Quinone dioximes, reduction

The reduction of benzofuroxans can lead to a variety of products, depending upon the conditions. Deoxygenation to benzofurazans (40) can be effected either directly, using trialkyl phosphites, -tributyl or triphenyl - phosphine, or indirectly, via o-quinone dioximes (41), using methanol and potassium hydroxide, or hydroxyl-amine and alkali. - - The dioximes may be isolated, but... 

Reduction of benzofuroxans is usually the most convenient route to benzofurazans and o-quinone dioximes (see Section VI, C). Reduction of 4-nitrobenzofuroxan would seem to be a method of synthesis of 1,2,3-triaminobenzene, while the haloalkoxy-substitution reaction (Section VTT,B) and the rearrangements of Section VIII provide compounds accessible only with difficulty by other methods. Apart from these reactions, the benzofuroxans appear to be of limited synthetic utility. 

Benzofuroxans can be prepared by oxidation of o-quinone dioximes with, for example, fer-ricyanide or hypohalite in a process which closely parallels the formation of monocychc furoxans from glyoximes. Its utility is restricted by the availability of the starting materials which are themselves often best made by reduction of the furoxan. However, it is a valuable approach when the parent quinone or its monooxime is accessible by other means. It was, for example, the route originally used for naphtho[l,2-c]furoxan, the first aromatic-fused derivative <1886CB176>, and it is the method of choice for acenaphthofuroxans (11). In other cases oxidation of o-nitroanilines or thermolysis of o-nitroaryl azides are more suitable. 

The utility of the method depends on the availability of the dioxime precursors. When the parent quinone is readily accessible, direct oximation provides a straightforward approach which has been used for the acenaphtho- and phenanthro-furazans, (22) and (94 n= 0), although in the former case mild conditions (SOCl2, 20 °C) are required for the subsequent cyclodehydration in order to avoid fragmentation of the product (73JOC1054) (see Section 4.22.3.1.1). o-Quinone dioximes can be generated by reduction of benzo-furoxans, but in many such cases direct deoxygenation to the benzofurazan is also possible. 

Benzofuroxans are formed from o-quinone dioximes by oxidation with, for example, alkaline ferricyanide, nitric acid, bromine water and chlorine, While the reaction is usually straightforward and high yielding the method is not generally applicable since the dioximes themselves are not readily obtainable and are often best prepared via reduction of the furoxan (see Section 4.22.3.1.3). However it can be used when the parent quinones or their monooximes (o-nitrosophenols) are available from other sources. Thus it is the method of choice for the acenaphtho- and phenanthro-furoxans, (18) and (94 n = 1), respectively. In other cases alternative routes, such as the oxidation of o-nitroanilines or the thermolysis of o-nitroaryl azides, are more commonly utilized. 

A special type of reduction is exhibited by the benzofuroxans (240) these are generally reduced186,255 in two or three steps. The first two-electron reduction results in ring opening to an o-quinone dioxime (241) which is further reduced at a more negative potential. 

The oxidation of o-quinone dioximes to benzofuroxans has been known since the early years,15 and it is an efficient route, but not often a practical one, since the most convenient way to prepare an o-quinone dioxime is usually by reduction of the benzofuroxan (see Section V,D). Other methods— from o-quinones or o-nitrosophenols with hydroxylamine—are known these were well established at the time of the earlier reviews, and the reaction needs no further mention here. Ferricyanide oxidation of the trioxime 63 gives the fused furoxan 64.102 Benzofuroxan appeared as a by-product in the reaction of o-benzoquinone dioxime with diselenium dichloride.320... 

Reduction of unsaturated oximes without effect on the C=C double bond is best carried out by sodium amalgam at not too high a temperature. Quinone dioximes are best converted into the amines by use of sodium sulfide in boiling alcohol.115 Amino carboxylic acids are prepared from the corresponding oxo carboxylic acid oximes, catalytic reduction being in these cases the method of choice. 

The properties of quinones are to a certain extent similar to those of carbonyl compoimds. They form oximes, thiosemicarbazones, and phenylhydrazones. The rate of the reaction of quinones with hydroxylamine varies, and a number of quinones do not react with it at all. Substituents already present in the quinone molecule play a predominant role (16). In acid media hydroxylamine can have an oxidative influence, parallel to oximation, for example in the case of quinol, which is converted to quinone dioxime (17) free hydroxylamine can, however, have a reductive influence it reduces quinone to hydroquinone (18). 

---