Azoxy derivatives, isomerization

Isomeric azoxy derivatives 227 and 228 have been prepared by condensation of the corresponding amino and nitroso substrates (Scheme 155) (88UP1). 

The oxidation of both aliphatic and aromatic azo compounds to the corresponding azoxy derivative may be carried out with a variety of reagents. While older techniques favored chromic or nitric acid as the oxidizing agent, newer methods make use of various organic peracids or hydrogen peroxide. In the oxidation of aliphatic azo compounds, relatively weak peracids are favored to reduce the possibility of acid-catalyzed isomerization of azo compounds to hydrazones. Under controlled conditions cis azo compounds may be converted into cis azoxy compounds. 

Light-catalyzed isomerization of azoxy derivatives to o-hydroxyazo-derivatives by exposure to sunlight, whereby the oxygen migrates from the nitrogen atom to which it is attached to the nucleus which is attached to the other nitrogen atom, seems to proceed via an intramolecular mechanism LXXVII I LXXX.12... 

In a similar way, azoxy derivatives cyclize to oxadiaziridines (see Equation 99.8, X = N). Here, however, the reaction is limited to aliphatic derivatives, both open chain °- and cyclic,even if the resulting fused oxadiaziridines may have hmited stabihty. At any rate, cycHzation is usually thermally and/or photochemically reversible and the transient formation of oxadiaziridines may be also involved when either E,Z- or positional isomerizations of the azoxy derivatives are the observed final result Likewise, products from further photochemical rearrangement or fragmentation of oxadiaziridines may be obtained. 

The Af-aryl-Af-acylhydroxylamines 289 and 290 rearrange to aminophenol derivatives in the presence of sulfonyl chlorides (equation 138) as well as of iodonium salts in a reaction similar to the benzidine rearrangement (equation 139). The A-aryl-N,0-diacylhydroxylamine 291 undergoes isomerization on heating to produce dibenzoylated aminophenol 292 (equation 140) . The Wallach rearrangement consists of isomerization of aromatic azoxy compounds 293 to form the hydroxyazobenzenes 294 on heating in... 

Thanks to the discovery of two isomeric derivatives that were markedly different in their chemical behavior, such as the two azoxy isomers depicted by Structures la and lb, where R and R" are two different aromatic or aliphatic groups. 

It is well known that transitions between cis- and trans-azobenzene can be induced photochemically [145] by excitation with appropriate wavelength (cf Fig. 20). Accordingly, colored pictures may be produced in a reversible way on cholesteric layers [143]. In the same way as azobenzene, the derivatives of azoxy-benzene undergo cis-trans (or trans cis) isomerization upon excitation and induce different color changes in cholesteric phases. 

---