Azoxy compounds aliphatic, reduction

Historically this reaction developed from the assumption that the formation of azoxy compounds by the reduction of aromatic nitro compounds probably involved the intermediate formation of C-nitroso compounds and hydroxylamines. In the all-aliphatic series, this reaction appears to be quite general. Symmetrically and unsymmetrically substituted azoxy compounds have been prepared by it, the only major problems being the usual ones of developing procedures that afford good yields and of determining the exact position of the azoxy oxygen in unsymmetrically substituted products. 

Among the reductive methods of preparing azoxy compounds is the reduction of aliphatic nitroso compounds with stannous chloride. Triethyl phosphite has been used for the bimolecular reduction of fully fluorinated aromatic nitroso compounds. 

The bimolecular reduction of aliphatic nitroso compounds is complex and somewhat unreliable. With careful control of reaction conditions, a-nitroso ketones (in dimeric form) may be reduced with stannous chloride in an acidic medium at room temperature to the azoxy compounds, while dimeric a-nitroso acid derivatives may be reduced at about 50°C [10, 35, 36]. Nitrosoalkanes, on the other hand, are decomposed at room temperature to alcohols and nitrogen, and are reduced to amines at 50°-60°C. It has been postulated that only the dimeric nitroso compounds can be reduced to azoxy compounds and, in fact, that the dimer has a covalent nitrogen-nitrogen bond. Equations (31)—(34) summarize these data [10]. 

As noted, the bimolecular reduction of aromatic nitro compounds may produce azoxy compounds, azo compounds, hydrazo compounds (1,2-diaryIhydrazines), benzidines or amines (Scheme 1) depending on the reaction conditions. Zinc reduction under basic conditions generates azo compounds, whereas the use of acetic anhydride/acetic acid as the solvent system affords symmetrical azoxy compounds. Although unsymmetrical azoxy compounds are accessible in the aliphatic series, aromatic reagents yield only sym-... 

Standard methods which existed in the mid-1970s for the reduction of azo and azoxy compounds to hydrazo compounds have been reviewed. The reduction of aliphatic azo compounds is not of much use for the synthesis of aliphatic hydrazo compounds because the azo compounds are usually prepared from them. Methods for the reduction of aliphatic azoxy compounds directly to the corresponding hydrazo compounds involve the use of tin(II) chloride in hydrochloric acid and catalytic hydrogenation over platinum oxide electrochemical reduction can also be used. 

Aliphatic azoxy compounds may be prepared from nitroalkanes by electrolytic reduction to alkylhydroxylamines as described earlier followed by anodic oxidation in alkaline solution. Azoxymethane and azoxycyclopropane have been prepared by this method [80]. 

Another method is the reduction of aliphatic azodioxy compounds [81], obtained by addition of nitrosyl chloride to alkenes, to azoxy compounds. 

Reduction of aliphatic azoxy compounds gives the symmetrical hydrazines in good yield. This makes it possible to prepare such hydrazines from nitroalkanes in a three-step synthesis by reversing the current twice but without isolating the intermediates [80]. 

In summary, the reactivity of various functional groups toward Li 9-BBNH is classified into four broad categories [18] (1) rapid- or fast-reduction aldehyde, ketone, ester, lactone, acylchloride, acid anhydride, epoxide, disulfide, -alkyli-odide, and tosylate (2) slow-reduction tertiary amide, alkylbromide, and aromatic nitrile (3) sluggish-reduction carboxylic acid, aliphatic nitrile, primary amide, nitro and azoxy compounds, and secondary alkylbromide and tosylate (4) inert olefin, oxime, alkylchloride, sulfoxide, azo-compound, sulfide, sulfone, and sulfonic acid. 

In this chapter there will be discussed the reduction of aliphatic and aromatic as well as iV-nitro compounds and the further reduction of the products obtained, such as nitroso, azoxy, azo compounds, and hydroxylamines. The electrochemistry of these compounds has been treated in reviews [3]. 

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