same molecule (see Chapter 19 for a discussion of selectivity in reduction reactions). Sodium in liquid ammonia also does not reduce ordinary double bonds, although it does reduce alkynes, allenes, conjugated dienes, and aromatic rings (15-14). 

The. selective hydrogenation of a nitro group in the presence of other reactive functionalities is a frequently encountered problem in fine chemicals manufacture. Ciba-Geigy (Novartis). scientists developed, in collaboration with a catalyst manufacturer, a new Pt/Pb on CaCO. catalyst that allows the chemoselective hydrogenation of an aromatic nitro group in the presence of C=C, C=0, C=N as well as Cl or Br substituents in selectivities > 95% (even C C groups react very slowly) (Bader et al., 1996). Eqn. (3) shows an example (Bader eJ a/., 1996). 

Electrochemically generated nickel is very selective for the reduction of aromatic nitro compounds into anilines, in which alkenyl, alkynyl, halo, cyano, formyl, and benzyloxy groups are not affected.84 Sodium sulfide has been used for the selective reduction of aromatic nitro group in the presence of aliphatic nitro groups (Eq. 6.44).85... 

Reduction of nitro groups. The lithium anion of phthalocyaninecobalt(I), Li[Co(I)Pc], selectively reduces aliphatic and aromatic nitro compounds to primary amines at room temperature in 65 95% yield. Double bonds, nitriles, carhonyl groups, and aryl halides are not reduced. 

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