Nitrosophenols, reduction

Bruciquinone forms a reddish-yellow monosemicarbazone and a yellowTsh-green monoxime (both isolated as perchlorates (159)), while at higher temperatures (80°) a reddish-violet hydrate of the monoxime is formed (probable structure XCIV). The formation of a monoxime, even ivith an excess of hydroxylamine, suggests that this derivative may exist as the tautomeric nitrosophenol. Reduction of either the monoxime or the monosemicarbazone by tin and hydrochloric acid yields aminohydroxy-strychnine (XCV), while the dihydro derivative of XCV results from the uptake of six atoms of hydrogen over PtOa (159). 

Nitrosoarenes are readily formed by the oxidation of primary N-hydroxy arylamines and several mechanisms appear to be involved. These include 1) the metal-catalyzed oxidation/reduction to nitrosoarenes, azoxyarenes and arylamines (144) 2) the 02-dependent, metal-catalyzed oxidation to nitrosoarenes (145) 3) the 02-dependent, hemoglobin-mediated co-oxidation to nitrosoarenes and methe-moglobin (146) and 4) the 0 2-dependent conversion of N-hydroxy arylamines to nitrosoarenes, nitrosophenols and nitroarenes (147,148). Each of these processes can involve intermediate nitroxide radicals, superoxide anion radicals, hydrogen peroxide and hydroxyl radicals, all of which have been observed in model systems (149,151). Although these radicals are electrophilic and have been suggested to result in DNA damage (151,152), a causal relationship has not yet been established. Nitrosoarenes, on the other hand, are readily formed in in vitro metabolic incubations (2,153) and have been shown to react covalently with lipids (154), proteins (28,155) and GSH (17,156-159). Nitrosoarenes are also readily reduced to N-hydroxy arylamines by ascorbic acid (17,160) and by reduced pyridine nucleotides (9,161). 

Azobenzene, 340/13/6.5 azoformamide, 260/194/925 diketene, 125/140/- 4-nitroisopropylbenzene, 250/> 182/830 4-nitrophenol, 280/> 199/1,030 4-nitrosophenol, 120/23/5. It is concluded that the first and last compounds are of relatively low hazard [1], Improved equipment has provided more accurate and detailed results for a further 7 compounds and has shown the effect of variations in the initial stage of decomposition on the final pressure attained, and of the increase in pressure causing a reduction in the rate of pressure rise. At 0.2 g/cm3 loadings, comparable results are -... 

The experimental model used to illustrate the ECE mechanism was the reduction of p-nitrosophenol at a mercury electrode, in which the chemical step is dehydration [54]. The experimental data have been analyzed by best-fitting curve pro-... 

Various nitrosoarenes have been utilized as benzofurazan precursors including o-azido derivatives generated from the o-chloro analogues <66JCS(B)1004>, and 1-amino-2-nitrosoarenes which can be oxidized with ferricyanide or hypochlorite. Treatment of o-nitrosophenols with hydroxylamine also affords the furazan, presumably via an oximation-dehydration pathway involving the tautomeric o-quinone monooxime. Other related approaches involve the reduction of o-dinitroarenes with borohydride, and the thermolysis of o-nitroanilines and o-nitroacetanilides. 

Examples of this mechanism are the reduction of o-nitrophenol, p-nitrosophenol, uranium complexes, and tocopherols, among many others [55], and also proteins containing two redox sites including the possibility of an intramolecular electron transfer [35]. 

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. 

Such reaction pathways are common for many systems, e.g., isomerizations, couplings, and homogeneous disproportionation, studied mainly in nonaqueous media [i-iii]. The reduction of p-nitrosophenol is a typical example [ii] ... 

Nitrophenols, alkylation, 148 o-Nitrophenol-p-sulfonic add, 107 p-Nitrophenyloxamic add, 93 o-NitrophenyBiydrazine, 97 p-NitrophenyDiydrazine, 95 Nitrophenylpyrazolones, 97 Nitrosobenzene, 78 p-Nitrosodiediylaniline, 309 p-Nitrosodimediylaniline, 308, 311 reduction, 78, 312 Nitroso- -naphthol, 80, 201 p-Nitrosophenol, 85 Nitrosylsulfuric add, 9, 247 o-Nitrotoluene, 53, 160 m-Nitrotoluene, 51 Nitrotoluenes, 3... 

Linear potential sweep and cyclic voltammetry are at their best for qualitative studies of the reactions occurring in a certain range of potential. In Fig. 5L, for example, we see the cyclic voltammogram obtain on a mercury-drop electrode in a solution of p-nitrosophenol in acetate buffer. Starting at a potential of 0.3 V versus SCE, and sweeping in the cathodic direction, one observes the first reduction peak at about - 0.1 V. This potential corresponds to the reduction of... 

One synthesis of the analgesic paracetamol (acetaminophen, 12) involves the nitrosation of phenol followed by reduction of the 4-nitrosophenol. Subsequent selective acetylation of the more reactive amino group completes the process ... 

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... 

The Baudisch reaction295 makes it possible simultaneously to introduce a nitroso and a hydroxyl group in the ortho-position to one another. In this reaction NOH radicals are produced by oxidation of hydroxylamine or reduction of nitrous acid these radicals, in conjunction with an oxidizing agent and in the presence of a copper salt as catalyst attack the aromatic nucleus. Yields are seldom very high, but few o-nitrosophenols are easily prepared in other ways. Cronheim296 has reported the first preparation of fifty mono-and di-substituted n-nitrosophenols by the Baudisch reaction. 

Nitration of dihydrovomicine proved more difficult (30% nitric acid), but a dinitro derivative has been characterized (16). Reduction of dinitro-dihydrovomicine with tin and hydrochloric acid yields the aminohydro-quinone, CXXX. This is quite understandable if the nitrosophenol (quinone monoxime), CXXIX, is assumed as the intermediate. 

Reaction of [Ru(CO)3(PPh3)2] with 2-nitrophenol in boiling gives the unusual Ru" complex (239) in which on nitrophenol ligand has undergone reduction to a nitrosophenolate anion and another coordinates via only the phenolate group. 

The experimental verification of the EQrrE mechanism was demonstrated on the reduction of 4-nitrosophenol [86]. Using LSV technique on a HMDE with the scan rate 0.02-30 V s a well defined peak was observed (at -0.08 V vs. SCE in buffered, pH 4.9, solvent mixture of water with ethanol). The reaction scheme... 

Chronocoulometry (CCM) of coupled chemical reactions The possibilities of CCM can be illustrated in the study of electrochemical reduction of 4-nitrophenol and 4-nitrosophenol [92]. The sequence of reactions (ECi E mechanism) is very close for both compounds as has been discussed above, in paragraph 4.2.3. The normalized chronoam-perometric working curves are constructed using the following dependence (cf [93]) ... 

Fig. 6.15 — Cyclic voltammogram for the reduction of p-nitrosophenol in an acetate buffer solution at a mercury drop electrode.

Scheme 10.11. The nitrosation of phenol (QH5OH) with nitrosyl chloride, NOCl (see Chapter 8, Section IV (b) D) to produce p-nitrosophenol (P-HOC6H4NO) and its subsequent reduction by thiophenol to the corresponding p-hydroxyphenylnitrosyl radical (p-HOCeH4NHO ). The oxidation product is the corresponding diphenyldithio ether (CeHsSSCeHs).

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