Oxidation of arylhydrazines

The oxidation of arylhydrazines with An2TeO is unsuitable for synthetic purposes, since it furnishes a complex mixture of products. 

Reaction with hydrazo compounds. Potassium superoxide reacts with various hydrazo and related compounds in a variety of ways as shown in the examples. Oxidation of arylhydrazines also results in biaryls derived from the solvent and thus involves free aryl radicals. 

In the hydrogenation, 200 g of acetophenone azine, 1000 ml of EtOAc and 5 g of 10% Pd-on-C was shaken at 30-50 psig for 10 h. Hydrogen absorption had ceased. About 7.6 g of phenylethylamine, formed by cleavage of the N—N bond, was obtained as a by-product. Oxidation of hydrazines can be done catalytically. Ethyl 2-arylhydrazine carboxylates were oxidized easily by bubbling air at 25 "C through a toluene or dioxane solution in the presence of Pd or Pt (5d). 

Other non-traditional preparations of 1,2,3-triazoles have been reported. The rearrangement in dioxane/water of (Z)-arylhydrazones of 5-amino-3-benzoyl-l,2,4-oxadiazole into (2-aryl-5-phenyl-27/-l,2,3-triazol-4-yl)ureas was investigated mechanistically in terms of substituents on different pathways <06JOC5616>. A general and efficient method for the preparation of 2,4-diary 1-1,2,3-triazoles 140 from a-hydroxyacetophenones 139 and arylhydrazines is reported <06SC2461>. 5-Alkylamino-] //-], 2,3-triazoles were obtained by base-mediated cleavage of cycloadducts of azides to cyclic ketene acetals <06S1943>. Oxidation of N-... 

The electrochemical oxidation of A, Ai-dibenzyl, Ai -arylhydrazines gives carbocations that react with alkenes affording five-membered heterocycles (Scheme 48) [70]. 

Photo-oxidation of l,l-dialkyl-2-arylhydrazines by single-electron transfer with trimethylsilyl cyanide (TMSCN) as cyanide ion source leads to regio- and stereoselective a-hydrazino nitriles. This stereoselective cyanation of hydrazines takes place on the more substituted carbon atom compared with the results obtained with tertiary amines (Scheme 5). 

The present author has used the method to obtain an approximate rate constant for the oxidation of an arylhydrazine with oxygen, and to show that the reaction occurred in film . A saturated solution of the hydrazine in an aqueous sodium chloride solution was reacted... 

Since oximes are more stable to oxidation and often more available than the corresponding carbonyl compounds they also can be used as starting material for the cyclizations. For instance, quinoxaline-2-aldehyde oxime 49a as well as ketoxime 49b are converted by hydrazine in acidic medium into flavazoles 50 (Scheme 14) (01JHC829). Hydroxylamine, which is liberated in the first step, may act as a hydride ion acceptor. This minimizes using a large excess of arylhydrazine or any another oxidant. 

Ce4+ can be also used for the same type of reaction, since it is a strong one-electron oxidant. Generation of sp2 carbon-centered radicals such as aryl radicals, is not so easy, except for the reactions of aryl halides with Bu3SnH or Ph4Si2H2. However, treatment of arylhydrazines with Cu2+ generates aryl radicals through the initial oxidation to the arenediazonium ion with Cu2+, and subsequent SET from Cu+. Aryl radicals are much more reactive than alkyl radicals, and rapidly react with alkenes or imines as shown below (eq. 4.22) [60-63]. 

Arylhydrazines undergo complex oxidation reactions from which a variety of products such as arylhydrocarbons, biarj ls, azo compounds, tetrazenes and phenols may be obtained. Hypochlorite oxidation of aryl semicarbazides, however, affords aryl azides possibly by... 

Azo Coupling. The coupling reaction between an aromatic diazo compound and a coupling component is the single most important synthetic route to azo dyes. Of the total dyes manufactured, about 60% are produced by this reaction. Other methods include oxidative coupling, reaction of arylhydrazine with quinones, and oxidation of aromatic amines. These methods, however, have limited industrial applications. 

Aromatic-aliphatic azo compounds can without exception be prepared by oxidation of the corresponding JV-alkyl-A -arylhydrazines. The oxidant must be one that reacts in neutral or alkaline conditions since the azo product rearranges to arylhydrazone in an acid medium. Mercury(n) oxide, introduced by Emil Fischer341 as oxidant, is almost always used for this reaction. 30% Hydrogen peroxide in hydrogen carbonate solution has been applied to dehydrogenation of A-aryl-A -(triphenylmethyl)hydrazines with good results.342... 

The mixed hydrazones (667), prepared from diacetyl monobenzoyl hydrazone and arylhydrazines, undergo oxidative cyclization to 2-aryl-jV-benzoyl-4,5-dimethyl-l,2,3-triazol-l-ylimines (668) in 32-76% yield upon treatment with lead tetraacetate in acetonitrile (Scheme 132) <92JOC2252>. The cychzation of bishydrazones to 1,2,3-triazoles can also occur in acidic or basic media. For instance, the tetrahydrobenzo[ /]triazol-4-one (669) is prepared by the base-catalyzed cyclization of the corresponding a-hydrazono oxime (Equation (53)) <85HCA1748>. 3-Methyl-1,2-cyclohexanedione reacts... 

Acyl substituents at the 3- and/or 4-positions result in decreased hydrolytic stability compared with the alkyl and aryl derivatives described above. Despite this constraint most of the usual reactions of the carbonyl group are possible. Aldehydes <9ILA1211> and ketones are oxidized to the carboxylic acid, borohydride reduction affords the expected alcohols, and epoxides are formed on reaction with diazomethane. Oximes and arylhydrazones are formed with hydroxylamine and arylhydrazines, and the products may subsequently undergo monocyclic rearrangement involving the oxadiazole to give the corresponding isomeric furazans and 1,2,3-triazoles (Section 4.05.5.1.4). 

Bis[4-methoxyphenyl] tellurium oxide was found to be a mild and highly selective oxidizing agent for the conversion of thiocarbonyl groups to carbonyl groups, thiols to disulfides, arylhydrazines to arenes, and 1,2- or 1,4-dihydroxyarenes to quinones. No reaction was observed with simple phenols, alcohols, enamines, amines (including pyrrole, indole, tryptophan, tyrosine, aniline, and dimethylaniline), oximes, dithiolanes, isonitriles, and 2,4-dinitrophenylhydrazones4. 

The addition of hydrazines to chalcones gives dihydropyrazoles, which can be functionalized further. Eor example, a,/3-unsaturated ketones 643 were condensed with arylhydrazines to yield dihydropyrazoles 644 these can be further alkylated and oxidized to l,3,5-triaryl-4-alkylpyrazoles 645, which are novel ligands for the estrogen receptor (Scheme 80) <20000L2833>. A parallel solution-phase synthesis of an N-substituted 2-pyrazoline library 648... 

Both O-alkyl hydroxylamines and, especially, arylhydrazines, are slightly oxidized compounds. The presence of any oxidizers in the reaction mixtures must be excluded. Nevertheless, in real practice, these mixtures very often contain some by-products (e.g., ArNH2, ArOH, ArH, etc.). Usually, there are no problems to reveal their chromatographic peaks, because aU of them have lower retention parameters than those for the initial reagents and, moreover, all target derivatives. The condensation reaction of the considered type can be characterized by statistically processed differences of retention indices of products and initial substrates. This mode of additive scheme permits us to estimate these analytical parameters for any new derivatives on standard nonpolar polydimethyl siloxanes. For the simplest reaction scheme, Ah— —>Bh—, AMW = MW(B) -MW(A) and ARl, = RI(B) RI(A) ... 

Deamination. On exposure to nitric oxide and a catalytic amount of oxygen the amino group of arylamines and heterarylamines is removed. Similarly, arylhydrazines are converted to the hydrocarbons although aryl azides are also formed as minor products. ... 

Maleic anhydride and hydrazine give the hydroxy-pyridazinone ( maleic hydrazide ) directly, " the additional unsaturation in the 1,4-dicarbonyl component meaning that an oxidative step is not reqnired conversion of 3-hydroxypyridazin-6-one into 3,6-dichloropyridazine makes this useful intermediate very easily available. Mucohalo acids (18.1.1.4), synthons for 4-carboxy-aldehydes, are an oxidation level down and produce l-aryl-pyridazin-3-ones on reaction with arylhydrazines. ... 

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