Nitro groups, aromatic nickel

Nitro Compounds. Under mild conditions, aromatic nitro compounds are hydrogenated easily to amines.518 The reaction may give partially reduced products, according to the circumstances. Palladium, platinum, and nickel are used frequently for this reaction. For example, nitro and benzyl ester functions are reduced on Pd(OH)2/C on THF and on Pd/C in EtOH.519 Aliphatic nitro groups are reduced more slowly. 

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 aromatic nitro group takes preference to the reduction of the aromatic ring. Under certain conditions, however, even the benzene ring was reduced. Hydrogenation of nitrobenzene over platinum oxide or rhodium-platinum oxide in ethanol yielded aniline while in acetic acid cyclohexylamine was produced [55]. Heating of nitrobenzene with formic acid in the presence of copper at 200° gave a 100% yield of aniline, whereas similar treatment in the presence of nickel afforded 67% of cyclohexylamine [71]. 

Amidine derivatives are effective dehalogenation inhibitors for the chemoselective hydrogenation of aromatic halonitro compounds with Raney nickel catalysts. The best modifiers are unsubstituted or N-alkyl substituted formamidine acetates and dicyandiamide which are able to prevent dehalogenation even of very sensitive substrates. Our results indicate that the dehalogenation occurs after the nitro group has been completely reduced i.e. as a consecutive reaction from the halogenated aniline. A possible explanation for these observations is the competitive adsorption between haloaniline, nitro compound, reaction intermediates and/or modifier. The measurement of the catalyst potential can be used to determine the endpoint of the desired nitro reduction very accurately. 

Secondary amines can be prepared from the primary amine and carbonyl compounds by way of the reduction of the derived Schiff bases, with or without the isolation of these intermediates. This procedure represents one aspect of the general method of reductive alkylation discussed in Section 5.16.3, p. 776. With aromatic primary amines and aromatic aldehydes the Schiff bases are usually readily isolable in the crystalline state and can then be subsequently subjected to a suitable reduction procedure, often by hydrogenation over a Raney nickel catalyst at moderate temperatures and pressures. A convenient procedure, which is illustrated in Expt 6.58, uses sodium borohydride in methanol, a reagent which owing to its selective reducing properties (Section 5.4.1, p. 519) does not affect other reducible functional groups (particularly the nitro group) which may be present in the Schiff base contrast the use of sodium borohydride in the presence of palladium-on-carbon, p. 894. 

Rhenium catalysts. H. Smith Broadbent and co-workers have reported the preparation of a number of oxides of rhenium (RcbOt. ReOj, ReOa, ReO), which are effective hydrogenation catalysts, particularly for the reduction of carboxylic acids to primary alcohols. Kor the reduction of aromatic, olefinic. carbonyl, and nitro groups they are less aclivc than nickel or plutimim calalysts hence selective hydrogenation is possible. Bcn/ylic hydroxyl groups are stable to hydrogenolysis. 

As we have already seen (Section 9.6B), the nitration of an aromatic ring introduces a NOg group. A particular value of nitration is the fact that the resulting nitro group can be reduced to a primary amino group, —NH2, by hydrogenation in the presence of a transition metal catalyst such as nickel, palladium, or platinum ... 

Procedures were proposed for the reduction of aromatic nitro groups to amines. Sodium dithionitei78 and a combination of sulfur and hydrazine on activated carboni79 are reported to provide anilines in good to excellent yields. From aliphatic nitro compounds (Eq. 66), primary amines could be obtained with "nickel boride", prepared from nickel chloride and sodium borohydride.i ... 

The reduction of nitroarenes to azoxy compounds ArN=N(0)Ar is promoted by bismuth trichloride/powdered zinc427. Aromatic amines are formed in excellent yields in the reduction of nitroarenes with BH3/NiCl2428 or nickel boride429. Acyl, ester, amide and cyano groups are not affected. Reaction of the nitro compounds RCH2N02 (R = Ph, Bz... 

Smith and Lieber (36) reported that, although the use of alkali activated Raney nickel for the hydrogenation of carbonyl groups, it had a deterring effect on the reduction of aromatic nitro compounds. In particular, alkali retarded the reduction of nitrobenzene and the three isomeric sodium nitrobenzoates, whereas it increased the velocity of reduction of the methyl and ethyl esters of nitrobenzoic acid. 

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