Free Radicals diazotates

When an aqueous solution of a diazonium salt is added to an alkaline solution of a phenol, coupling occurs with formation of an azo-compound (p. 188). If ho vc cr the ntiueous solution of the diazonium salt, t. . ., />-bromohenzene diazonium chloride, is mixed with an excess of an aromatic hydrocarbon, and aqueous sodium hydroxide then added to the vigorously stirred mixture, the diazotate which is formed, e.g., BrC,H N OH, dissolves in the hydrocarbon and there undergoes decomposition with the formation of nitrogen and two free radicals. The aryl free radical then reacts with the hydrocarbon to give a... 

The free radical mechanism is confirmed by the fact that if a substituted aromatic hydrocarbon is used in this reaction, the incoming group (derived from the diazotate) may not necessarily occupy the position in the benzene ring normally determined by the substituent present—a characteristic of free radical reactions. 

Free radicals can be generated on the cellulose chain by hydrogen abstraction, oxidation, the ceric ion method, diazotization, introduction of unsaturated groups, or by y-irradiation. 

New interesting applications have been in the epoxidation of difficult olefin compounds (including hexafluoropropene) with NaOCl, side-chain chlorination of substituted toluenes, diazotization of pentafluoroaniline, polymerization with free radicals, etc. 

A limitation of both methods is that the second component must be liquid at the temperature of-the reaction, which is 5-10° for the diazohydroxide reaction and room temperature or slightly higher for the nitrosoacetylamine reaction. Experiments with solid reactants in solution have not been very successful, because of the difficulty of finding a suitable solvent. The solvent should be neutral and immiscible with water, have a high solvent action and reasonably low boiling point, and be inert to the free radicals which result from the diazo compound. The last qualification is the most difficult one to satisfy. Of the solvents which have been tried, carbon tetrachloride and chloroform appear to be the most suitable.18 From diazotized aniline and biphenyl in these solvents, some p-terphenyl is obtained, and from diazotized p-nitroaniline and biphenyl a small amount of 4-nitro-4 -phenylbiphenyl is formed. In these reactions an appreciable amount of tfie aryl halide (chlorobenzene and p-nitrochlorobenzene) is produced as a by-product. In general, the yields of products obtained by coupling with reactants in solution are extremely low. 

Action of aromatic nitro compounds on polymerization Reduction of aromatic nitro compounds Formation of nitroso aimpounds Reduction of aromatic ring Diazotization of amino nitro compounds 1,3-Cycloadditiun of nitro compr unds Thermal stability of aromatic nitro compounds Free radicals Furoxanes References... 

Diazotization of a-aminonitriles has been reported to generate free radicals, presumably from the diazonium ion. The cyanide group is clearly important, as this reaction is usually observed only with cyano diazonium ions. Diazotization of 2-amino-2-methylpropanenitrile (179) yields 2-nitrosopropanenitrile (180) which can be trapped to give 181, presumably by reaction of the radical 182 with NO. Further evidence of the existance of 182 is provided by observation of its dimerization product. 

Murphy utilizies the free radical chemistry of diazonium salts rather than aryl halides in a new indole synthesis <97TL7295>. Thus, diazotization of the anilines 65 to afford the diazonium ions 66 afforded the indoles 67 after treatment with sodium iodide. This methodology is a useful complement to Murphy s approach to indolines by radical-polar crossover reactions <97JCS(P1)1549>. 

Polyaminostyrene can undergo typical reactions of aromatic amines, such as diazotization. The diazonium salt decomposes with ferrous ions to yield polymeric free-radicals ... 

The reaction apparently involves free-radical mechanism, but arylcopper compounds take a part, at least under certain reaction conditions, as clearly demonstrated through Cohen s concise results [99,100]. The reaction was discovered as a side-reaction during probes of the Gatterman synthesis of aryl halides from diazonium salts and copper(l) halides. Probably the most known example is very practical preparation of diphenic acid (57) starting from anthranilic acid (58). The reaction is usually conducted by adding an aqueous diazotized anthranilic acid solution (diazonium salt 59) to the copper(l) reagent, in situ obtained by reduction of CUSO4 with an equimolar amount of hydroxylaminc in aqueous sodium hydroxide solution, to produce diphenic acid with a 80-90% yield [99,100], Scheme 25. 

Diazotization in tetrahydrofuran. Tetrahydrofuran is recommended as a solvent for diazotization where a) the amine forms salts (including the diazonium salts) of low solubility in dil. mineral acids, and which on account of low solubility in acetic acid cannot be used satisfactorily in the procedure of Hodgson and Mahadevan (s. Synth. Meth. A, 287) and b) where the subsequent reaction for which the diazonium salt soln. is to be used, is fast relative to the deamination. For slow reactions such as arylation of arenes via diazonium acetate, the presence of the ether can be of disadvantage, because of competition from it for the free radicals, in addition to the deamination Reaction.—E Aq. HCl added to a soln. of 2-amino-4 -phenylbiphenyl in tetrahydrofuran, treated with aq. NaNOg at 5-10°, stirred 20 min., treated with Kl-soln., heated and stirred for 2hrs. 2-iodo-4 -phenylbiphenyl. Y 88%. F. e. s. J. A. Cade and A. Pilbeam, Chem. Ind. 1959, 1578. 

The Sandmeyer reaction is not only useful for the preparation of arylbromides. The Boger group used a Sandmeyer reaction to install both a hydroxyl and chloro substituent in their synthesis of the teicoplanin aglycon (29). Diazotization of 25 followed immediately by hydroxylation with cuprous oxide (halide-free/radical conditions) fiimished phenol 26 in excellent yield, even with several sensitive functionalities present in the molecule. Several steps were then required to convert 25 into aniline 27. At this point, a Sandmeyer chlorination proceeded in moderate yield to furnish the aryl chloride 28. This particular reaction is noteworthy, as the reaction proceeded well, in the presence of a variety of functional groups, many of which were unprotected. 

The grafting reactions initiated by free radicals discussed here have generally been identifiable, at least in part, by ESR spectroscopy. Other methods include chemical oxidation of cellulose by thermal decomposition of peroxides, ultrasonic radiation, electric-arc discharge (also called corona discharge), electrolysis, mechanical milling and oxidation of products of chemical reactions in the presence of vinyl monomers. Chemical modification of cellulose, e,g. by diazotization and thiocar-bonation, increases the rate of oxidation and of graft copolymer formation in the presence of vinyl monomers. ... 

Bases, Neutral Salts.— As a base it forms salts, in which form the diazo compound is obtained by diazotization, and which though also unstable has been isolated in small quantities and the composition and properties determined. Of the three salts, the sulphate, chloride and nitrate, the first is the most stable and the last is the least stable. They are colorless crystalline neutral compounds soluble in water, difficultly soluble in alcohol and insoluble in ether. After being prepared by the ordinary diazo reaction, with sodium nitrite in cold acid water solution, they may be precipitated in crystalline form by the addition of alcohol and ether. If the diazotization is effected in alcohol solution by means of amyl nitrite or ethyl nitrite the crystals of the diazonium salt separate at once. These salts of diazo benzene all show true salt characteristics, e.g., they lower the freezing point of solutions. The diazo radical, (CeHs—N2—) is thus basic toward strong acids, and the hydroxide, the non-isolated hypothetical diazo benzene, CeHs—N2—OH, is the free base. It may be considered as the simplest aromatic diazo compound and the mother substance of all other members of the class. 

Radical Reactions.—Thiazolyl, pyridyl, and other heteroaryl radicals formed by aprotic diazotization of the corresponding heterocyclic amines substitute homolytically on thiophen with the formation of 2-heteroaryl-thiophens as the main products in 20—50% yield. The results of competitive experiments indicate that the reactivity of thiophen in this reaction at 70—80 °C is slightly higher than that of benzene. The currently accepted mechanism of the decomposition of benzoyl peroxide in thiophen has been criticized on the basis of new experimental results. No free thienyl radicals are involved in the reaction, as demonstrated by scavenging experiments, and the bithienyls formed are probably derived from dimerization of a benzoyloxythiophen radical a-complex, with subsequent loss of benzoic acid. Nitrene insertion into the thiophen ring has been observed in the thermal decomposition of 2-(2-azidobenzyl)thiophen and similar compounds, leading to thieno[3,2-6]quinoline derivatives. ... 

---