Hydroxylamine unsymmetrical preparation

Grignard reagents have reacted with diimide dioxides prepared from nitrosohydroxylamines and with toluenesulfonyl derivatives of nitroso-hydroxylamines to prepare unsymmetrical azoxy compounds, including aliphatic-aromatic types. 

In a reaction similar to 12-50, azoxy compounds can be prepared by the condensation of a nitroso compound with a hydroxylamine. The position of the oxygen in the final product is determined by the nature of the R groups, not by which R groups came from which starting compound. Both R and R can be alkyl or aryl, but when two different aryl groups are involved, mixtures of azoxy compounds (ArNONAr, ArNONAr, and Ar NONAr ) are obtained and the unsymmetrical product (ArNONAr ) is likely to be formed in the smallest amount. This behavior is probably caused by an equilibration between the starting compounds prior to the actual reaction (ArNO -I- Ar NHOH Ar NO - - ArNHOH). The mechanism has been investigated in the presence of base. Under these conditions both reactants are converted to radical anions, which couple ... 

A convenient synthetic procedure for the preparation of azo compounds, particularly unsymmetrically substituted ones, involves the reaction of aromatic nitroso compounds with aromatic amines [31a, b]. The reaction is of particular interest because the replacement of the amine by the corresponding hydroxylamine leads to the formation of the related azoxy compounds (see Chapter 15, Azoxy Compounds ). 

It is self-evident that one of the simpler methods of preparing unsymmetrically substituted azoxy compounds must involve the condensation of two distinctly different starting materials. In principle, the reaction of C-nitroso compounds with hydroxylamines meets this requirement (Eq. 1). 

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. 

A very flexible method of preparing unsymmetrical azo compounds makes use of the condensation of C-nitroso compounds with amines. Thionylamines have also been condensed with substituted hydroxylamines to produce azo compounds not usually accessible by other means. Treatment of dialkylsulfuric diamides with sodium hypochlorite is one means of preparing aliphatic azo compounds. Aromatic amines and aromatic nitro compounds at high temperature produce azo compounds. 

The reduction of aromatic nitro compounds is believed to proceed to an intermediate mixture of nitroso compounds and substituted hydroxylamines which are not isolated but condense to form an azoxy compound which, in turn, is reduced to an azo compound. Contributing evidence to substantiate this mechanism is that the reduction of a mixture of two aromatic nitro compounds leads to a mixture of azo compounds consistent with that predicted if each of the nitro compounds were reduced to a nitroso compound and a hydroxylamine and these, in turn, reacted with each other in all possible combinations. This observation also implies that the bimolecular reduction of nitro compounds is practical only from the preparative standpoint for the production of symmetrically substituted azo compounds. Spectrophotometric studies of the reaction kinetics of the reduction of variously substituted nitro compounds may, however, uncover reasonable procedures for the synthesis of unsymmetrical azo compounds. 

As an alternative to an olefination method, the Cope elimination can be used as a preparative method for unsymmetrical hydroxylamines. Addition of unsymmetrical secondary amine 41 to a,p-unsaturated compounds gave access to compound 42, which underwent oxidation and Cope elimination to yield the unsymmetrical hydroxylamine product 44.16... 

The trihydroxybenzene which has the unsymmetrical structure C6H3(0H)3(1, 3, 4) is called hydroxyquinol. It can be prepared by fusing quinol with potassium hydroxide. Like pyrogallol it does not react with hydroxylamine. 

The crude product obtained was purified by column chromatography over silica gel with 1 1 ethyl acetate-hexane as the eluant to secure the final compound (229) in 55% yield. Careful isolahon and characterization showed that one of the major by-products of this reaction to be the bisurea derivative (259) in yields of up to 20%. We explored phe-nylchloroformate as a substitute to triphosgene in the preparation of unsymmetrical urea derivatives. Phenylchloroformate (260) in the presence of an appropriate base on reaction with an amine (241) gave the corresponding carbamate, which on further reaction with A-methyl hydroxylamine gave a urea derivative (229). However, the formation of bisurea... 

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