Hydrazo compounds: reduction

Reductions with zinc are carried out in aqueous [160 as well as anhydrous solvents [163 and at different pHs of the medium. The choice of the reaction conditions is very important since entirely different results may be obtained under different conditions. While reduction of aromatic nitro groups in alkali hydroxides or aqueous ammonia gives hydrazo compounds, reduction in aqueous ammonium chloride gives hydroxylamines, and reduction in acidic medium amines (p. 73). Of organic solvents the most efficient seem to be dimethyl formamide [164 and acetic anhydride [755]. However, alcohols have... 

Azo-compounds can be obtained by reduction of nitro-compounds, or by oxidation of hydrazo-compounds. They are usually prepared, however, by reacting a phenol or amine with a diazonium salt. The coupling usually takes place in the position para to the hydroxyl or amino group, but if this position is occupied it goes to the ortho position, e.g. 

A great variety of solvents has been used with success. Reactive solvents, such as acetic anhydride, will react with the amine as formed. Basic solvents cause the formation of azo, azoxy. and hydrazo compounds, paralleling chemical reductions (39,73). 

Isocyanates and isothiocyanates are reduced to methylamines on treatment with LiAlH4. Lithium aluminium hydride does not usually reduce azo compounds (indeed these are the products from LiAlH4 reduction of nitro compounds, 19-59), but these can be reduced to hydrazo compounds by catalytic hydrogenation or with... 

Nitro compounds can be further reduced to hydrazo compounds with zinc and sodium hydroxide, with hydrazine hydrate and Raney nickel,or with LiAlH4 mixed with a metal chloride such as TiCU or VCl3. The reduction has also been accomplished electrochemically. 

Alkaline reduction of o-nitro-anisole leads (as in the case of nitrobenzene) to the hydrazo-compound, which undergoes a benzidine trans-... 

The reduction of azo compounds to hydrazo compounds is also achieved by means of aryltellurols or sodium hydrogen telluride. The last reagent (generated from NaBH4 and... 

Aromatic nitro compounds were among the first organic compounds ever reduced. The nitro group is readily converted to a series of functions of various degrees of reduction very exceptionally to a nitroso group, more often to a hydroxylamino group and most frequently to the amino group. In addition azoxy, azo and hydrazo compounds are formed by combination of two molecules of the reduction intermediates (Scheme 58). 

Reduction of nitro compounds with zinc in alkali hydroxides gives azo and hydrazo compounds. With an excess of zinc hydrazo compounds are obtained which are easily reoxidized to azo compounds Procedure 32, p. 213). 

Hydrazo compounds were hydrogenolyzed to primary amines by catalytic hydrogenation over palladium [740]. Since hydrazo compounds are intermediates in the reductive cleavage of azo compounds to amines it is very likely that all the reducing agents converting azo compounds to amines cleave also the hydrazo compounds (p. 96). 

Reduction of azobenzenes in an aprotic solvent generates nucleophilic nitrogen species, which can react with an added alkyl halide. In this way, cyclic hydrazo compounds can be generated from a,o>-dibromoalkanes [t07]. 

The bimolecular reduction of nitro compounds is believed to involve reduction of some of the starting material to a nitroso compound and another portion to either a substituted hydroxylamine or an amine. These intermediates, in turn, condense to form the azo compound. The exact mechanism of the reaction requires critical study. On the one hand, reducing conditions are always on the alkaline side to prevent the benzidine rearrangement of an intermediate hydrazo compound under acidic conditions, yet it is difficult to visualize the formation of hydrazo compounds by the indicated condensation. As a practical matter, this method is of value only if symmetrically substituted azo compounds are desired. 

Further complications of the reduction of aromatic nitro compounds are the possibility of complete reduction to aromatic amines (which may condense with nitroso compounds to give the desired azo compounds), reduction of azo compounds to the corresponding hydrazo compounds, followed by a benzidine (or semidine) rearrangement. It is clear, therefore, that the level of reducing agent used and other reaction conditions are quite critical. 

The preparation of azobenzene by reduction of nitrobenzene with zinc dust and sodium hydroxide has been well described [60] except that no specific mention is made of the desirability of carrying the reaction out in the presence of air. As a matter of fact, in the older literature, mention is made of blowing air through the filtrate after the reduction has been completed and the insoluble salts have been filtered off [59], This procedure has been recommended more recently also. Actually, air has been drawn through the product solution for as long as 6 hr to oxidize any hydrazo compound which may be present [61]. However, as shown by Blackadder and Hinshelwood [50], we believe that the presence of modest amounts of air toward the end of the reduction cycle should be sufficient for obtaining adequate yields. 

The bimolecular reduction of aromatic nitro compounds, depending on reaction conditions, may proceed by way of azoxy and azo compounds to 1,2-diarylhydrazines (also referred to as hydrazo compounds). This may be... 

The bimolecular reduction of aromatic nitro compounds, depending on reaction conditions, may produce azoxy compounds, azo compounds, hydrazo compounds (1,2-diarylhydrazines), benzidines, or amines. Whereas the reduction with zinc and sodium hydroxide leads to azo compounds, zinc and acetic acid/acetic anhydride produces azoxy compounds. Other reducing agents suggested are stannous chloride, magnesium with anhydrous methanol, a sodium-lead alloy in ethanol, thallium in ethanol, and sodium arsenite. 

The details of the mechanism for the conversion of nitrobenzene into azoxy-benzene need further amplification. It also should be pointed out that, in the preparation of azo compounds by bimolecular reduction, hydrazo compounds seem to form invariably since the directions invariably call for the reoxidation of the hydrazo product with air (see Chapter 14, Azo Compounds and Meisenheimer and Witte [39]). 

Reduction of Nitro to Hydrazo Compounds /V-Hydrogen-de-bisoxygen-coupling... 

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