Formation of azo compounds

Secondly, Leyva et al found that the solution phase photochemistry of phenylazide (PA) was temperature dependent. Photolysis of PA in the presence of diethylamine at ambient temperature yields azepine 2, first prepared by Doering and Odum. Lowering the temperature suppresses the yield of 2 and encomages the formation of azo compound. Thus, high temperatures favor reactions of singlet state intermediates, whilst low temperatures favor reactions associated with triplet phenylnitrene. 

The oxidation of aromatic amines with peracids had been the subject of some dispute. It has now been demonstrated that simple oxidation of aromatic amines with peracids produces azoxy compounds without the intermediate formation of azo compounds [71]. To be sure, small amounts of azo compounds were isolated from the reaction mixture, but this was considered a side reaction. 

In contrast to classical Meerwein arylations, non-activated alkenes are well suited for this reaction type for two reasons. First, due to the relatively slow formation of azo compounds by addition of aryl radical 49 to 48, this undesired pathway cannot compete successfully with the attack of 49 on the alkene to give radical adduct 50. Second, a nucleophilic alkyl radical 50 arises from the addition step, which is effectively trapped by electrophilic salt 48 to give azo compound 51. As a result of several improvements, the methodology is now applicable for a wide range of polar to non-polar alkenes with almost no restrictions on the substitution pattern of the diazonium salt [101, 102]. Moderate diastereoselectivities have been obtained in first attempts with chiral auxiliaries [103]. The azo compounds accessible, such as 51, can be converted to carboamination products 52 by hydrogenation and to various other heterocycles. 

In basic solution head-to-head coupling of the deprotonated radicals result in the formation of azo compounds (21) from Ai -unsubstituted anilines. This occurs via the formation of hydrazo derivatives. p-Substituted anilines also preferentially undergo head-to-head coupling to yield azo derivatives. 

Swenton found a somewhat analogous behaviour in the photodecomposition of 2-azidobiphenyl. Direct photolysis resulted in ring closure to carbazole (71%), acetophenone sensitized photolysis produced 43% of the azo compound and less than 8% carbazole. The presence of piperylene, a triplet quencher, during photolysis reduced the formation of azo compound to 4% and enhanced the carbazole yield to 89%. 

One of the most ubiquitous reactions is the formation of azo compounds by the attack of nitrene on the azido group. This process accounts for the high overall quantum yield of nitrogen evolution in the gas phase photolysis of hydrazoic acid and of methyl azide It has also been demonstrated in the gas phase pyrolysis of phenyl azide and of or//jo-trifiuoromethyl azide where azobenzenes arc the major reaction products. Azo compounds are formed in near quantitative yield in the solution photolysis of / -methoxyphenyl azide and... 

The photolysis of mixtures of aromatic azides in benzene produced mixed azo derivatives and this was taken as evidence for a nitrene mechanism in the formation of azo compounds . 4-Azidobiphenyl (128) and />-methoxyphenyl azide (124) yielded the mixed azo compound 129 as well as the expected symmetrical derivatives 125 and... 

As we have seen, the ring of a phenol is reactive enough to undergo attack by diazonium salts, with the formation of azo compounds. This reaction is discussed in detail in Sec. 23.17. 

Tlie formation of free radical HN by decomposition of hydrazoic acid has been suggested by a number of authors since 1928 (see Vol. HI, p. 167). This was substantiated by experiments on the decomposition of HNj by the flasli photolysis of Thrush (Vol. Ill, p. 167). The formation of nitrene radicals from azides by flash photolysis was reported simultaneously and independently by Koto (56], Reiser et al. [57—59]. Reiser rationalized the reaction derived from flash pl otolysis of formation of azo compounds (5) ... 

Thus, the application of urea herbicides docs not actually involve the danger of the formation of azo compounds in the environment or in food. 

The temperature at which photolysis of this azide is carried out has been shown to have a profound effect upon the nature of the products. Formation of azo compound on room temperature photolysis in preference to 85 was considered to be the reaction of a lazy nitrene. Raising the photolysis temperature makes the photolytically generated triplet sufficiently energetic to undergo C-H insertion to give 85. The importance of the right choice of conditions to be used in the decomposition of aryl azides for the purpose of heterocyclic synthesis has been stressed. 

The remaining two reactions, which will be described in this section, are related through the well-known formation of azo compounds 39 by the addition of nucleophiles to diazonium salts (40). The former compounds lose nitrogen by homolytic and the latter by heterolytic processes to give, respectively, aryl radicals and sometimes, as has been recently demonstrated, aryl cations. Because the equilibrium between 39 and 40 is so dependent on solvent, gegenion, and additives, both reactions [equations (8) and (9)] often occur simultaneously. ... 

Formation of azo compounds by condensation reactions is a less common means of synthesis but can occur by the reaction of primary aromatic amines with nitrosobenzenes (equation 23)—the Mills reactionAliphatic azo compounds have not been prepared in this manner. 

Secondary aromatic amines and, in part, aromatic diamines are very efficient hydrogen donors (H donors). The reaction products can react further with the radicals, similar to phenols. The use of amine stabilizers can lead to discoloration of the plastic due to the formation of azo compounds (chinonimines). Therefore, the use of aromatic amines is limited to carbon-black filled elastomers as well as other substrates where discoloration is not an issue. 

---