A number of aldehyde arylhydrazones have been oxidised with lead tetra-acetate. In many cases diacylhydrazines, RCO-NHaNAcAr, can be isolated in good yield. With benzaldehyde phenylhydrazone, provided that precautions are taken to avoid autoxidation, a-phenylazobenzyl acetate can be isolated in up to 27% yield. Evidence has been obtained that this compound is not the main precursor of the diacyl derivative PhCO*NH*NAcPh or of further oxidation products, but that these arise via the nitrilimine PhCrN-NPh ; this I ,3-dipolar compound has been trapped with acrylonitrile, to form 1.3-diphenylpyrazole-5-carbonitrile together with a small quantity of the isomeric 4-carbonitrile. Analogous nitrilimines have been similarly trapped during the oxidation of the p-nitrophenylhydrazones of benzaldehyde, propionaldehyde, benzoin, and benzil. Oxidations with lead tetrabenzoate, and with lead tetra-acetate in methanol, have also been carried out; they lead to azo-dibenzoates and azo-dimethyl ethers, respectively, and benzaldehyde phenylhydrazone also forms an azo-benzoate.
Two series of p-substituted N'-arylbenzohydrazonyl bromides were prepared. On solvolysis in aqueous dioxan these compounds yielded the equivalent aroylhydrazides in good yields. The solvolysis of p-substituted N'-pnitrophenylbenzohydrazonyl bromides in 80% dioxan-20% water proved suitable for kinetic studies, and a p of -0-92 was obtained when the results were subjected to a Hammett po plot. These results were interpreted in terms of an intermediate azo-carbonium ion stabilised by resonance into both aryl rings. The aroylhydrazides were cleaved by hydrochloric acid in ethanol to the aroic acids and arylhydrazines. This proved an effective method for the synthesis of Z-bromo-4-nitrophenyIhydraz!ne and a series of arylidene-2-bromo-4-nitrophenylhydrazones were prepared from this hydrazine.OTHER than in preliminary communications lagb no work has been reported on the mechanism of solvolysis of hydrazonyl halides. For such a study, we considered N'-arylhydrazonyl halides of type (I) as desirable substrates amenable, for example, to Hammett treatment for substituent variation in both Arl and ArZ. Some representatives of these materials had been described, being prepared, for example, by the action of bromine on arylidenearylhydrazones (a reaction whose mechanism we reported 2). In the bromination of such hydrazones, with even relatively deactivated arylhydrazines (e.g., with Ar2 = +-NO,) both hydrazonic and concurrent nuclear bromination were reported. Thus, Burgess and Gibson observed that treatment of benzaldehyde pnitrophenylhydrazone with bromine in acetic acid afforded N'-(2-bromo-4-nitrophenyl)benzohydrazonyl bromide (11), but they offered no proof of its structure.We treated a series of para-substituted benzaldehyde 9-nitrophenylhydrazones (1 equiv. , suspended or dissolved) with bromine (4 mol. in glacial acetic acid) at room temperature. With aldehyde derivatives (111) containing electron-withdrawing groups (such as C1, Br, and NO,) in the para (X) position, reactions were slow compared with those of the corresponding p-tolyl, panisyl, and phenyl compounds and more bromine (10 mol.) and longer times (up to three weeks) were required. [With the less reactive compounds, use of the (initial) milder conditions afforded monobromo-compounds (IV)] .The dibromo-compounds (11) did not liberate iodine from acidified potassium iodide solution and thus neither bromine atom was on nitrogen. We confirmed the position of the nuclear bromine in each hydrazonyl bromide by the sequence : unambiguous synthesis of 2-bromo-4-nitrophenylhydrazine (V) by Chattaway and Irving's m e t h~d ,~ preparation of the appropriate hydrazones (VI), and reaction of these with bromine. The last step afforded materials identical with those obtained by direct bromination of the 9-nitrophenylhydrazones (111). We verified that under our solvolysis conditions the nuclear bromine atoms in compounds (VI) [and thus in compounds (II)] were not labile, since t Present address: the compounds were refluxed in both 95% ethanol and 50% aqueous dioxan for 7 hr...
Five aroylhydrazines have been oxidised by lead tetra-acetate, usually a t room temperature, to give, after hydrolysis, high yields of the corresponding aroic acids. A study of the reaction of benzhydrazide under various conditions has provided evidence that reaction occurs by way of the aroyldi-imide, and this can be diverted in part to give benzaldehyde and thence, by further reactions, 2,5-diphenyloxadiazole. When the hydrazide is in considerable excess, it can trap an intermediate in the oxidation to give NN'-dibenzoylhydrazine. The oxidation of benzhydrazide by mercuric acetate occurs in a similar manner, although not under such mild conditions. MONOACYLHYDRAZINES are susceptible to oxidation by a or 1 equivalent of either sulphur monochloride or variety of reagents. In some conditions the correspond-N-bromosuccinimide.4 In other conditions, other types ing NN'-diacylhydrazine is formed, as with iodine of product can be obtained: for example, oxidation ( B ) , 1968, 994.
Boron trifluoride has been used to catalyse the reactions of lead tetra-acetate with benzene, toluene, and anisole.The mechanisms of these reactions are discussed. The boron trifluoride-catalysed decomposition of phenyl-lead triacetate, and p-methoxyphenyl-lead triacetate in aromatic solvents has been investigated.
The oxidation of phenylhydrazine with lead tetra-acetate in the solvents benzene, chlorobenzene, nitrobenzene, and dichloromethane has been investigated. At low temperatures in dichloromethane the benzenediazonium ion was formed. At room temperature, benzene, azobenzene, biphenyl, and, where aromatic solvents were used, biaryls, were isolated and identified. The formation of these products can be explained in terms of the formation of phenyldi-imide and its subsequent homolytic and heterolytic breakdown.THE oxidation of phenylhydrazine was first studied by Fischer.l He oxidised phenylhydrazine in ether with mercuric oxide and isolated aniline and biphenyl.1 Chattaway 2-4 further investigated the reaction with different oxidising agents. Use of oxygen yielded benzene and nitrogen,2 use of metallic oxides gave benzene, azobenzene, and biphenyl, and use of chlorine or bromine at -15" gave the benzenediazonium These oxidations were investigated more thoroughly by Hardie and Th~mpson,~ who used a variety of different oxidising agents in aqueous and aromatic solvents. They found that oxidation with metallic oxides, benzoquinone, or chloranil in aromatic solvents yielded benzene, biphenyl, azobenzene, and substituted biaryls. These products were accounted for in terms of the phenyl radical which was postulated to arise from phenyldi-imide by homolytic cleavage : Ph2 t J-ArH
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