Hydrogenation, catalytic nitro group

Reductions at noble metal electrodes in acidic protic media often form adsorbed hydrogen, which is the actual reductant. For example, reduction of nitrobenzene at a Pd/C electrode in acetic acid-methanol mixtures affords aniline via adsorbed hydrogen28. This reaction is more closely related to catalytic hydrogenation of nitro groups than to the... 

The reduction of the nitro group to yield aniline is the most commercially important reaction of nitrobenzene. Usually the reaction is carried out by the catalytic hydrogenation of nitrobenzene, either in the gas phase or in solution, or by using iron borings and dilute hydrochloric acid (the Bechamp process). Depending on the conditions, the reduction of nitrobenzene can lead to a variety of products. The series of reduction products is shown in Figure 1 (see Amines byreduction). Nitrosobenzene, /V-pbenylbydroxylamine, and aniline are primary reduction products. Azoxybenzene is formed by the condensation of nitrosobenzene and /V-pbenylbydroxylamine in alkaline solutions, and azoxybenzene can be reduced to form azobenzene and hydrazobenzene. The reduction products of nitrobenzene under various conditions ate given in Table 2. 

Hydrogenation Catalysts. The key to catalytic hydrogenation is the catalyst, which promotes a reaction which otherwise would occur too slowly to be useful. Catalysts for the hydrogenation of nitro compounds and nitriles are generally based on one or more of the group VIII metals. The metals most commonly used are cobalt, nickel, palladium, platinum, rhodium, and mthenium, but others, including copper (16), iron (17), and tellurium... 

Nitropyridazines are reduced catalytically either over platinum, Raney nickel or palladium-charcoal catalyst. When an N-oxide function is present, palladium-charcoal in neutral solution is used in order to obtain the corresponding amino N-oxide. On the other hand, when hydrogenation is carried out in aqueous or alcoholic hydrochloric acid and palladium-charcoal or Raney nickel are used for the reduction of the nitro group, deoxygenation of the N- oxide takes place simultaneously. Halonitropyridazines and their N- oxides are reduced, dehalogenated and deoxygenated to aminopyridazines or to aminopyridazine N- oxides under analogous conditions. 

Nitro groups are readily reduced to primary amines by a variety of methods. Catalytic hydrogenation over platinum, palladium, or nickel is often used, as is reduction by non or tin in hydrochloric acid. The ease with which nitro groups are reduced is especially useful in the preparation of arylanines, where the sequence ArH ArN02 ArNH2 is the standard route to these compounds. 

Reduction of aryl nitro compounds (Section 22.9) The standard method for the preparation of an arylamine is by nitration of an aromatic ring, followed by reduction of the nitro group. Typical reducing agents include iron or tin in hydrochloric acid or catalytic hydrogenation. 

The catalytic effect of aromatic nitro groups in the substrate and product or in an added inert nitro compoimd (e.g., w-dinitrobenzene in 18) has been observed in the reaction of 2,4-dinitrochlorobenzene with an amine in chloroform. Hydrogen bonding to benzil or to dimethyl sulfone and sulfoxide also provided catalysis. It is clear that the type of catalysis of proton transfer shown in structure 18 will be more effective when hydrogen bonding is to an azine-nitrogen. 

Reduction of 8-nitro-l,3,4,6,l 1,1 la-hexahydro[l,4]oxazino[4,3-/)]isoqui-nolin-4-one with BH3 in THF afforded 8-nitro-1,3,4,6,11,1 la-hexahydro [l,4]oxazino[4,3-/)]isoquinoline (97MIP4). Then the nitro group was reduced by catalytic hydrogenation over 5% Pd/C catalyst in acidified MeOH to yield 8-amino derivative. Catalytic hydrogenation of 3-nitro-6,6a,7,... 

A somewhat more complex application of this notion is represented by the CNS stimulant fencamfine (83). Diels-Alder addition of cyclopentadiene and nitrostyrene affords the norbomene derivative, 80. Catalytic hydrogenation reduces both the remaining double bond and the nitro group (81). ° Condensation with acetaldehyde gives the corresponding imine (82) a second reduction step completes the synthesis of fencamfine (83). ... 

Arylamines are usually prepared by nitration of an aromatic starting material, followed by reduction of the nitro group (Section 16.2). The reduction step can be carried out in many different ways, depending on the circumstances. Catalytic hydrogenation over platinum works well but is often incompatible with... 

Amino-2-methoxyphenyl)perhydropyrido[l,2-tf]pyrazine was prepared from a 2-(5-nitro-2-methoxyphenyl)-3-one derivative by catalytic hydrogenation over Pd/C catalyst, followed by the reduction of the 3-oxo group by treatment with BH3-THF complex <1999WO99/042465>. A nitro group was reduced to an amino group in 2-[4-(3-nitrophenyl)piperazin-l-yl]butyl]perhydropyrido[l,2-tf]pyrazine-l,4-dione <2001JME186>, in 8-hydroxy-... 

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